Adding upstream version 6.6.15.upstream/6.6.15

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

3 files changed, 4230 insertions, 0 deletions

diff --git a/Documentation/sound/kernel-api/alsa-driver-api.rst b/Documentation/sound/kernel-api/alsa-driver-api.rst
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+===================
+The ALSA Driver API
+===================
+
+Management of Cards and Devices
+===============================
+
+Card Management
+---------------
+.. kernel-doc:: sound/core/init.c
+
+Device Components
+-----------------
+.. kernel-doc:: sound/core/device.c
+
+Module requests and Device File Entries
+---------------------------------------
+.. kernel-doc:: sound/core/sound.c
+
+Memory Management Helpers
+-------------------------
+.. kernel-doc:: sound/core/memory.c
+.. kernel-doc:: sound/core/memalloc.c
+
+
+PCM API
+=======
+
+PCM Core
+--------
+.. kernel-doc:: sound/core/pcm.c
+.. kernel-doc:: sound/core/pcm_lib.c
+.. kernel-doc:: sound/core/pcm_native.c
+.. kernel-doc:: include/sound/pcm.h
+
+PCM Format Helpers
+------------------
+.. kernel-doc:: sound/core/pcm_misc.c
+
+PCM Memory Management
+---------------------
+.. kernel-doc:: sound/core/pcm_memory.c
+
+PCM DMA Engine API
+------------------
+.. kernel-doc:: sound/core/pcm_dmaengine.c
+.. kernel-doc:: include/sound/dmaengine_pcm.h
+
+Control/Mixer API
+=================
+
+General Control Interface
+-------------------------
+.. kernel-doc:: sound/core/control.c
+
+AC97 Codec API
+--------------
+.. kernel-doc:: sound/pci/ac97/ac97_codec.c
+.. kernel-doc:: sound/pci/ac97/ac97_pcm.c
+
+Virtual Master Control API
+--------------------------
+.. kernel-doc:: sound/core/vmaster.c
+.. kernel-doc:: include/sound/control.h
+
+MIDI API
+========
+
+Raw MIDI API
+------------
+.. kernel-doc:: sound/core/rawmidi.c
+
+MPU401-UART API
+---------------
+.. kernel-doc:: sound/drivers/mpu401/mpu401_uart.c
+
+Proc Info API
+=============
+
+Proc Info Interface
+-------------------
+.. kernel-doc:: sound/core/info.c
+
+Compress Offload
+================
+
+Compress Offload API
+--------------------
+.. kernel-doc:: sound/core/compress_offload.c
+.. kernel-doc:: include/uapi/sound/compress_offload.h
+.. kernel-doc:: include/uapi/sound/compress_params.h
+.. kernel-doc:: include/sound/compress_driver.h
+
+ASoC
+====
+
+ASoC Core API
+-------------
+.. kernel-doc:: include/sound/soc.h
+.. kernel-doc:: sound/soc/soc-core.c
+.. kernel-doc:: sound/soc/soc-devres.c
+.. kernel-doc:: sound/soc/soc-component.c
+.. kernel-doc:: sound/soc/soc-pcm.c
+.. kernel-doc:: sound/soc/soc-ops.c
+.. kernel-doc:: sound/soc/soc-compress.c
+
+ASoC DAPM API
+-------------
+.. kernel-doc:: sound/soc/soc-dapm.c
+
+ASoC DMA Engine API
+-------------------
+.. kernel-doc:: sound/soc/soc-generic-dmaengine-pcm.c
+
+Miscellaneous Functions
+=======================
+
+Hardware-Dependent Devices API
+------------------------------
+.. kernel-doc:: sound/core/hwdep.c
+
+Jack Abstraction Layer API
+--------------------------
+.. kernel-doc:: include/sound/jack.h
+.. kernel-doc:: sound/core/jack.c
+.. kernel-doc:: sound/soc/soc-jack.c
+
+ISA DMA Helpers
+---------------
+.. kernel-doc:: sound/core/isadma.c
+
+Other Helper Macros
+-------------------
+.. kernel-doc:: include/sound/core.h
+.. kernel-doc:: sound/sound_core.c
diff --git a/Documentation/sound/kernel-api/index.rst b/Documentation/sound/kernel-api/index.rst
new file mode 100644
index 000000000..d0e6df35b
--- /dev/null
+++ b/Documentation/sound/kernel-api/index.rst
@@ -0,0 +1,8 @@
+ALSA Kernel API Documentation
+=============================
+
+.. toctree::
+   :maxdepth: 2
+
+   alsa-driver-api
+   writing-an-alsa-driver
diff --git a/Documentation/sound/kernel-api/writing-an-alsa-driver.rst b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
new file mode 100644
index 000000000..cd4218564
--- /dev/null
+++ b/Documentation/sound/kernel-api/writing-an-alsa-driver.rst
@@ -0,0 +1,4087 @@
+======================
+Writing an ALSA Driver
+======================
+
+:Author: Takashi Iwai <tiwai@suse.de>
+
+Preface
+=======
+
+This document describes how to write an `ALSA (Advanced Linux Sound
+Architecture) <http://www.alsa-project.org/>`__ driver. The document
+focuses mainly on PCI soundcards. In the case of other device types, the
+API might be different, too. However, at least the ALSA kernel API is
+consistent, and therefore it would be still a bit help for writing them.
+
+This document targets people who already have enough C language skills
+and have basic linux kernel programming knowledge. This document doesn't
+explain the general topic of linux kernel coding and doesn't cover
+low-level driver implementation details. It only describes the standard
+way to write a PCI sound driver on ALSA.
+
+File Tree Structure
+===================
+
+General
+-------
+
+The file tree structure of ALSA driver is depicted below::
+
+            sound
+                    /core
+                            /oss
+                            /seq
+                                    /oss
+                    /include
+                    /drivers
+                            /mpu401
+                            /opl3
+                    /i2c
+                    /synth
+                            /emux
+                    /pci
+                            /(cards)
+                    /isa
+                            /(cards)
+                    /arm
+                    /ppc
+                    /sparc
+                    /usb
+                    /pcmcia /(cards)
+                    /soc
+                    /oss
+
+
+core directory
+--------------
+
+This directory contains the middle layer which is the heart of ALSA
+drivers. In this directory, the native ALSA modules are stored. The
+sub-directories contain different modules and are dependent upon the
+kernel config.
+
+core/oss
+~~~~~~~~
+
+The code for OSS PCM and mixer emulation modules is stored in this
+directory. The OSS rawmidi emulation is included in the ALSA rawmidi
+code since it's quite small. The sequencer code is stored in
+``core/seq/oss`` directory (see `below <core/seq/oss_>`__).
+
+core/seq
+~~~~~~~~
+
+This directory and its sub-directories are for the ALSA sequencer. This
+directory contains the sequencer core and primary sequencer modules such
+as snd-seq-midi, snd-seq-virmidi, etc. They are compiled only when
+``CONFIG_SND_SEQUENCER`` is set in the kernel config.
+
+core/seq/oss
+~~~~~~~~~~~~
+
+This contains the OSS sequencer emulation code.
+
+include directory
+-----------------
+
+This is the place for the public header files of ALSA drivers, which are
+to be exported to user-space, or included by several files in different
+directories. Basically, the private header files should not be placed in
+this directory, but you may still find files there, due to historical
+reasons :)
+
+drivers directory
+-----------------
+
+This directory contains code shared among different drivers on different
+architectures. They are hence supposed not to be architecture-specific.
+For example, the dummy PCM driver and the serial MIDI driver are found
+in this directory. In the sub-directories, there is code for components
+which are independent from bus and cpu architectures.
+
+drivers/mpu401
+~~~~~~~~~~~~~~
+
+The MPU401 and MPU401-UART modules are stored here.
+
+drivers/opl3 and opl4
+~~~~~~~~~~~~~~~~~~~~~
+
+The OPL3 and OPL4 FM-synth stuff is found here.
+
+i2c directory
+-------------
+
+This contains the ALSA i2c components.
+
+Although there is a standard i2c layer on Linux, ALSA has its own i2c
+code for some cards, because the soundcard needs only a simple operation
+and the standard i2c API is too complicated for such a purpose.
+
+synth directory
+---------------
+
+This contains the synth middle-level modules.
+
+So far, there is only Emu8000/Emu10k1 synth driver under the
+``synth/emux`` sub-directory.
+
+pci directory
+-------------
+
+This directory and its sub-directories hold the top-level card modules
+for PCI soundcards and the code specific to the PCI BUS.
+
+The drivers compiled from a single file are stored directly in the pci
+directory, while the drivers with several source files are stored on
+their own sub-directory (e.g. emu10k1, ice1712).
+
+isa directory
+-------------
+
+This directory and its sub-directories hold the top-level card modules
+for ISA soundcards.
+
+arm, ppc, and sparc directories
+-------------------------------
+
+They are used for top-level card modules which are specific to one of
+these architectures.
+
+usb directory
+-------------
+
+This directory contains the USB-audio driver.
+The USB MIDI driver is integrated in the usb-audio driver.
+
+pcmcia directory
+----------------
+
+The PCMCIA, especially PCCard drivers will go here. CardBus drivers will
+be in the pci directory, because their API is identical to that of
+standard PCI cards.
+
+soc directory
+-------------
+
+This directory contains the codes for ASoC (ALSA System on Chip)
+layer including ASoC core, codec and machine drivers.
+
+oss directory
+-------------
+
+This contains OSS/Lite code.
+At the time of writing, all code has been removed except for dmasound
+on m68k.
+
+
+Basic Flow for PCI Drivers
+==========================
+
+Outline
+-------
+
+The minimum flow for PCI soundcards is as follows:
+
+-  define the PCI ID table (see the section `PCI Entries`_).
+
+-  create ``probe`` callback.
+
+-  create ``remove`` callback.
+
+-  create a struct pci_driver structure
+   containing the three pointers above.
+
+-  create an ``init`` function just calling the
+   :c:func:`pci_register_driver()` to register the pci_driver
+   table defined above.
+
+-  create an ``exit`` function to call the
+   :c:func:`pci_unregister_driver()` function.
+
+Full Code Example
+-----------------
+
+The code example is shown below. Some parts are kept unimplemented at
+this moment but will be filled in the next sections. The numbers in the
+comment lines of the :c:func:`snd_mychip_probe()` function refer
+to details explained in the following section.
+
+::
+
+      #include <linux/init.h>
+      #include <linux/pci.h>
+      #include <linux/slab.h>
+      #include <sound/core.h>
+      #include <sound/initval.h>
+
+      /* module parameters (see "Module Parameters") */
+      /* SNDRV_CARDS: maximum number of cards supported by this module */
+      static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+      static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+      static bool enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+      /* definition of the chip-specific record */
+      struct mychip {
+              struct snd_card *card;
+              /* the rest of the implementation will be in section
+               * "PCI Resource Management"
+               */
+      };
+
+      /* chip-specific destructor
+       * (see "PCI Resource Management")
+       */
+      static int snd_mychip_free(struct mychip *chip)
+      {
+              .... /* will be implemented later... */
+      }
+
+      /* component-destructor
+       * (see "Management of Cards and Components")
+       */
+      static int snd_mychip_dev_free(struct snd_device *device)
+      {
+              return snd_mychip_free(device->device_data);
+      }
+
+      /* chip-specific constructor
+       * (see "Management of Cards and Components")
+       */
+      static int snd_mychip_create(struct snd_card *card,
+                                   struct pci_dev *pci,
+                                   struct mychip **rchip)
+      {
+              struct mychip *chip;
+              int err;
+              static const struct snd_device_ops ops = {
+                     .dev_free = snd_mychip_dev_free,
+              };
+
+              *rchip = NULL;
+
+              /* check PCI availability here
+               * (see "PCI Resource Management")
+               */
+              ....
+
+              /* allocate a chip-specific data with zero filled */
+              chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+              if (chip == NULL)
+                      return -ENOMEM;
+
+              chip->card = card;
+
+              /* rest of initialization here; will be implemented
+               * later, see "PCI Resource Management"
+               */
+              ....
+
+              err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+              if (err < 0) {
+                      snd_mychip_free(chip);
+                      return err;
+              }
+
+              *rchip = chip;
+              return 0;
+      }
+
+      /* constructor -- see "Driver Constructor" sub-section */
+      static int snd_mychip_probe(struct pci_dev *pci,
+                                  const struct pci_device_id *pci_id)
+      {
+              static int dev;
+              struct snd_card *card;
+              struct mychip *chip;
+              int err;
+
+              /* (1) */
+              if (dev >= SNDRV_CARDS)
+                      return -ENODEV;
+              if (!enable[dev]) {
+                      dev++;
+                      return -ENOENT;
+              }
+
+              /* (2) */
+              err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                                 0, &card);
+              if (err < 0)
+                      return err;
+
+              /* (3) */
+              err = snd_mychip_create(card, pci, &chip);
+              if (err < 0)
+                      goto error;
+
+              /* (4) */
+              strcpy(card->driver, "My Chip");
+              strcpy(card->shortname, "My Own Chip 123");
+              sprintf(card->longname, "%s at 0x%lx irq %i",
+                      card->shortname, chip->port, chip->irq);
+
+              /* (5) */
+              .... /* implemented later */
+
+              /* (6) */
+              err = snd_card_register(card);
+              if (err < 0)
+                      goto error;
+
+              /* (7) */
+              pci_set_drvdata(pci, card);
+              dev++;
+              return 0;
+
+      error:
+              snd_card_free(card);
+              return err;
+      }
+
+      /* destructor -- see the "Destructor" sub-section */
+      static void snd_mychip_remove(struct pci_dev *pci)
+      {
+              snd_card_free(pci_get_drvdata(pci));
+      }
+
+
+
+Driver Constructor
+------------------
+
+The real constructor of PCI drivers is the ``probe`` callback. The
+``probe`` callback and other component-constructors which are called
+from the ``probe`` callback cannot be used with the ``__init`` prefix
+because any PCI device could be a hotplug device.
+
+In the ``probe`` callback, the following scheme is often used.
+
+1) Check and increment the device index.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  static int dev;
+  ....
+  if (dev >= SNDRV_CARDS)
+          return -ENODEV;
+  if (!enable[dev]) {
+          dev++;
+          return -ENOENT;
+  }
+
+
+where ``enable[dev]`` is the module option.
+
+Each time the ``probe`` callback is called, check the availability of
+the device. If not available, simply increment the device index and
+return. dev will be incremented also later (`step 7
+<7) Set the PCI driver data and return zero._>`__).
+
+2) Create a card instance
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  struct snd_card *card;
+  int err;
+  ....
+  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                     0, &card);
+
+
+The details will be explained in the section `Management of Cards and
+Components`_.
+
+3) Create a main component
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In this part, the PCI resources are allocated::
+
+  struct mychip *chip;
+  ....
+  err = snd_mychip_create(card, pci, &chip);
+  if (err < 0)
+          goto error;
+
+The details will be explained in the section `PCI Resource
+Management`_.
+
+When something goes wrong, the probe function needs to deal with the
+error.  In this example, we have a single error handling path placed
+at the end of the function::
+
+  error:
+          snd_card_free(card);
+          return err;
+
+Since each component can be properly freed, the single
+:c:func:`snd_card_free()` call should suffice in most cases.
+
+
+4) Set the driver ID and name strings.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  strcpy(card->driver, "My Chip");
+  strcpy(card->shortname, "My Own Chip 123");
+  sprintf(card->longname, "%s at 0x%lx irq %i",
+          card->shortname, chip->port, chip->irq);
+
+The driver field holds the minimal ID string of the chip. This is used
+by alsa-lib's configurator, so keep it simple but unique. Even the
+same driver can have different driver IDs to distinguish the
+functionality of each chip type.
+
+The shortname field is a string shown as more verbose name. The longname
+field contains the information shown in ``/proc/asound/cards``.
+
+5) Create other components, such as mixer, MIDI, etc.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Here you define the basic components such as `PCM <PCM Interface_>`__,
+mixer (e.g. `AC97 <API for AC97 Codec_>`__), MIDI (e.g.
+`MPU-401 <MIDI (MPU401-UART) Interface_>`__), and other interfaces.
+Also, if you want a `proc file <Proc Interface_>`__, define it here,
+too.
+
+6) Register the card instance.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  err = snd_card_register(card);
+  if (err < 0)
+          goto error;
+
+Will be explained in the section `Management of Cards and
+Components`_, too.
+
+7) Set the PCI driver data and return zero.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+  pci_set_drvdata(pci, card);
+  dev++;
+  return 0;
+
+In the above, the card record is stored. This pointer is used in the
+remove callback and power-management callbacks, too.
+
+Destructor
+----------
+
+The destructor, the remove callback, simply releases the card instance.
+Then the ALSA middle layer will release all the attached components
+automatically.
+
+It would be typically just calling :c:func:`snd_card_free()`::
+
+  static void snd_mychip_remove(struct pci_dev *pci)
+  {
+          snd_card_free(pci_get_drvdata(pci));
+  }
+
+
+The above code assumes that the card pointer is set to the PCI driver
+data.
+
+Header Files
+------------
+
+For the above example, at least the following include files are
+necessary::
+
+  #include <linux/init.h>
+  #include <linux/pci.h>
+  #include <linux/slab.h>
+  #include <sound/core.h>
+  #include <sound/initval.h>
+
+where the last one is necessary only when module options are defined
+in the source file. If the code is split into several files, the files
+without module options don't need them.
+
+In addition to these headers, you'll need ``<linux/interrupt.h>`` for
+interrupt handling, and ``<linux/io.h>`` for I/O access. If you use the
+:c:func:`mdelay()` or :c:func:`udelay()` functions, you'll need
+to include ``<linux/delay.h>`` too.
+
+The ALSA interfaces like the PCM and control APIs are defined in other
+``<sound/xxx.h>`` header files. They have to be included after
+``<sound/core.h>``.
+
+Management of Cards and Components
+==================================
+
+Card Instance
+-------------
+
+For each soundcard, a “card” record must be allocated.
+
+A card record is the headquarters of the soundcard. It manages the whole
+list of devices (components) on the soundcard, such as PCM, mixers,
+MIDI, synthesizer, and so on. Also, the card record holds the ID and the
+name strings of the card, manages the root of proc files, and controls
+the power-management states and hotplug disconnections. The component
+list on the card record is used to manage the correct release of
+resources at destruction.
+
+As mentioned above, to create a card instance, call
+:c:func:`snd_card_new()`::
+
+  struct snd_card *card;
+  int err;
+  err = snd_card_new(&pci->dev, index, id, module, extra_size, &card);
+
+
+The function takes six arguments: the parent device pointer, the
+card-index number, the id string, the module pointer (usually
+``THIS_MODULE``), the size of extra-data space, and the pointer to
+return the card instance. The extra_size argument is used to allocate
+card->private_data for the chip-specific data. Note that these data are
+allocated by :c:func:`snd_card_new()`.
+
+The first argument, the pointer of struct device, specifies the parent
+device. For PCI devices, typically ``&pci->`` is passed there.
+
+Components
+----------
+
+After the card is created, you can attach the components (devices) to
+the card instance. In an ALSA driver, a component is represented as a
+struct snd_device object. A component
+can be a PCM instance, a control interface, a raw MIDI interface, etc.
+Each such instance has one component entry.
+
+A component can be created via the :c:func:`snd_device_new()`
+function::
+
+  snd_device_new(card, SNDRV_DEV_XXX, chip, &ops);
+
+This takes the card pointer, the device-level (``SNDRV_DEV_XXX``), the
+data pointer, and the callback pointers (``&ops``). The device-level
+defines the type of components and the order of registration and
+de-registration. For most components, the device-level is already
+defined. For a user-defined component, you can use
+``SNDRV_DEV_LOWLEVEL``.
+
+This function itself doesn't allocate the data space. The data must be
+allocated manually beforehand, and its pointer is passed as the
+argument. This pointer (``chip`` in the above example) is used as the
+identifier for the instance.
+
+Each pre-defined ALSA component such as AC97 and PCM calls
+:c:func:`snd_device_new()` inside its constructor. The destructor
+for each component is defined in the callback pointers. Hence, you don't
+need to take care of calling a destructor for such a component.
+
+If you wish to create your own component, you need to set the destructor
+function to the dev_free callback in the ``ops``, so that it can be
+released automatically via :c:func:`snd_card_free()`. The next
+example will show an implementation of chip-specific data.
+
+Chip-Specific Data
+------------------
+
+Chip-specific information, e.g. the I/O port address, its resource
+pointer, or the irq number, is stored in the chip-specific record::
+
+  struct mychip {
+          ....
+  };
+
+
+In general, there are two ways of allocating the chip record.
+
+1. Allocating via :c:func:`snd_card_new()`.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+As mentioned above, you can pass the extra-data-length to the 5th
+argument of :c:func:`snd_card_new()`, e.g.::
+
+  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                     sizeof(struct mychip), &card);
+
+struct mychip is the type of the chip record.
+
+In return, the allocated record can be accessed as
+
+::
+
+  struct mychip *chip = card->private_data;
+
+With this method, you don't have to allocate twice. The record is
+released together with the card instance.
+
+2. Allocating an extra device.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+After allocating a card instance via :c:func:`snd_card_new()`
+(with ``0`` on the 4th arg), call :c:func:`kzalloc()`::
+
+  struct snd_card *card;
+  struct mychip *chip;
+  err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                     0, &card);
+  .....
+  chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+
+The chip record should have the field to hold the card pointer at least,
+
+::
+
+  struct mychip {
+          struct snd_card *card;
+          ....
+  };
+
+
+Then, set the card pointer in the returned chip instance::
+
+  chip->card = card;
+
+Next, initialize the fields, and register this chip record as a
+low-level device with a specified ``ops``::
+
+  static const struct snd_device_ops ops = {
+          .dev_free =        snd_mychip_dev_free,
+  };
+  ....
+  snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+
+:c:func:`snd_mychip_dev_free()` is the device-destructor
+function, which will call the real destructor::
+
+  static int snd_mychip_dev_free(struct snd_device *device)
+  {
+          return snd_mychip_free(device->device_data);
+  }
+
+where :c:func:`snd_mychip_free()` is the real destructor.
+
+The demerit of this method is the obviously larger amount of code.
+The merit is, however, that you can trigger your own callback at
+registering and disconnecting the card via a setting in snd_device_ops.
+About registering and disconnecting the card, see the subsections
+below.
+
+
+Registration and Release
+------------------------
+
+After all components are assigned, register the card instance by calling
+:c:func:`snd_card_register()`. Access to the device files is
+enabled at this point. That is, before
+:c:func:`snd_card_register()` is called, the components are safely
+inaccessible from external side. If this call fails, exit the probe
+function after releasing the card via :c:func:`snd_card_free()`.
+
+For releasing the card instance, you can call simply
+:c:func:`snd_card_free()`. As mentioned earlier, all components
+are released automatically by this call.
+
+For a device which allows hotplugging, you can use
+:c:func:`snd_card_free_when_closed()`. This one will postpone
+the destruction until all devices are closed.
+
+PCI Resource Management
+=======================
+
+Full Code Example
+-----------------
+
+In this section, we'll complete the chip-specific constructor,
+destructor and PCI entries. Example code is shown first, below::
+
+      struct mychip {
+              struct snd_card *card;
+              struct pci_dev *pci;
+
+              unsigned long port;
+              int irq;
+      };
+
+      static int snd_mychip_free(struct mychip *chip)
+      {
+              /* disable hardware here if any */
+              .... /* (not implemented in this document) */
+
+              /* release the irq */
+              if (chip->irq >= 0)
+                      free_irq(chip->irq, chip);
+              /* release the I/O ports & memory */
+              pci_release_regions(chip->pci);
+              /* disable the PCI entry */
+              pci_disable_device(chip->pci);
+              /* release the data */
+              kfree(chip);
+              return 0;
+      }
+
+      /* chip-specific constructor */
+      static int snd_mychip_create(struct snd_card *card,
+                                   struct pci_dev *pci,
+                                   struct mychip **rchip)
+      {
+              struct mychip *chip;
+              int err;
+              static const struct snd_device_ops ops = {
+                     .dev_free = snd_mychip_dev_free,
+              };
+
+              *rchip = NULL;
+
+              /* initialize the PCI entry */
+              err = pci_enable_device(pci);
+              if (err < 0)
+                      return err;
+              /* check PCI availability (28bit DMA) */
+              if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+                  pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+                      printk(KERN_ERR "error to set 28bit mask DMA\n");
+                      pci_disable_device(pci);
+                      return -ENXIO;
+              }
+
+              chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+              if (chip == NULL) {
+                      pci_disable_device(pci);
+                      return -ENOMEM;
+              }
+
+              /* initialize the stuff */
+              chip->card = card;
+              chip->pci = pci;
+              chip->irq = -1;
+
+              /* (1) PCI resource allocation */
+              err = pci_request_regions(pci, "My Chip");
+              if (err < 0) {
+                      kfree(chip);
+                      pci_disable_device(pci);
+                      return err;
+              }
+              chip->port = pci_resource_start(pci, 0);
+              if (request_irq(pci->irq, snd_mychip_interrupt,
+                              IRQF_SHARED, KBUILD_MODNAME, chip)) {
+                      printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+                      snd_mychip_free(chip);
+                      return -EBUSY;
+              }
+              chip->irq = pci->irq;
+              card->sync_irq = chip->irq;
+
+              /* (2) initialization of the chip hardware */
+              .... /*   (not implemented in this document) */
+
+              err = snd_device_new(card, SNDRV_DEV_LOWLEVEL, chip, &ops);
+              if (err < 0) {
+                      snd_mychip_free(chip);
+                      return err;
+              }
+
+              *rchip = chip;
+              return 0;
+      }
+
+      /* PCI IDs */
+      static struct pci_device_id snd_mychip_ids[] = {
+              { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+                PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+              ....
+              { 0, }
+      };
+      MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+      /* pci_driver definition */
+      static struct pci_driver driver = {
+              .name = KBUILD_MODNAME,
+              .id_table = snd_mychip_ids,
+              .probe = snd_mychip_probe,
+              .remove = snd_mychip_remove,
+      };
+
+      /* module initialization */
+      static int __init alsa_card_mychip_init(void)
+      {
+              return pci_register_driver(&driver);
+      }
+
+      /* module clean up */
+      static void __exit alsa_card_mychip_exit(void)
+      {
+              pci_unregister_driver(&driver);
+      }
+
+      module_init(alsa_card_mychip_init)
+      module_exit(alsa_card_mychip_exit)
+
+      EXPORT_NO_SYMBOLS; /* for old kernels only */
+
+Some Hafta's
+------------
+
+The allocation of PCI resources is done in the ``probe`` function, and
+usually an extra :c:func:`xxx_create()` function is written for this
+purpose.
+
+In the case of PCI devices, you first have to call the
+:c:func:`pci_enable_device()` function before allocating
+resources. Also, you need to set the proper PCI DMA mask to limit the
+accessed I/O range. In some cases, you might need to call
+:c:func:`pci_set_master()` function, too.
+
+Suppose a 28bit mask, the code to be added would look like::
+
+  err = pci_enable_device(pci);
+  if (err < 0)
+          return err;
+  if (pci_set_dma_mask(pci, DMA_BIT_MASK(28)) < 0 ||
+      pci_set_consistent_dma_mask(pci, DMA_BIT_MASK(28)) < 0) {
+          printk(KERN_ERR "error to set 28bit mask DMA\n");
+          pci_disable_device(pci);
+          return -ENXIO;
+  }
+  
+
+Resource Allocation
+-------------------
+
+The allocation of I/O ports and irqs is done via standard kernel
+functions.  These resources must be released in the destructor
+function (see below).
+
+Now assume that the PCI device has an I/O port with 8 bytes and an
+interrupt. Then struct mychip will have the
+following fields::
+
+  struct mychip {
+          struct snd_card *card;
+
+          unsigned long port;
+          int irq;
+  };
+
+
+For an I/O port (and also a memory region), you need to have the
+resource pointer for the standard resource management. For an irq, you
+have to keep only the irq number (integer). But you need to initialize
+this number to -1 before actual allocation, since irq 0 is valid. The
+port address and its resource pointer can be initialized as null by
+:c:func:`kzalloc()` automatically, so you don't have to take care of
+resetting them.
+
+The allocation of an I/O port is done like this::
+
+  err = pci_request_regions(pci, "My Chip");
+  if (err < 0) { 
+          kfree(chip);
+          pci_disable_device(pci);
+          return err;
+  }
+  chip->port = pci_resource_start(pci, 0);
+
+It will reserve the I/O port region of 8 bytes of the given PCI device.
+The returned value, ``chip->res_port``, is allocated via
+:c:func:`kmalloc()` by :c:func:`request_region()`. The pointer
+must be released via :c:func:`kfree()`, but there is a problem with
+this. This issue will be explained later.
+
+The allocation of an interrupt source is done like this::
+
+  if (request_irq(pci->irq, snd_mychip_interrupt,
+                  IRQF_SHARED, KBUILD_MODNAME, chip)) {
+          printk(KERN_ERR "cannot grab irq %d\n", pci->irq);
+          snd_mychip_free(chip);
+          return -EBUSY;
+  }
+  chip->irq = pci->irq;
+
+where :c:func:`snd_mychip_interrupt()` is the interrupt handler
+defined `later <PCM Interrupt Handler_>`__. Note that
+``chip->irq`` should be defined only when :c:func:`request_irq()`
+succeeded.
+
+On the PCI bus, interrupts can be shared. Thus, ``IRQF_SHARED`` is used
+as the interrupt flag of :c:func:`request_irq()`.
+
+The last argument of :c:func:`request_irq()` is the data pointer
+passed to the interrupt handler. Usually, the chip-specific record is
+used for that, but you can use what you like, too.
+
+I won't give details about the interrupt handler at this point, but at
+least its appearance can be explained now. The interrupt handler looks
+usually as follows::
+
+  static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+  {
+          struct mychip *chip = dev_id;
+          ....
+          return IRQ_HANDLED;
+  }
+
+After requesting the IRQ, you can passed it to ``card->sync_irq``
+field::
+
+          card->irq = chip->irq;
+
+This allows the PCM core to automatically call
+:c:func:`synchronize_irq()` at the right time, like before ``hw_free``.
+See the later section `sync_stop callback`_ for details.
+
+Now let's write the corresponding destructor for the resources above.
+The role of destructor is simple: disable the hardware (if already
+activated) and release the resources. So far, we have no hardware part,
+so the disabling code is not written here.
+
+To release the resources, the “check-and-release” method is a safer way.
+For the interrupt, do like this::
+
+  if (chip->irq >= 0)
+          free_irq(chip->irq, chip);
+
+Since the irq number can start from 0, you should initialize
+``chip->irq`` with a negative value (e.g. -1), so that you can check
+the validity of the irq number as above.
+
+When you requested I/O ports or memory regions via
+:c:func:`pci_request_region()` or
+:c:func:`pci_request_regions()` like in this example, release the
+resource(s) using the corresponding function,
+:c:func:`pci_release_region()` or
+:c:func:`pci_release_regions()`::
+
+  pci_release_regions(chip->pci);
+
+When you requested manually via :c:func:`request_region()` or
+:c:func:`request_mem_region()`, you can release it via
+:c:func:`release_resource()`. Suppose that you keep the resource
+pointer returned from :c:func:`request_region()` in
+chip->res_port, the release procedure looks like::
+
+  release_and_free_resource(chip->res_port);
+
+Don't forget to call :c:func:`pci_disable_device()` before the
+end.
+
+And finally, release the chip-specific record::
+
+  kfree(chip);
+
+We didn't implement the hardware disabling part above. If you
+need to do this, please note that the destructor may be called even
+before the initialization of the chip is completed. It would be better
+to have a flag to skip hardware disabling if the hardware was not
+initialized yet.
+
+When the chip-data is assigned to the card using
+:c:func:`snd_device_new()` with ``SNDRV_DEV_LOWLELVEL``, its
+destructor is called last. That is, it is assured that all other
+components like PCMs and controls have already been released. You don't
+have to stop PCMs, etc. explicitly, but just call low-level hardware
+stopping.
+
+The management of a memory-mapped region is almost as same as the
+management of an I/O port. You'll need two fields as follows::
+
+  struct mychip {
+          ....
+          unsigned long iobase_phys;
+          void __iomem *iobase_virt;
+  };
+
+and the allocation would look like below::
+
+  err = pci_request_regions(pci, "My Chip");
+  if (err < 0) {
+          kfree(chip);
+          return err;
+  }
+  chip->iobase_phys = pci_resource_start(pci, 0);
+  chip->iobase_virt = ioremap(chip->iobase_phys,
+                                      pci_resource_len(pci, 0));
+
+and the corresponding destructor would be::
+
+  static int snd_mychip_free(struct mychip *chip)
+  {
+          ....
+          if (chip->iobase_virt)
+                  iounmap(chip->iobase_virt);
+          ....
+          pci_release_regions(chip->pci);
+          ....
+  }
+
+Of course, a modern way with :c:func:`pci_iomap()` will make things a
+bit easier, too::
+
+  err = pci_request_regions(pci, "My Chip");
+  if (err < 0) {
+          kfree(chip);
+          return err;
+  }
+  chip->iobase_virt = pci_iomap(pci, 0, 0);
+
+which is paired with :c:func:`pci_iounmap()` at destructor.
+
+
+PCI Entries
+-----------
+
+So far, so good. Let's finish the missing PCI stuff. At first, we need a
+struct pci_device_id table for
+this chipset. It's a table of PCI vendor/device ID number, and some
+masks.
+
+For example::
+
+  static struct pci_device_id snd_mychip_ids[] = {
+          { PCI_VENDOR_ID_FOO, PCI_DEVICE_ID_BAR,
+            PCI_ANY_ID, PCI_ANY_ID, 0, 0, 0, },
+          ....
+          { 0, }
+  };
+  MODULE_DEVICE_TABLE(pci, snd_mychip_ids);
+
+The first and second fields of the struct pci_device_id are the vendor
+and device IDs. If you have no reason to filter the matching devices, you can
+leave the remaining fields as above. The last field of the
+struct pci_device_id contains private data for this entry. You can specify
+any value here, for example, to define specific operations for supported
+device IDs. Such an example is found in the intel8x0 driver.
+
+The last entry of this list is the terminator. You must specify this
+all-zero entry.
+
+Then, prepare the struct pci_driver
+record::
+
+  static struct pci_driver driver = {
+          .name = KBUILD_MODNAME,
+          .id_table = snd_mychip_ids,
+          .probe = snd_mychip_probe,
+          .remove = snd_mychip_remove,
+  };
+
+The ``probe`` and ``remove`` functions have already been defined in
+the previous sections. The ``name`` field is the name string of this
+device. Note that you must not use slashes (“/”) in this string.
+
+And at last, the module entries::
+
+  static int __init alsa_card_mychip_init(void)
+  {
+          return pci_register_driver(&driver);
+  }
+
+  static void __exit alsa_card_mychip_exit(void)
+  {
+          pci_unregister_driver(&driver);
+  }
+
+  module_init(alsa_card_mychip_init)
+  module_exit(alsa_card_mychip_exit)
+
+Note that these module entries are tagged with ``__init`` and ``__exit``
+prefixes.
+
+That's all!
+
+PCM Interface
+=============
+
+General
+-------
+
+The PCM middle layer of ALSA is quite powerful and it is only necessary
+for each driver to implement the low-level functions to access its
+hardware.
+
+To access the PCM layer, you need to include ``<sound/pcm.h>``
+first. In addition, ``<sound/pcm_params.h>`` might be needed if you
+access some functions related with hw_param.
+
+Each card device can have up to four PCM instances. A PCM instance
+corresponds to a PCM device file. The limitation of number of instances
+comes only from the available bit size of Linux' device numbers.
+Once 64bit device numbers are used, we'll have more PCM instances
+available.
+
+A PCM instance consists of PCM playback and capture streams, and each
+PCM stream consists of one or more PCM substreams. Some soundcards
+support multiple playback functions. For example, emu10k1 has a PCM
+playback of 32 stereo substreams. In this case, at each open, a free
+substream is (usually) automatically chosen and opened. Meanwhile, when
+only one substream exists and it was already opened, a subsequent open
+will either block or error with ``EAGAIN`` according to the file open
+mode. But you don't have to care about such details in your driver. The
+PCM middle layer will take care of such work.
+
+Full Code Example
+-----------------
+
+The example code below does not include any hardware access routines but
+shows only the skeleton, how to build up the PCM interfaces::
+
+      #include <sound/pcm.h>
+      ....
+
+      /* hardware definition */
+      static struct snd_pcm_hardware snd_mychip_playback_hw = {
+              .info = (SNDRV_PCM_INFO_MMAP |
+                       SNDRV_PCM_INFO_INTERLEAVED |
+                       SNDRV_PCM_INFO_BLOCK_TRANSFER |
+                       SNDRV_PCM_INFO_MMAP_VALID),
+              .formats =          SNDRV_PCM_FMTBIT_S16_LE,
+              .rates =            SNDRV_PCM_RATE_8000_48000,
+              .rate_min =         8000,
+              .rate_max =         48000,
+              .channels_min =     2,
+              .channels_max =     2,
+              .buffer_bytes_max = 32768,
+              .period_bytes_min = 4096,
+              .period_bytes_max = 32768,
+              .periods_min =      1,
+              .periods_max =      1024,
+      };
+
+      /* hardware definition */
+      static struct snd_pcm_hardware snd_mychip_capture_hw = {
+              .info = (SNDRV_PCM_INFO_MMAP |
+                       SNDRV_PCM_INFO_INTERLEAVED |
+                       SNDRV_PCM_INFO_BLOCK_TRANSFER |
+                       SNDRV_PCM_INFO_MMAP_VALID),
+              .formats =          SNDRV_PCM_FMTBIT_S16_LE,
+              .rates =            SNDRV_PCM_RATE_8000_48000,
+              .rate_min =         8000,
+              .rate_max =         48000,
+              .channels_min =     2,
+              .channels_max =     2,
+              .buffer_bytes_max = 32768,
+              .period_bytes_min = 4096,
+              .period_bytes_max = 32768,
+              .periods_min =      1,
+              .periods_max =      1024,
+      };
+
+      /* open callback */
+      static int snd_mychip_playback_open(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              struct snd_pcm_runtime *runtime = substream->runtime;
+
+              runtime->hw = snd_mychip_playback_hw;
+              /* more hardware-initialization will be done here */
+              ....
+              return 0;
+      }
+
+      /* close callback */
+      static int snd_mychip_playback_close(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              /* the hardware-specific codes will be here */
+              ....
+              return 0;
+
+      }
+
+      /* open callback */
+      static int snd_mychip_capture_open(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              struct snd_pcm_runtime *runtime = substream->runtime;
+
+              runtime->hw = snd_mychip_capture_hw;
+              /* more hardware-initialization will be done here */
+              ....
+              return 0;
+      }
+
+      /* close callback */
+      static int snd_mychip_capture_close(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              /* the hardware-specific codes will be here */
+              ....
+              return 0;
+      }
+
+      /* hw_params callback */
+      static int snd_mychip_pcm_hw_params(struct snd_pcm_substream *substream,
+                                   struct snd_pcm_hw_params *hw_params)
+      {
+              /* the hardware-specific codes will be here */
+              ....
+              return 0;
+      }
+
+      /* hw_free callback */
+      static int snd_mychip_pcm_hw_free(struct snd_pcm_substream *substream)
+      {
+              /* the hardware-specific codes will be here */
+              ....
+              return 0;
+      }
+
+      /* prepare callback */
+      static int snd_mychip_pcm_prepare(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              struct snd_pcm_runtime *runtime = substream->runtime;
+
+              /* set up the hardware with the current configuration
+               * for example...
+               */
+              mychip_set_sample_format(chip, runtime->format);
+              mychip_set_sample_rate(chip, runtime->rate);
+              mychip_set_channels(chip, runtime->channels);
+              mychip_set_dma_setup(chip, runtime->dma_addr,
+                                   chip->buffer_size,
+                                   chip->period_size);
+              return 0;
+      }
+
+      /* trigger callback */
+      static int snd_mychip_pcm_trigger(struct snd_pcm_substream *substream,
+                                        int cmd)
+      {
+              switch (cmd) {
+              case SNDRV_PCM_TRIGGER_START:
+                      /* do something to start the PCM engine */
+                      ....
+                      break;
+              case SNDRV_PCM_TRIGGER_STOP:
+                      /* do something to stop the PCM engine */
+                      ....
+                      break;
+              default:
+                      return -EINVAL;
+              }
+      }
+
+      /* pointer callback */
+      static snd_pcm_uframes_t
+      snd_mychip_pcm_pointer(struct snd_pcm_substream *substream)
+      {
+              struct mychip *chip = snd_pcm_substream_chip(substream);
+              unsigned int current_ptr;
+
+              /* get the current hardware pointer */
+              current_ptr = mychip_get_hw_pointer(chip);
+              return current_ptr;
+      }
+
+      /* operators */
+      static struct snd_pcm_ops snd_mychip_playback_ops = {
+              .open =        snd_mychip_playback_open,
+              .close =       snd_mychip_playback_close,
+              .hw_params =   snd_mychip_pcm_hw_params,
+              .hw_free =     snd_mychip_pcm_hw_free,
+              .prepare =     snd_mychip_pcm_prepare,
+              .trigger =     snd_mychip_pcm_trigger,
+              .pointer =     snd_mychip_pcm_pointer,
+      };
+
+      /* operators */
+      static struct snd_pcm_ops snd_mychip_capture_ops = {
+              .open =        snd_mychip_capture_open,
+              .close =       snd_mychip_capture_close,
+              .hw_params =   snd_mychip_pcm_hw_params,
+              .hw_free =     snd_mychip_pcm_hw_free,
+              .prepare =     snd_mychip_pcm_prepare,
+              .trigger =     snd_mychip_pcm_trigger,
+              .pointer =     snd_mychip_pcm_pointer,
+      };
+
+      /*
+       *  definitions of capture are omitted here...
+       */
+
+      /* create a pcm device */
+      static int snd_mychip_new_pcm(struct mychip *chip)
+      {
+              struct snd_pcm *pcm;
+              int err;
+
+              err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+              if (err < 0)
+                      return err;
+              pcm->private_data = chip;
+              strcpy(pcm->name, "My Chip");
+              chip->pcm = pcm;
+              /* set operators */
+              snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+                              &snd_mychip_playback_ops);
+              snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+                              &snd_mychip_capture_ops);
+              /* pre-allocation of buffers */
+              /* NOTE: this may fail */
+              snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                             &chip->pci->dev,
+                                             64*1024, 64*1024);
+              return 0;
+      }
+
+
+PCM Constructor
+---------------
+
+A PCM instance is allocated by the :c:func:`snd_pcm_new()`
+function. It would be better to create a constructor for the PCM, namely::
+
+  static int snd_mychip_new_pcm(struct mychip *chip)
+  {
+          struct snd_pcm *pcm;
+          int err;
+
+          err = snd_pcm_new(chip->card, "My Chip", 0, 1, 1, &pcm);
+          if (err < 0) 
+                  return err;
+          pcm->private_data = chip;
+          strcpy(pcm->name, "My Chip");
+          chip->pcm = pcm;
+          ...
+          return 0;
+  }
+
+The :c:func:`snd_pcm_new()` function takes six arguments. The
+first argument is the card pointer to which this PCM is assigned, and
+the second is the ID string.
+
+The third argument (``index``, 0 in the above) is the index of this new
+PCM. It begins from zero. If you create more than one PCM instances,
+specify the different numbers in this argument. For example, ``index =
+1`` for the second PCM device.
+
+The fourth and fifth arguments are the number of substreams for playback
+and capture, respectively. Here 1 is used for both arguments. When no
+playback or capture substreams are available, pass 0 to the
+corresponding argument.
+
+If a chip supports multiple playbacks or captures, you can specify more
+numbers, but they must be handled properly in open/close, etc.
+callbacks. When you need to know which substream you are referring to,
+then it can be obtained from struct snd_pcm_substream data passed to each
+callback as follows::
+
+  struct snd_pcm_substream *substream;
+  int index = substream->number;
+
+
+After the PCM is created, you need to set operators for each PCM stream::
+
+  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_PLAYBACK,
+                  &snd_mychip_playback_ops);
+  snd_pcm_set_ops(pcm, SNDRV_PCM_STREAM_CAPTURE,
+                  &snd_mychip_capture_ops);
+
+The operators are defined typically like this::
+
+  static struct snd_pcm_ops snd_mychip_playback_ops = {
+          .open =        snd_mychip_pcm_open,
+          .close =       snd_mychip_pcm_close,
+          .hw_params =   snd_mychip_pcm_hw_params,
+          .hw_free =     snd_mychip_pcm_hw_free,
+          .prepare =     snd_mychip_pcm_prepare,
+          .trigger =     snd_mychip_pcm_trigger,
+          .pointer =     snd_mychip_pcm_pointer,
+  };
+
+All the callbacks are described in the Operators_ subsection.
+
+After setting the operators, you probably will want to pre-allocate the
+buffer and set up the managed allocation mode.
+For that, simply call the following::
+
+  snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                 &chip->pci->dev,
+                                 64*1024, 64*1024);
+
+It will allocate a buffer up to 64kB by default. Buffer management
+details will be described in the later section `Buffer and Memory
+Management`_.
+
+Additionally, you can set some extra information for this PCM in
+``pcm->info_flags``. The available values are defined as
+``SNDRV_PCM_INFO_XXX`` in ``<sound/asound.h>``, which is used for the
+hardware definition (described later). When your soundchip supports only
+half-duplex, specify it like this::
+
+  pcm->info_flags = SNDRV_PCM_INFO_HALF_DUPLEX;
+
+
+... And the Destructor?
+-----------------------
+
+The destructor for a PCM instance is not always necessary. Since the PCM
+device will be released by the middle layer code automatically, you
+don't have to call the destructor explicitly.
+
+The destructor would be necessary if you created special records
+internally and needed to release them. In such a case, set the
+destructor function to ``pcm->private_free``::
+
+      static void mychip_pcm_free(struct snd_pcm *pcm)
+      {
+              struct mychip *chip = snd_pcm_chip(pcm);
+              /* free your own data */
+              kfree(chip->my_private_pcm_data);
+              /* do what you like else */
+              ....
+      }
+
+      static int snd_mychip_new_pcm(struct mychip *chip)
+      {
+              struct snd_pcm *pcm;
+              ....
+              /* allocate your own data */
+              chip->my_private_pcm_data = kmalloc(...);
+              /* set the destructor */
+              pcm->private_data = chip;
+              pcm->private_free = mychip_pcm_free;
+              ....
+      }
+
+
+
+Runtime Pointer - The Chest of PCM Information
+----------------------------------------------
+
+When the PCM substream is opened, a PCM runtime instance is allocated
+and assigned to the substream. This pointer is accessible via
+``substream->runtime``. This runtime pointer holds most information you
+need to control the PCM: a copy of hw_params and sw_params
+configurations, the buffer pointers, mmap records, spinlocks, etc.
+
+The definition of runtime instance is found in ``<sound/pcm.h>``. Here
+is the relevant part of this file::
+
+  struct _snd_pcm_runtime {
+          /* -- Status -- */
+          struct snd_pcm_substream *trigger_master;
+          snd_timestamp_t trigger_tstamp;	/* trigger timestamp */
+          int overrange;
+          snd_pcm_uframes_t avail_max;
+          snd_pcm_uframes_t hw_ptr_base;	/* Position at buffer restart */
+          snd_pcm_uframes_t hw_ptr_interrupt; /* Position at interrupt time*/
+  
+          /* -- HW params -- */
+          snd_pcm_access_t access;	/* access mode */
+          snd_pcm_format_t format;	/* SNDRV_PCM_FORMAT_* */
+          snd_pcm_subformat_t subformat;	/* subformat */
+          unsigned int rate;		/* rate in Hz */
+          unsigned int channels;		/* channels */
+          snd_pcm_uframes_t period_size;	/* period size */
+          unsigned int periods;		/* periods */
+          snd_pcm_uframes_t buffer_size;	/* buffer size */
+          unsigned int tick_time;		/* tick time */
+          snd_pcm_uframes_t min_align;	/* Min alignment for the format */
+          size_t byte_align;
+          unsigned int frame_bits;
+          unsigned int sample_bits;
+          unsigned int info;
+          unsigned int rate_num;
+          unsigned int rate_den;
+  
+          /* -- SW params -- */
+          struct timespec tstamp_mode;	/* mmap timestamp is updated */
+          unsigned int period_step;
+          unsigned int sleep_min;		/* min ticks to sleep */
+          snd_pcm_uframes_t start_threshold;
+          /*
+           * The following two thresholds alleviate playback buffer underruns; when
+           * hw_avail drops below the threshold, the respective action is triggered:
+           */
+          snd_pcm_uframes_t stop_threshold;	/* - stop playback */
+          snd_pcm_uframes_t silence_threshold;	/* - pre-fill buffer with silence */
+          snd_pcm_uframes_t silence_size;       /* max size of silence pre-fill; when >= boundary,
+                                                 * fill played area with silence immediately */
+          snd_pcm_uframes_t boundary;	/* pointers wrap point */
+  
+          /* internal data of auto-silencer */
+          snd_pcm_uframes_t silence_start; /* starting pointer to silence area */
+          snd_pcm_uframes_t silence_filled; /* size filled with silence */
+  
+          snd_pcm_sync_id_t sync;		/* hardware synchronization ID */
+  
+          /* -- mmap -- */
+          volatile struct snd_pcm_mmap_status *status;
+          volatile struct snd_pcm_mmap_control *control;
+          atomic_t mmap_count;
+  
+          /* -- locking / scheduling -- */
+          spinlock_t lock;
+          wait_queue_head_t sleep;
+          struct timer_list tick_timer;
+          struct fasync_struct *fasync;
+
+          /* -- private section -- */
+          void *private_data;
+          void (*private_free)(struct snd_pcm_runtime *runtime);
+  
+          /* -- hardware description -- */
+          struct snd_pcm_hardware hw;
+          struct snd_pcm_hw_constraints hw_constraints;
+  
+          /* -- timer -- */
+          unsigned int timer_resolution;	/* timer resolution */
+  
+          /* -- DMA -- */           
+          unsigned char *dma_area;	/* DMA area */
+          dma_addr_t dma_addr;		/* physical bus address (not accessible from main CPU) */
+          size_t dma_bytes;		/* size of DMA area */
+  
+          struct snd_dma_buffer *dma_buffer_p;	/* allocated buffer */
+  
+  #if defined(CONFIG_SND_PCM_OSS) || defined(CONFIG_SND_PCM_OSS_MODULE)
+          /* -- OSS things -- */
+          struct snd_pcm_oss_runtime oss;
+  #endif
+  };
+
+
+For the operators (callbacks) of each sound driver, most of these
+records are supposed to be read-only. Only the PCM middle-layer changes
+/ updates them. The exceptions are the hardware description (hw) DMA
+buffer information and the private data. Besides, if you use the
+standard managed buffer allocation mode, you don't need to set the
+DMA buffer information by yourself.
+
+In the sections below, important records are explained.
+
+Hardware Description
+~~~~~~~~~~~~~~~~~~~~
+
+The hardware descriptor (struct snd_pcm_hardware) contains the definitions of
+the fundamental hardware configuration. Above all, you'll need to define this
+in the `PCM open callback`_. Note that the runtime instance holds a copy of
+the descriptor, not a pointer to the existing descriptor. That is,
+in the open callback, you can modify the copied descriptor
+(``runtime->hw``) as you need. For example, if the maximum number of
+channels is 1 only on some chip models, you can still use the same
+hardware descriptor and change the channels_max later::
+
+          struct snd_pcm_runtime *runtime = substream->runtime;
+          ...
+          runtime->hw = snd_mychip_playback_hw; /* common definition */
+          if (chip->model == VERY_OLD_ONE)
+                  runtime->hw.channels_max = 1;
+
+Typically, you'll have a hardware descriptor as below::
+
+  static struct snd_pcm_hardware snd_mychip_playback_hw = {
+          .info = (SNDRV_PCM_INFO_MMAP |
+                   SNDRV_PCM_INFO_INTERLEAVED |
+                   SNDRV_PCM_INFO_BLOCK_TRANSFER |
+                   SNDRV_PCM_INFO_MMAP_VALID),
+          .formats =          SNDRV_PCM_FMTBIT_S16_LE,
+          .rates =            SNDRV_PCM_RATE_8000_48000,
+          .rate_min =         8000,
+          .rate_max =         48000,
+          .channels_min =     2,
+          .channels_max =     2,
+          .buffer_bytes_max = 32768,
+          .period_bytes_min = 4096,
+          .period_bytes_max = 32768,
+          .periods_min =      1,
+          .periods_max =      1024,
+  };
+
+-  The ``info`` field contains the type and capabilities of this
+   PCM. The bit flags are defined in ``<sound/asound.h>`` as
+   ``SNDRV_PCM_INFO_XXX``. Here, at least, you have to specify whether
+   mmap is supported and which interleaving formats are
+   supported. When the hardware supports mmap, add the
+   ``SNDRV_PCM_INFO_MMAP`` flag here. When the hardware supports the
+   interleaved or the non-interleaved formats, the
+   ``SNDRV_PCM_INFO_INTERLEAVED`` or ``SNDRV_PCM_INFO_NONINTERLEAVED``
+   flag must be set, respectively. If both are supported, you can set
+   both, too.
+
+   In the above example, ``MMAP_VALID`` and ``BLOCK_TRANSFER`` are
+   specified for the OSS mmap mode. Usually both are set. Of course,
+   ``MMAP_VALID`` is set only if mmap is really supported.
+
+   The other possible flags are ``SNDRV_PCM_INFO_PAUSE`` and
+   ``SNDRV_PCM_INFO_RESUME``. The ``PAUSE`` bit means that the PCM
+   supports the “pause” operation, while the ``RESUME`` bit means that
+   the PCM supports the full “suspend/resume” operation. If the
+   ``PAUSE`` flag is set, the ``trigger`` callback below must handle
+   the corresponding (pause push/release) commands. The suspend/resume
+   trigger commands can be defined even without the ``RESUME``
+   flag. See the `Power Management`_ section for details.
+
+   When the PCM substreams can be synchronized (typically,
+   synchronized start/stop of a playback and a capture stream), you
+   can give ``SNDRV_PCM_INFO_SYNC_START``, too. In this case, you'll
+   need to check the linked-list of PCM substreams in the trigger
+   callback. This will be described in a later section.
+
+-  The ``formats`` field contains the bit-flags of supported formats
+   (``SNDRV_PCM_FMTBIT_XXX``). If the hardware supports more than one
+   format, give all or'ed bits. In the example above, the signed 16bit
+   little-endian format is specified.
+
+-  The ``rates`` field contains the bit-flags of supported rates
+   (``SNDRV_PCM_RATE_XXX``). When the chip supports continuous rates,
+   pass the ``CONTINUOUS`` bit additionally. The pre-defined rate bits
+   are provided only for typical rates. If your chip supports
+   unconventional rates, you need to add the ``KNOT`` bit and set up
+   the hardware constraint manually (explained later).
+
+-  ``rate_min`` and ``rate_max`` define the minimum and maximum sample
+   rate. This should correspond somehow to ``rates`` bits.
+
+-  ``channels_min`` and ``channels_max`` define, as you might have already
+   expected, the minimum and maximum number of channels.
+
+-  ``buffer_bytes_max`` defines the maximum buffer size in
+   bytes. There is no ``buffer_bytes_min`` field, since it can be
+   calculated from the minimum period size and the minimum number of
+   periods. Meanwhile, ``period_bytes_min`` and ``period_bytes_max``
+   define the minimum and maximum size of the period in bytes.
+   ``periods_max`` and ``periods_min`` define the maximum and minimum
+   number of periods in the buffer.
+
+   The “period” is a term that corresponds to a fragment in the OSS
+   world. The period defines the point at which a PCM interrupt is
+   generated. This point strongly depends on the hardware. Generally,
+   a smaller period size will give you more interrupts, which results
+   in being able to fill/drain the buffer more timely. In the case of
+   capture, this size defines the input latency. On the other hand,
+   the whole buffer size defines the output latency for the playback
+   direction.
+
+-  There is also a field ``fifo_size``. This specifies the size of the
+   hardware FIFO, but currently it is neither used by the drivers nor
+   in the alsa-lib. So, you can ignore this field.
+
+PCM Configurations
+~~~~~~~~~~~~~~~~~~
+
+Ok, let's go back again to the PCM runtime records. The most
+frequently referred records in the runtime instance are the PCM
+configurations. The PCM configurations are stored in the runtime
+instance after the application sends ``hw_params`` data via
+alsa-lib. There are many fields copied from hw_params and sw_params
+structs. For example, ``format`` holds the format type chosen by the
+application. This field contains the enum value
+``SNDRV_PCM_FORMAT_XXX``.
+
+One thing to be noted is that the configured buffer and period sizes
+are stored in “frames” in the runtime. In the ALSA world, ``1 frame =
+channels \* samples-size``. For conversion between frames and bytes,
+you can use the :c:func:`frames_to_bytes()` and
+:c:func:`bytes_to_frames()` helper functions::
+
+  period_bytes = frames_to_bytes(runtime, runtime->period_size);
+
+Also, many software parameters (sw_params) are stored in frames, too.
+Please check the type of the field. ``snd_pcm_uframes_t`` is for
+frames as unsigned integer while ``snd_pcm_sframes_t`` is for
+frames as signed integer.
+
+DMA Buffer Information
+~~~~~~~~~~~~~~~~~~~~~~
+
+The DMA buffer is defined by the following four fields: ``dma_area``,
+``dma_addr``, ``dma_bytes`` and ``dma_private``. ``dma_area``
+holds the buffer pointer (the logical address). You can call
+:c:func:`memcpy()` from/to this pointer. Meanwhile, ``dma_addr`` holds
+the physical address of the buffer. This field is specified only when
+the buffer is a linear buffer. ``dma_bytes`` holds the size of the
+buffer in bytes. ``dma_private`` is used for the ALSA DMA allocator.
+
+If you use either the managed buffer allocation mode or the standard
+API function :c:func:`snd_pcm_lib_malloc_pages()` for allocating the buffer,
+these fields are set by the ALSA middle layer, and you should *not*
+change them by yourself. You can read them but not write them. On the
+other hand, if you want to allocate the buffer by yourself, you'll
+need to manage it in the hw_params callback. At least, ``dma_bytes`` is
+mandatory. ``dma_area`` is necessary when the buffer is mmapped. If
+your driver doesn't support mmap, this field is not
+necessary. ``dma_addr`` is also optional. You can use dma_private as
+you like, too.
+
+Running Status
+~~~~~~~~~~~~~~
+
+The running status can be referred via ``runtime->status``. This is
+a pointer to a struct snd_pcm_mmap_status record.
+For example, you can get the current
+DMA hardware pointer via ``runtime->status->hw_ptr``.
+
+The DMA application pointer can be referred via ``runtime->control``,
+which points to a struct snd_pcm_mmap_control record.
+However, accessing this value directly is not recommended.
+
+Private Data
+~~~~~~~~~~~~
+
+You can allocate a record for the substream and store it in
+``runtime->private_data``. Usually, this is done in the `PCM open
+callback`_. Don't mix this with ``pcm->private_data``. The
+``pcm->private_data`` usually points to the chip instance assigned
+statically at creation time of the PCM device, while
+``runtime->private_data``
+points to a dynamic data structure created in the PCM open
+callback::
+
+  static int snd_xxx_open(struct snd_pcm_substream *substream)
+  {
+          struct my_pcm_data *data;
+          ....
+          data = kmalloc(sizeof(*data), GFP_KERNEL);
+          substream->runtime->private_data = data;
+          ....
+  }
+
+
+The allocated object must be released in the `close callback`_.
+
+Operators
+---------
+
+OK, now let me give details about each PCM callback (``ops``). In
+general, every callback must return 0 if successful, or a negative
+error number such as ``-EINVAL``. To choose an appropriate error
+number, it is advised to check what value other parts of the kernel
+return when the same kind of request fails.
+
+Each callback function takes at least one argument containing a
+struct snd_pcm_substream pointer. To retrieve the chip
+record from the given substream instance, you can use the following
+macro::
+
+  int xxx(...) {
+          struct mychip *chip = snd_pcm_substream_chip(substream);
+          ....
+  }
+
+The macro reads ``substream->private_data``, which is a copy of
+``pcm->private_data``. You can override the former if you need to
+assign different data records per PCM substream. For example, the
+cmi8330 driver assigns different ``private_data`` for playback and
+capture directions, because it uses two different codecs (SB- and
+AD-compatible) for different directions.
+
+PCM open callback
+~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_open(struct snd_pcm_substream *substream);
+
+This is called when a PCM substream is opened.
+
+At least, here you have to initialize the ``runtime->hw``
+record. Typically, this is done like this::
+
+  static int snd_xxx_open(struct snd_pcm_substream *substream)
+  {
+          struct mychip *chip = snd_pcm_substream_chip(substream);
+          struct snd_pcm_runtime *runtime = substream->runtime;
+
+          runtime->hw = snd_mychip_playback_hw;
+          return 0;
+  }
+
+where ``snd_mychip_playback_hw`` is the pre-defined hardware
+description.
+
+You can allocate private data in this callback, as described in the
+`Private Data`_ section.
+
+If the hardware configuration needs more constraints, set the hardware
+constraints here, too. See Constraints_ for more details.
+
+close callback
+~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_close(struct snd_pcm_substream *substream);
+
+
+Obviously, this is called when a PCM substream is closed.
+
+Any private instance for a PCM substream allocated in the ``open``
+callback will be released here::
+
+  static int snd_xxx_close(struct snd_pcm_substream *substream)
+  {
+          ....
+          kfree(substream->runtime->private_data);
+          ....
+  }
+
+ioctl callback
+~~~~~~~~~~~~~~
+
+This is used for any special call to PCM ioctls. But usually you can
+leave it NULL, then the PCM core calls the generic ioctl callback
+function :c:func:`snd_pcm_lib_ioctl()`.  If you need to deal with a
+unique setup of channel info or reset procedure, you can pass your own
+callback function here.
+
+hw_params callback
+~~~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_hw_params(struct snd_pcm_substream *substream,
+                               struct snd_pcm_hw_params *hw_params);
+
+This is called when the hardware parameters (``hw_params``) are set up
+by the application, that is, once when the buffer size, the period
+size, the format, etc. are defined for the PCM substream.
+
+Many hardware setups should be done in this callback, including the
+allocation of buffers.
+
+Parameters to be initialized are retrieved by the
+:c:func:`params_xxx()` macros.
+
+When you choose managed buffer allocation mode for the substream,
+a buffer is already allocated before this callback gets
+called. Alternatively, you can call a helper function below for
+allocating the buffer::
+
+  snd_pcm_lib_malloc_pages(substream, params_buffer_bytes(hw_params));
+
+:c:func:`snd_pcm_lib_malloc_pages()` is available only when the
+DMA buffers have been pre-allocated. See the section `Buffer Types`_
+for more details.
+
+Note that this one and the ``prepare`` callback may be called multiple
+times per initialization. For example, the OSS emulation may call these
+callbacks at each change via its ioctl.
+
+Thus, you need to be careful not to allocate the same buffers many
+times, which will lead to memory leaks! Calling the helper function
+above many times is OK. It will release the previous buffer
+automatically when it was already allocated.
+
+Another note is that this callback is non-atomic (schedulable) by
+default, i.e. when no ``nonatomic`` flag set. This is important,
+because the ``trigger`` callback is atomic (non-schedulable). That is,
+mutexes or any schedule-related functions are not available in the
+``trigger`` callback. Please see the subsection Atomicity_ for
+details.
+
+hw_free callback
+~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_hw_free(struct snd_pcm_substream *substream);
+
+This is called to release the resources allocated via
+``hw_params``.
+
+This function is always called before the close callback is called.
+Also, the callback may be called multiple times, too. Keep track
+whether each resource was already released.
+
+When you have chosen managed buffer allocation mode for the PCM
+substream, the allocated PCM buffer will be automatically released
+after this callback gets called.  Otherwise you'll have to release the
+buffer manually.  Typically, when the buffer was allocated from the
+pre-allocated pool, you can use the standard API function
+:c:func:`snd_pcm_lib_malloc_pages()` like::
+
+  snd_pcm_lib_free_pages(substream);
+
+prepare callback
+~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_prepare(struct snd_pcm_substream *substream);
+
+This callback is called when the PCM is “prepared”. You can set the
+format type, sample rate, etc. here. The difference from ``hw_params``
+is that the ``prepare`` callback will be called each time
+:c:func:`snd_pcm_prepare()` is called, i.e. when recovering after
+underruns, etc.
+
+Note that this callback is non-atomic. You can use
+schedule-related functions safely in this callback.
+
+In this and the following callbacks, you can refer to the values via
+the runtime record, ``substream->runtime``. For example, to get the
+current rate, format or channels, access to ``runtime->rate``,
+``runtime->format`` or ``runtime->channels``, respectively. The
+physical address of the allocated buffer is set to
+``runtime->dma_area``. The buffer and period sizes are in
+``runtime->buffer_size`` and ``runtime->period_size``, respectively.
+
+Be careful that this callback will be called many times at each setup,
+too.
+
+trigger callback
+~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_trigger(struct snd_pcm_substream *substream, int cmd);
+
+This is called when the PCM is started, stopped or paused.
+
+The action is specified in the second argument, ``SNDRV_PCM_TRIGGER_XXX``
+defined in ``<sound/pcm.h>``. At least, the ``START``
+and ``STOP`` commands must be defined in this callback::
+
+  switch (cmd) {
+  case SNDRV_PCM_TRIGGER_START:
+          /* do something to start the PCM engine */
+          break;
+  case SNDRV_PCM_TRIGGER_STOP:
+          /* do something to stop the PCM engine */
+          break;
+  default:
+          return -EINVAL;
+  }
+
+When the PCM supports the pause operation (given in the info field of
+the hardware table), the ``PAUSE_PUSH`` and ``PAUSE_RELEASE`` commands
+must be handled here, too. The former is the command to pause the PCM,
+and the latter to restart the PCM again.
+
+When the PCM supports the suspend/resume operation, regardless of full
+or partial suspend/resume support, the ``SUSPEND`` and ``RESUME``
+commands must be handled, too. These commands are issued when the
+power-management status is changed. Obviously, the ``SUSPEND`` and
+``RESUME`` commands suspend and resume the PCM substream, and usually,
+they are identical to the ``STOP`` and ``START`` commands, respectively.
+See the `Power Management`_ section for details.
+
+As mentioned, this callback is atomic by default unless the ``nonatomic``
+flag set, and you cannot call functions which may sleep. The
+``trigger`` callback should be as minimal as possible, just really
+triggering the DMA. The other stuff should be initialized in
+``hw_params`` and ``prepare`` callbacks properly beforehand.
+
+sync_stop callback
+~~~~~~~~~~~~~~~~~~
+
+::
+
+  static int snd_xxx_sync_stop(struct snd_pcm_substream *substream);
+
+This callback is optional, and NULL can be passed.  It's called after
+the PCM core stops the stream, before it changes the stream state via
+``prepare``, ``hw_params`` or ``hw_free``.
+Since the IRQ handler might be still pending, we need to wait until
+the pending task finishes before moving to the next step; otherwise it
+might lead to a crash due to resource conflicts or access to freed
+resources.  A typical behavior is to call a synchronization function
+like :c:func:`synchronize_irq()` here.
+
+For the majority of drivers that need only a call of
+:c:func:`synchronize_irq()`, there is a simpler setup, too.
+While keeping the ``sync_stop`` PCM callback NULL, the driver can set
+the ``card->sync_irq`` field to the returned interrupt number after
+requesting an IRQ, instead.   Then PCM core will call
+:c:func:`synchronize_irq()` with the given IRQ appropriately.
+
+If the IRQ handler is released by the card destructor, you don't need
+to clear ``card->sync_irq``, as the card itself is being released.
+So, usually you'll need to add just a single line for assigning
+``card->sync_irq`` in the driver code unless the driver re-acquires
+the IRQ.  When the driver frees and re-acquires the IRQ dynamically
+(e.g. for suspend/resume), it needs to clear and re-set
+``card->sync_irq`` again appropriately.
+
+pointer callback
+~~~~~~~~~~~~~~~~
+
+::
+
+  static snd_pcm_uframes_t snd_xxx_pointer(struct snd_pcm_substream *substream)
+
+This callback is called when the PCM middle layer inquires the current
+hardware position in the buffer. The position must be returned in
+frames, ranging from 0 to ``buffer_size - 1``. 
+
+This is usually called from the buffer-update routine in the PCM
+middle layer, which is invoked when :c:func:`snd_pcm_period_elapsed()`
+is called by the interrupt routine. Then the PCM middle layer updates
+the position and calculates the available space, and wakes up the
+sleeping poll threads, etc.
+
+This callback is also atomic by default.
+
+copy and fill_silence ops
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+These callbacks are not mandatory, and can be omitted in most cases.
+These callbacks are used when the hardware buffer cannot be in the
+normal memory space. Some chips have their own buffer in the hardware
+which is not mappable. In such a case, you have to transfer the data
+manually from the memory buffer to the hardware buffer. Or, if the
+buffer is non-contiguous on both physical and virtual memory spaces,
+these callbacks must be defined, too.
+
+If these two callbacks are defined, copy and set-silence operations
+are done by them. The details will be described in the later section
+`Buffer and Memory Management`_.
+
+ack callback
+~~~~~~~~~~~~
+
+This callback is also not mandatory. This callback is called when the
+``appl_ptr`` is updated in read or write operations. Some drivers like
+emu10k1-fx and cs46xx need to track the current ``appl_ptr`` for the
+internal buffer, and this callback is useful only for such a purpose.
+
+The callback function may return 0 or a negative error. When the
+return value is ``-EPIPE``, PCM core treats that as a buffer XRUN,
+and changes the state to ``SNDRV_PCM_STATE_XRUN`` automatically.
+
+This callback is atomic by default.
+
+page callback
+~~~~~~~~~~~~~
+
+This callback is optional too. The mmap calls this callback to get the
+page fault address.
+
+You need no special callback for the standard SG-buffer or vmalloc-
+buffer. Hence this callback should be rarely used.
+
+mmap callback
+~~~~~~~~~~~~~
+
+This is another optional callback for controlling mmap behavior.
+When defined, the PCM core calls this callback when a page is
+memory-mapped, instead of using the standard helper.
+If you need special handling (due to some architecture or
+device-specific issues), implement everything here as you like.
+
+
+PCM Interrupt Handler
+---------------------
+
+The remainder of the PCM stuff is the PCM interrupt handler. The role
+of the PCM
+interrupt handler in the sound driver is to update the buffer position
+and to tell the PCM middle layer when the buffer position goes across
+the specified period boundary. To inform about this, call the
+:c:func:`snd_pcm_period_elapsed()` function.
+
+There are several ways sound chips can generate interrupts.
+
+Interrupts at the period (fragment) boundary
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This is the most frequently found type: the hardware generates an
+interrupt at each period boundary. In this case, you can call
+:c:func:`snd_pcm_period_elapsed()` at each interrupt.
+
+:c:func:`snd_pcm_period_elapsed()` takes the substream pointer as
+its argument. Thus, you need to keep the substream pointer accessible
+from the chip instance. For example, define ``substream`` field in the
+chip record to hold the current running substream pointer, and set the
+pointer value at ``open`` callback (and reset at ``close`` callback).
+
+If you acquire a spinlock in the interrupt handler, and the lock is used
+in other PCM callbacks, too, then you have to release the lock before
+calling :c:func:`snd_pcm_period_elapsed()`, because
+:c:func:`snd_pcm_period_elapsed()` calls other PCM callbacks
+inside.
+
+Typical code would look like::
+
+
+      static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+      {
+              struct mychip *chip = dev_id;
+              spin_lock(&chip->lock);
+              ....
+              if (pcm_irq_invoked(chip)) {
+                      /* call updater, unlock before it */
+                      spin_unlock(&chip->lock);
+                      snd_pcm_period_elapsed(chip->substream);
+                      spin_lock(&chip->lock);
+                      /* acknowledge the interrupt if necessary */
+              }
+              ....
+              spin_unlock(&chip->lock);
+              return IRQ_HANDLED;
+      }
+
+Also, when the device can detect a buffer underrun/overrun, the driver
+can notify the XRUN status to the PCM core by calling
+:c:func:`snd_pcm_stop_xrun()`. This function stops the stream and sets
+the PCM state to ``SNDRV_PCM_STATE_XRUN``. Note that it must be called
+outside the PCM stream lock, hence it can't be called from the atomic
+callback.
+
+
+High frequency timer interrupts
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+This happens when the hardware doesn't generate interrupts at the period
+boundary but issues timer interrupts at a fixed timer rate (e.g. es1968
+or ymfpci drivers). In this case, you need to check the current hardware
+position and accumulate the processed sample length at each interrupt.
+When the accumulated size exceeds the period size, call
+:c:func:`snd_pcm_period_elapsed()` and reset the accumulator.
+
+Typical code would look as follows::
+
+
+      static irqreturn_t snd_mychip_interrupt(int irq, void *dev_id)
+      {
+              struct mychip *chip = dev_id;
+              spin_lock(&chip->lock);
+              ....
+              if (pcm_irq_invoked(chip)) {
+                      unsigned int last_ptr, size;
+                      /* get the current hardware pointer (in frames) */
+                      last_ptr = get_hw_ptr(chip);
+                      /* calculate the processed frames since the
+                       * last update
+                       */
+                      if (last_ptr < chip->last_ptr)
+                              size = runtime->buffer_size + last_ptr
+                                       - chip->last_ptr;
+                      else
+                              size = last_ptr - chip->last_ptr;
+                      /* remember the last updated point */
+                      chip->last_ptr = last_ptr;
+                      /* accumulate the size */
+                      chip->size += size;
+                      /* over the period boundary? */
+                      if (chip->size >= runtime->period_size) {
+                              /* reset the accumulator */
+                              chip->size %= runtime->period_size;
+                              /* call updater */
+                              spin_unlock(&chip->lock);
+                              snd_pcm_period_elapsed(substream);
+                              spin_lock(&chip->lock);
+                      }
+                      /* acknowledge the interrupt if necessary */
+              }
+              ....
+              spin_unlock(&chip->lock);
+              return IRQ_HANDLED;
+      }
+
+
+
+On calling :c:func:`snd_pcm_period_elapsed()`
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In both cases, even if more than one period has elapsed, you don't have
+to call :c:func:`snd_pcm_period_elapsed()` many times. Call only
+once. And the PCM layer will check the current hardware pointer and
+update to the latest status.
+
+Atomicity
+---------
+
+One of the most important (and thus difficult to debug) problems in
+kernel programming are race conditions. In the Linux kernel, they are
+usually avoided via spin-locks, mutexes or semaphores. In general, if a
+race condition can happen in an interrupt handler, it has to be managed
+atomically, and you have to use a spinlock to protect the critical
+section. If the critical section is not in interrupt handler code and if
+taking a relatively long time to execute is acceptable, you should use
+mutexes or semaphores instead.
+
+As already seen, some PCM callbacks are atomic and some are not. For
+example, the ``hw_params`` callback is non-atomic, while the ``trigger``
+callback is atomic. This means, the latter is called already in a
+spinlock held by the PCM middle layer, the PCM stream lock. Please
+take this atomicity into account when you choose a locking scheme in
+the callbacks.
+
+In the atomic callbacks, you cannot use functions which may call
+:c:func:`schedule()` or go to :c:func:`sleep()`. Semaphores and
+mutexes can sleep, and hence they cannot be used inside the atomic
+callbacks (e.g. ``trigger`` callback). To implement some delay in such a
+callback, please use :c:func:`udelay()` or :c:func:`mdelay()`.
+
+All three atomic callbacks (trigger, pointer, and ack) are called with
+local interrupts disabled.
+
+However, it is possible to request all PCM operations to be non-atomic.
+This assumes that all call sites are in
+non-atomic contexts. For example, the function
+:c:func:`snd_pcm_period_elapsed()` is called typically from the
+interrupt handler. But, if you set up the driver to use a threaded
+interrupt handler, this call can be in non-atomic context, too. In such
+a case, you can set the ``nonatomic`` field of the struct snd_pcm object
+after creating it. When this flag is set, mutex and rwsem are used internally
+in the PCM core instead of spin and rwlocks, so that you can call all PCM
+functions safely in a non-atomic
+context.
+
+Also, in some cases, you might need to call
+:c:func:`snd_pcm_period_elapsed()` in the atomic context (e.g. the
+period gets elapsed during ``ack`` or other callback). There is a
+variant that can be called inside the PCM stream lock
+:c:func:`snd_pcm_period_elapsed_under_stream_lock()` for that purpose,
+too.
+
+Constraints
+-----------
+
+Due to physical limitations, hardware is not infinitely configurable.
+These limitations are expressed by setting constraints.
+
+For example, in order to restrict the sample rates to some supported
+values, use :c:func:`snd_pcm_hw_constraint_list()`. You need to
+call this function in the open callback::
+
+      static unsigned int rates[] =
+              {4000, 10000, 22050, 44100};
+      static struct snd_pcm_hw_constraint_list constraints_rates = {
+              .count = ARRAY_SIZE(rates),
+              .list = rates,
+              .mask = 0,
+      };
+
+      static int snd_mychip_pcm_open(struct snd_pcm_substream *substream)
+      {
+              int err;
+              ....
+              err = snd_pcm_hw_constraint_list(substream->runtime, 0,
+                                               SNDRV_PCM_HW_PARAM_RATE,
+                                               &constraints_rates);
+              if (err < 0)
+                      return err;
+              ....
+      }
+
+There are many different constraints. Look at ``sound/pcm.h`` for a
+complete list. You can even define your own constraint rules. For
+example, let's suppose my_chip can manage a substream of 1 channel if
+and only if the format is ``S16_LE``, otherwise it supports any format
+specified in struct snd_pcm_hardware (or in any other
+constraint_list). You can build a rule like this::
+
+      static int hw_rule_channels_by_format(struct snd_pcm_hw_params *params,
+                                            struct snd_pcm_hw_rule *rule)
+      {
+              struct snd_interval *c = hw_param_interval(params,
+                            SNDRV_PCM_HW_PARAM_CHANNELS);
+              struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+              struct snd_interval ch;
+
+              snd_interval_any(&ch);
+              if (f->bits[0] == SNDRV_PCM_FMTBIT_S16_LE) {
+                      ch.min = ch.max = 1;
+                      ch.integer = 1;
+                      return snd_interval_refine(c, &ch);
+              }
+              return 0;
+      }
+
+
+Then you need to call this function to add your rule::
+
+  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_CHANNELS,
+                      hw_rule_channels_by_format, NULL,
+                      SNDRV_PCM_HW_PARAM_FORMAT, -1);
+
+The rule function is called when an application sets the PCM format, and
+it refines the number of channels accordingly. But an application may
+set the number of channels before setting the format. Thus you also need
+to define the inverse rule::
+
+      static int hw_rule_format_by_channels(struct snd_pcm_hw_params *params,
+                                            struct snd_pcm_hw_rule *rule)
+      {
+              struct snd_interval *c = hw_param_interval(params,
+                    SNDRV_PCM_HW_PARAM_CHANNELS);
+              struct snd_mask *f = hw_param_mask(params, SNDRV_PCM_HW_PARAM_FORMAT);
+              struct snd_mask fmt;
+
+              snd_mask_any(&fmt);    /* Init the struct */
+              if (c->min < 2) {
+                      fmt.bits[0] &= SNDRV_PCM_FMTBIT_S16_LE;
+                      return snd_mask_refine(f, &fmt);
+              }
+              return 0;
+      }
+
+
+... and in the open callback::
+
+  snd_pcm_hw_rule_add(substream->runtime, 0, SNDRV_PCM_HW_PARAM_FORMAT,
+                      hw_rule_format_by_channels, NULL,
+                      SNDRV_PCM_HW_PARAM_CHANNELS, -1);
+
+One typical usage of the hw constraints is to align the buffer size
+with the period size.  By default, ALSA PCM core doesn't enforce the
+buffer size to be aligned with the period size.  For example, it'd be
+possible to have a combination like 256 period bytes with 999 buffer
+bytes.
+
+Many device chips, however, require the buffer to be a multiple of
+periods.  In such a case, call
+:c:func:`snd_pcm_hw_constraint_integer()` for
+``SNDRV_PCM_HW_PARAM_PERIODS``::
+
+  snd_pcm_hw_constraint_integer(substream->runtime,
+                                SNDRV_PCM_HW_PARAM_PERIODS);
+
+This assures that the number of periods is integer, hence the buffer
+size is aligned with the period size.
+
+The hw constraint is a very powerful mechanism to define the
+preferred PCM configuration, and there are relevant helpers.
+I won't give more details here, rather I would like to say, “Luke, use
+the source.”
+
+Control Interface
+=================
+
+General
+-------
+
+The control interface is used widely for many switches, sliders, etc.
+which are accessed from user-space. Its most important use is the mixer
+interface. In other words, since ALSA 0.9.x, all the mixer stuff is
+implemented on the control kernel API.
+
+ALSA has a well-defined AC97 control module. If your chip supports only
+the AC97 and nothing else, you can skip this section.
+
+The control API is defined in ``<sound/control.h>``. Include this file
+if you want to add your own controls.
+
+Definition of Controls
+----------------------
+
+To create a new control, you need to define the following three
+callbacks: ``info``, ``get`` and ``put``. Then, define a
+struct snd_kcontrol_new record, such as::
+
+
+      static struct snd_kcontrol_new my_control = {
+              .iface = SNDRV_CTL_ELEM_IFACE_MIXER,
+              .name = "PCM Playback Switch",
+              .index = 0,
+              .access = SNDRV_CTL_ELEM_ACCESS_READWRITE,
+              .private_value = 0xffff,
+              .info = my_control_info,
+              .get = my_control_get,
+              .put = my_control_put
+      };
+
+
+The ``iface`` field specifies the control type,
+``SNDRV_CTL_ELEM_IFACE_XXX``, which is usually ``MIXER``. Use ``CARD``
+for global controls that are not logically part of the mixer. If the
+control is closely associated with some specific device on the sound
+card, use ``HWDEP``, ``PCM``, ``RAWMIDI``, ``TIMER``, or ``SEQUENCER``,
+and specify the device number with the ``device`` and ``subdevice``
+fields.
+
+The ``name`` is the name identifier string. Since ALSA 0.9.x, the
+control name is very important, because its role is classified from
+its name. There are pre-defined standard control names. The details
+are described in the `Control Names`_ subsection.
+
+The ``index`` field holds the index number of this control. If there
+are several different controls with the same name, they can be
+distinguished by the index number. This is the case when several
+codecs exist on the card. If the index is zero, you can omit the
+definition above. 
+
+The ``access`` field contains the access type of this control. Give
+the combination of bit masks, ``SNDRV_CTL_ELEM_ACCESS_XXX``,
+there. The details will be explained in the `Access Flags`_
+subsection.
+
+The ``private_value`` field contains an arbitrary long integer value
+for this record. When using the generic ``info``, ``get`` and ``put``
+callbacks, you can pass a value through this field. If several small
+numbers are necessary, you can combine them in bitwise. Or, it's
+possible to store a pointer (casted to unsigned long) of some record in
+this field, too. 
+
+The ``tlv`` field can be used to provide metadata about the control;
+see the `Metadata`_ subsection.
+
+The other three are `Control Callbacks`_.
+
+Control Names
+-------------
+
+There are some standards to define the control names. A control is
+usually defined from the three parts as “SOURCE DIRECTION FUNCTION”.
+
+The first, ``SOURCE``, specifies the source of the control, and is a
+string such as “Master”, “PCM”, “CD” and “Line”. There are many
+pre-defined sources.
+
+The second, ``DIRECTION``, is one of the following strings according to
+the direction of the control: “Playback”, “Capture”, “Bypass Playback”
+and “Bypass Capture”. Or, it can be omitted, meaning both playback and
+capture directions.
+
+The third, ``FUNCTION``, is one of the following strings according to
+the function of the control: “Switch”, “Volume” and “Route”.
+
+The example of control names are, thus, “Master Capture Switch” or “PCM
+Playback Volume”.
+
+There are some exceptions:
+
+Global capture and playback
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+“Capture Source”, “Capture Switch” and “Capture Volume” are used for the
+global capture (input) source, switch and volume. Similarly, “Playback
+Switch” and “Playback Volume” are used for the global output gain switch
+and volume.
+
+Tone-controls
+~~~~~~~~~~~~~
+
+tone-control switch and volumes are specified like “Tone Control - XXX”,
+e.g. “Tone Control - Switch”, “Tone Control - Bass”, “Tone Control -
+Center”.
+
+3D controls
+~~~~~~~~~~~
+
+3D-control switches and volumes are specified like “3D Control - XXX”,
+e.g. “3D Control - Switch”, “3D Control - Center”, “3D Control - Space”.
+
+Mic boost
+~~~~~~~~~
+
+Mic-boost switch is set as “Mic Boost” or “Mic Boost (6dB)”.
+
+More precise information can be found in
+``Documentation/sound/designs/control-names.rst``.
+
+Access Flags
+------------
+
+The access flag is the bitmask which specifies the access type of the
+given control. The default access type is
+``SNDRV_CTL_ELEM_ACCESS_READWRITE``, which means both read and write are
+allowed to this control. When the access flag is omitted (i.e. = 0), it
+is considered as ``READWRITE`` access by default.
+
+When the control is read-only, pass ``SNDRV_CTL_ELEM_ACCESS_READ``
+instead. In this case, you don't have to define the ``put`` callback.
+Similarly, when the control is write-only (although it's a rare case),
+you can use the ``WRITE`` flag instead, and you don't need the ``get``
+callback.
+
+If the control value changes frequently (e.g. the VU meter),
+``VOLATILE`` flag should be given. This means that the control may be
+changed without `Change notification`_. Applications should poll such
+a control constantly.
+
+When the control may be updated, but currently has no effect on anything,
+setting the ``INACTIVE`` flag may be appropriate. For example, PCM
+controls should be inactive while no PCM device is open.
+
+There are ``LOCK`` and ``OWNER`` flags to change the write permissions.
+
+Control Callbacks
+-----------------
+
+info callback
+~~~~~~~~~~~~~
+
+The ``info`` callback is used to get detailed information on this
+control. This must store the values of the given
+struct snd_ctl_elem_info object. For example,
+for a boolean control with a single element::
+
+
+      static int snd_myctl_mono_info(struct snd_kcontrol *kcontrol,
+                              struct snd_ctl_elem_info *uinfo)
+      {
+              uinfo->type = SNDRV_CTL_ELEM_TYPE_BOOLEAN;
+              uinfo->count = 1;
+              uinfo->value.integer.min = 0;
+              uinfo->value.integer.max = 1;
+              return 0;
+      }
+
+
+
+The ``type`` field specifies the type of the control. There are
+``BOOLEAN``, ``INTEGER``, ``ENUMERATED``, ``BYTES``, ``IEC958`` and
+``INTEGER64``. The ``count`` field specifies the number of elements in
+this control. For example, a stereo volume would have count = 2. The
+``value`` field is a union, and the values stored depend on the
+type. The boolean and integer types are identical.
+
+The enumerated type is a bit different from the others. You'll need to
+set the string for the selectec item index::
+
+  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+                          struct snd_ctl_elem_info *uinfo)
+  {
+          static char *texts[4] = {
+                  "First", "Second", "Third", "Fourth"
+          };
+          uinfo->type = SNDRV_CTL_ELEM_TYPE_ENUMERATED;
+          uinfo->count = 1;
+          uinfo->value.enumerated.items = 4;
+          if (uinfo->value.enumerated.item > 3)
+                  uinfo->value.enumerated.item = 3;
+          strcpy(uinfo->value.enumerated.name,
+                 texts[uinfo->value.enumerated.item]);
+          return 0;
+  }
+
+The above callback can be simplified with a helper function,
+:c:func:`snd_ctl_enum_info()`. The final code looks like below.
+(You can pass ``ARRAY_SIZE(texts)`` instead of 4 in the third argument;
+it's a matter of taste.)
+
+::
+
+  static int snd_myctl_enum_info(struct snd_kcontrol *kcontrol,
+                          struct snd_ctl_elem_info *uinfo)
+  {
+          static char *texts[4] = {
+                  "First", "Second", "Third", "Fourth"
+          };
+          return snd_ctl_enum_info(uinfo, 1, 4, texts);
+  }
+
+
+Some common info callbacks are available for your convenience:
+:c:func:`snd_ctl_boolean_mono_info()` and
+:c:func:`snd_ctl_boolean_stereo_info()`. Obviously, the former
+is an info callback for a mono channel boolean item, just like
+:c:func:`snd_myctl_mono_info()` above, and the latter is for a
+stereo channel boolean item.
+
+get callback
+~~~~~~~~~~~~
+
+This callback is used to read the current value of the control, so it
+can be returned to user-space.
+
+For example::
+
+      static int snd_myctl_get(struct snd_kcontrol *kcontrol,
+                               struct snd_ctl_elem_value *ucontrol)
+      {
+              struct mychip *chip = snd_kcontrol_chip(kcontrol);
+              ucontrol->value.integer.value[0] = get_some_value(chip);
+              return 0;
+      }
+
+
+
+The ``value`` field depends on the type of control as well as on the
+info callback. For example, the sb driver uses this field to store the
+register offset, the bit-shift and the bit-mask. The ``private_value``
+field is set as follows::
+
+  .private_value = reg | (shift << 16) | (mask << 24)
+
+and is retrieved in callbacks like::
+
+  static int snd_sbmixer_get_single(struct snd_kcontrol *kcontrol,
+                                    struct snd_ctl_elem_value *ucontrol)
+  {
+          int reg = kcontrol->private_value & 0xff;
+          int shift = (kcontrol->private_value >> 16) & 0xff;
+          int mask = (kcontrol->private_value >> 24) & 0xff;
+          ....
+  }
+
+In the ``get`` callback, you have to fill all the elements if the
+control has more than one element, i.e. ``count > 1``. In the example
+above, we filled only one element (``value.integer.value[0]``) since
+``count = 1`` is assumed.
+
+put callback
+~~~~~~~~~~~~
+
+This callback is used to write a value coming from user-space.
+
+For example::
+
+      static int snd_myctl_put(struct snd_kcontrol *kcontrol,
+                               struct snd_ctl_elem_value *ucontrol)
+      {
+              struct mychip *chip = snd_kcontrol_chip(kcontrol);
+              int changed = 0;
+              if (chip->current_value !=
+                   ucontrol->value.integer.value[0]) {
+                      change_current_value(chip,
+                                  ucontrol->value.integer.value[0]);
+                      changed = 1;
+              }
+              return changed;
+      }
+
+
+
+As seen above, you have to return 1 if the value is changed. If the
+value is not changed, return 0 instead. If any fatal error happens,
+return a negative error code as usual.
+
+As in the ``get`` callback, when the control has more than one
+element, all elements must be evaluated in this callback, too.
+
+Callbacks are not atomic
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+All these three callbacks are not-atomic.
+
+Control Constructor
+-------------------
+
+When everything is ready, finally we can create a new control. To create
+a control, there are two functions to be called,
+:c:func:`snd_ctl_new1()` and :c:func:`snd_ctl_add()`.
+
+In the simplest way, you can do it like this::
+
+  err = snd_ctl_add(card, snd_ctl_new1(&my_control, chip));
+  if (err < 0)
+          return err;
+
+where ``my_control`` is the struct snd_kcontrol_new object defined above,
+and chip is the object pointer to be passed to kcontrol->private_data which
+can be referred to in callbacks.
+
+:c:func:`snd_ctl_new1()` allocates a new struct snd_kcontrol instance, and
+:c:func:`snd_ctl_add()` assigns the given control component to the
+card.
+
+Change Notification
+-------------------
+
+If you need to change and update a control in the interrupt routine, you
+can call :c:func:`snd_ctl_notify()`. For example::
+
+  snd_ctl_notify(card, SNDRV_CTL_EVENT_MASK_VALUE, id_pointer);
+
+This function takes the card pointer, the event-mask, and the control id
+pointer for the notification. The event-mask specifies the types of
+notification, for example, in the above example, the change of control
+values is notified. The id pointer is the pointer of struct snd_ctl_elem_id
+to be notified. You can find some examples in ``es1938.c`` or ``es1968.c``
+for hardware volume interrupts.
+
+Metadata
+--------
+
+To provide information about the dB values of a mixer control, use one of
+the ``DECLARE_TLV_xxx`` macros from ``<sound/tlv.h>`` to define a
+variable containing this information, set the ``tlv.p`` field to point to
+this variable, and include the ``SNDRV_CTL_ELEM_ACCESS_TLV_READ`` flag
+in the ``access`` field; like this::
+
+  static DECLARE_TLV_DB_SCALE(db_scale_my_control, -4050, 150, 0);
+
+  static struct snd_kcontrol_new my_control = {
+          ...
+          .access = SNDRV_CTL_ELEM_ACCESS_READWRITE |
+                    SNDRV_CTL_ELEM_ACCESS_TLV_READ,
+          ...
+          .tlv.p = db_scale_my_control,
+  };
+
+
+The :c:func:`DECLARE_TLV_DB_SCALE()` macro defines information
+about a mixer control where each step in the control's value changes the
+dB value by a constant dB amount. The first parameter is the name of the
+variable to be defined. The second parameter is the minimum value, in
+units of 0.01 dB. The third parameter is the step size, in units of 0.01
+dB. Set the fourth parameter to 1 if the minimum value actually mutes
+the control.
+
+The :c:func:`DECLARE_TLV_DB_LINEAR()` macro defines information
+about a mixer control where the control's value affects the output
+linearly. The first parameter is the name of the variable to be defined.
+The second parameter is the minimum value, in units of 0.01 dB. The
+third parameter is the maximum value, in units of 0.01 dB. If the
+minimum value mutes the control, set the second parameter to
+``TLV_DB_GAIN_MUTE``.
+
+API for AC97 Codec
+==================
+
+General
+-------
+
+The ALSA AC97 codec layer is a well-defined one, and you don't have to
+write much code to control it. Only low-level control routines are
+necessary. The AC97 codec API is defined in ``<sound/ac97_codec.h>``.
+
+Full Code Example
+-----------------
+
+::
+
+      struct mychip {
+              ....
+              struct snd_ac97 *ac97;
+              ....
+      };
+
+      static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+                                                 unsigned short reg)
+      {
+              struct mychip *chip = ac97->private_data;
+              ....
+              /* read a register value here from the codec */
+              return the_register_value;
+      }
+
+      static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+                                       unsigned short reg, unsigned short val)
+      {
+              struct mychip *chip = ac97->private_data;
+              ....
+              /* write the given register value to the codec */
+      }
+
+      static int snd_mychip_ac97(struct mychip *chip)
+      {
+              struct snd_ac97_bus *bus;
+              struct snd_ac97_template ac97;
+              int err;
+              static struct snd_ac97_bus_ops ops = {
+                      .write = snd_mychip_ac97_write,
+                      .read = snd_mychip_ac97_read,
+              };
+
+              err = snd_ac97_bus(chip->card, 0, &ops, NULL, &bus);
+              if (err < 0)
+                      return err;
+              memset(&ac97, 0, sizeof(ac97));
+              ac97.private_data = chip;
+              return snd_ac97_mixer(bus, &ac97, &chip->ac97);
+      }
+
+
+AC97 Constructor
+----------------
+
+To create an ac97 instance, first call :c:func:`snd_ac97_bus()`
+with an ``ac97_bus_ops_t`` record with callback functions::
+
+  struct snd_ac97_bus *bus;
+  static struct snd_ac97_bus_ops ops = {
+        .write = snd_mychip_ac97_write,
+        .read = snd_mychip_ac97_read,
+  };
+
+  snd_ac97_bus(card, 0, &ops, NULL, &pbus);
+
+The bus record is shared among all belonging ac97 instances.
+
+And then call :c:func:`snd_ac97_mixer()` with a struct snd_ac97_template
+record together with the bus pointer created above::
+
+  struct snd_ac97_template ac97;
+  int err;
+
+  memset(&ac97, 0, sizeof(ac97));
+  ac97.private_data = chip;
+  snd_ac97_mixer(bus, &ac97, &chip->ac97);
+
+where chip->ac97 is a pointer to a newly created ``ac97_t``
+instance. In this case, the chip pointer is set as the private data,
+so that the read/write callback functions can refer to this chip
+instance. This instance is not necessarily stored in the chip
+record. If you need to change the register values from the driver, or
+need the suspend/resume of ac97 codecs, keep this pointer to pass to
+the corresponding functions.
+
+AC97 Callbacks
+--------------
+
+The standard callbacks are ``read`` and ``write``. Obviously they
+correspond to the functions for read and write accesses to the
+hardware low-level codes.
+
+The ``read`` callback returns the register value specified in the
+argument::
+
+  static unsigned short snd_mychip_ac97_read(struct snd_ac97 *ac97,
+                                             unsigned short reg)
+  {
+          struct mychip *chip = ac97->private_data;
+          ....
+          return the_register_value;
+  }
+
+Here, the chip can be cast from ``ac97->private_data``.
+
+Meanwhile, the ``write`` callback is used to set the register
+value::
+
+  static void snd_mychip_ac97_write(struct snd_ac97 *ac97,
+                       unsigned short reg, unsigned short val)
+
+
+These callbacks are non-atomic like the control API callbacks.
+
+There are also other callbacks: ``reset``, ``wait`` and ``init``.
+
+The ``reset`` callback is used to reset the codec. If the chip
+requires a special kind of reset, you can define this callback.
+
+The ``wait`` callback is used to add some waiting time in the standard
+initialization of the codec. If the chip requires the extra waiting
+time, define this callback.
+
+The ``init`` callback is used for additional initialization of the
+codec.
+
+Updating Registers in The Driver
+--------------------------------
+
+If you need to access to the codec from the driver, you can call the
+following functions: :c:func:`snd_ac97_write()`,
+:c:func:`snd_ac97_read()`, :c:func:`snd_ac97_update()` and
+:c:func:`snd_ac97_update_bits()`.
+
+Both :c:func:`snd_ac97_write()` and
+:c:func:`snd_ac97_update()` functions are used to set a value to
+the given register (``AC97_XXX``). The difference between them is that
+:c:func:`snd_ac97_update()` doesn't write a value if the given
+value has been already set, while :c:func:`snd_ac97_write()`
+always rewrites the value::
+
+  snd_ac97_write(ac97, AC97_MASTER, 0x8080);
+  snd_ac97_update(ac97, AC97_MASTER, 0x8080);
+
+:c:func:`snd_ac97_read()` is used to read the value of the given
+register. For example::
+
+  value = snd_ac97_read(ac97, AC97_MASTER);
+
+:c:func:`snd_ac97_update_bits()` is used to update some bits in
+the given register::
+
+  snd_ac97_update_bits(ac97, reg, mask, value);
+
+Also, there is a function to change the sample rate (of a given register
+such as ``AC97_PCM_FRONT_DAC_RATE``) when VRA or DRA is supported by the
+codec: :c:func:`snd_ac97_set_rate()`::
+
+  snd_ac97_set_rate(ac97, AC97_PCM_FRONT_DAC_RATE, 44100);
+
+
+The following registers are available to set the rate:
+``AC97_PCM_MIC_ADC_RATE``, ``AC97_PCM_FRONT_DAC_RATE``,
+``AC97_PCM_LR_ADC_RATE``, ``AC97_SPDIF``. When ``AC97_SPDIF`` is
+specified, the register is not really changed but the corresponding
+IEC958 status bits will be updated.
+
+Clock Adjustment
+----------------
+
+In some chips, the clock of the codec isn't 48000 but using a PCI clock
+(to save a quartz!). In this case, change the field ``bus->clock`` to
+the corresponding value. For example, intel8x0 and es1968 drivers have
+their own function to read from the clock.
+
+Proc Files
+----------
+
+The ALSA AC97 interface will create a proc file such as
+``/proc/asound/card0/codec97#0/ac97#0-0`` and ``ac97#0-0+regs``. You
+can refer to these files to see the current status and registers of
+the codec.
+
+Multiple Codecs
+---------------
+
+When there are several codecs on the same card, you need to call
+:c:func:`snd_ac97_mixer()` multiple times with ``ac97.num=1`` or
+greater. The ``num`` field specifies the codec number.
+
+If you set up multiple codecs, you either need to write different
+callbacks for each codec or check ``ac97->num`` in the callback
+routines.
+
+MIDI (MPU401-UART) Interface
+============================
+
+General
+-------
+
+Many soundcards have built-in MIDI (MPU401-UART) interfaces. When the
+soundcard supports the standard MPU401-UART interface, most likely you
+can use the ALSA MPU401-UART API. The MPU401-UART API is defined in
+``<sound/mpu401.h>``.
+
+Some soundchips have a similar but slightly different implementation of
+mpu401 stuff. For example, emu10k1 has its own mpu401 routines.
+
+MIDI Constructor
+----------------
+
+To create a rawmidi object, call :c:func:`snd_mpu401_uart_new()`::
+
+  struct snd_rawmidi *rmidi;
+  snd_mpu401_uart_new(card, 0, MPU401_HW_MPU401, port, info_flags,
+                      irq, &rmidi);
+
+
+The first argument is the card pointer, and the second is the index of
+this component. You can create up to 8 rawmidi devices.
+
+The third argument is the type of the hardware, ``MPU401_HW_XXX``. If
+it's not a special one, you can use ``MPU401_HW_MPU401``.
+
+The 4th argument is the I/O port address. Many backward-compatible
+MPU401 have an I/O port such as 0x330. Or, it might be a part of its own
+PCI I/O region. It depends on the chip design.
+
+The 5th argument is a bitflag for additional information. When the I/O
+port address above is part of the PCI I/O region, the MPU401 I/O port
+might have been already allocated (reserved) by the driver itself. In
+such a case, pass a bit flag ``MPU401_INFO_INTEGRATED``, and the
+mpu401-uart layer will allocate the I/O ports by itself.
+
+When the controller supports only the input or output MIDI stream, pass
+the ``MPU401_INFO_INPUT`` or ``MPU401_INFO_OUTPUT`` bitflag,
+respectively. Then the rawmidi instance is created as a single stream.
+
+``MPU401_INFO_MMIO`` bitflag is used to change the access method to MMIO
+(via readb and writeb) instead of iob and outb. In this case, you have
+to pass the iomapped address to :c:func:`snd_mpu401_uart_new()`.
+
+When ``MPU401_INFO_TX_IRQ`` is set, the output stream isn't checked in
+the default interrupt handler. The driver needs to call
+:c:func:`snd_mpu401_uart_interrupt_tx()` by itself to start
+processing the output stream in the irq handler.
+
+If the MPU-401 interface shares its interrupt with the other logical
+devices on the card, set ``MPU401_INFO_IRQ_HOOK`` (see
+`below <MIDI Interrupt Handler_>`__).
+
+Usually, the port address corresponds to the command port and port + 1
+corresponds to the data port. If not, you may change the ``cport``
+field of struct snd_mpu401 manually afterward.
+However, struct snd_mpu401 pointer is
+not returned explicitly by :c:func:`snd_mpu401_uart_new()`. You
+need to cast ``rmidi->private_data`` to struct snd_mpu401 explicitly::
+
+  struct snd_mpu401 *mpu;
+  mpu = rmidi->private_data;
+
+and reset the ``cport`` as you like::
+
+  mpu->cport = my_own_control_port;
+
+The 6th argument specifies the ISA irq number that will be allocated. If
+no interrupt is to be allocated (because your code is already allocating
+a shared interrupt, or because the device does not use interrupts), pass
+-1 instead. For a MPU-401 device without an interrupt, a polling timer
+will be used instead.
+
+MIDI Interrupt Handler
+----------------------
+
+When the interrupt is allocated in
+:c:func:`snd_mpu401_uart_new()`, an exclusive ISA interrupt
+handler is automatically used, hence you don't have anything else to do
+than creating the mpu401 stuff. Otherwise, you have to set
+``MPU401_INFO_IRQ_HOOK``, and call
+:c:func:`snd_mpu401_uart_interrupt()` explicitly from your own
+interrupt handler when it has determined that a UART interrupt has
+occurred.
+
+In this case, you need to pass the private_data of the returned rawmidi
+object from :c:func:`snd_mpu401_uart_new()` as the second
+argument of :c:func:`snd_mpu401_uart_interrupt()`::
+
+  snd_mpu401_uart_interrupt(irq, rmidi->private_data, regs);
+
+
+RawMIDI Interface
+=================
+
+Overview
+--------
+
+The raw MIDI interface is used for hardware MIDI ports that can be
+accessed as a byte stream. It is not used for synthesizer chips that do
+not directly understand MIDI.
+
+ALSA handles file and buffer management. All you have to do is to write
+some code to move data between the buffer and the hardware.
+
+The rawmidi API is defined in ``<sound/rawmidi.h>``.
+
+RawMIDI Constructor
+-------------------
+
+To create a rawmidi device, call the :c:func:`snd_rawmidi_new()`
+function::
+
+  struct snd_rawmidi *rmidi;
+  err = snd_rawmidi_new(chip->card, "MyMIDI", 0, outs, ins, &rmidi);
+  if (err < 0)
+          return err;
+  rmidi->private_data = chip;
+  strcpy(rmidi->name, "My MIDI");
+  rmidi->info_flags = SNDRV_RAWMIDI_INFO_OUTPUT |
+                      SNDRV_RAWMIDI_INFO_INPUT |
+                      SNDRV_RAWMIDI_INFO_DUPLEX;
+
+The first argument is the card pointer, the second argument is the ID
+string.
+
+The third argument is the index of this component. You can create up to
+8 rawmidi devices.
+
+The fourth and fifth arguments are the number of output and input
+substreams, respectively, of this device (a substream is the equivalent
+of a MIDI port).
+
+Set the ``info_flags`` field to specify the capabilities of the
+device. Set ``SNDRV_RAWMIDI_INFO_OUTPUT`` if there is at least one
+output port, ``SNDRV_RAWMIDI_INFO_INPUT`` if there is at least one
+input port, and ``SNDRV_RAWMIDI_INFO_DUPLEX`` if the device can handle
+output and input at the same time.
+
+After the rawmidi device is created, you need to set the operators
+(callbacks) for each substream. There are helper functions to set the
+operators for all the substreams of a device::
+
+  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_OUTPUT, &snd_mymidi_output_ops);
+  snd_rawmidi_set_ops(rmidi, SNDRV_RAWMIDI_STREAM_INPUT, &snd_mymidi_input_ops);
+
+The operators are usually defined like this::
+
+  static struct snd_rawmidi_ops snd_mymidi_output_ops = {
+          .open =    snd_mymidi_output_open,
+          .close =   snd_mymidi_output_close,
+          .trigger = snd_mymidi_output_trigger,
+  };
+
+These callbacks are explained in the `RawMIDI Callbacks`_ section.
+
+If there are more than one substream, you should give a unique name to
+each of them::
+
+  struct snd_rawmidi_substream *substream;
+  list_for_each_entry(substream,
+                      &rmidi->streams[SNDRV_RAWMIDI_STREAM_OUTPUT].substreams,
+                      list {
+          sprintf(substream->name, "My MIDI Port %d", substream->number + 1);
+  }
+  /* same for SNDRV_RAWMIDI_STREAM_INPUT */
+
+RawMIDI Callbacks
+-----------------
+
+In all the callbacks, the private data that you've set for the rawmidi
+device can be accessed as ``substream->rmidi->private_data``.
+
+If there is more than one port, your callbacks can determine the port
+index from the struct snd_rawmidi_substream data passed to each
+callback::
+
+  struct snd_rawmidi_substream *substream;
+  int index = substream->number;
+
+RawMIDI open callback
+~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static int snd_xxx_open(struct snd_rawmidi_substream *substream);
+
+
+This is called when a substream is opened. You can initialize the
+hardware here, but you shouldn't start transmitting/receiving data yet.
+
+RawMIDI close callback
+~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static int snd_xxx_close(struct snd_rawmidi_substream *substream);
+
+Guess what.
+
+The ``open`` and ``close`` callbacks of a rawmidi device are
+serialized with a mutex, and can sleep.
+
+Rawmidi trigger callback for output substreams
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static void snd_xxx_output_trigger(struct snd_rawmidi_substream *substream, int up);
+
+
+This is called with a nonzero ``up`` parameter when there is some data
+in the substream buffer that must be transmitted.
+
+To read data from the buffer, call
+:c:func:`snd_rawmidi_transmit_peek()`. It will return the number
+of bytes that have been read; this will be less than the number of bytes
+requested when there are no more data in the buffer. After the data have
+been transmitted successfully, call
+:c:func:`snd_rawmidi_transmit_ack()` to remove the data from the
+substream buffer::
+
+  unsigned char data;
+  while (snd_rawmidi_transmit_peek(substream, &data, 1) == 1) {
+          if (snd_mychip_try_to_transmit(data))
+                  snd_rawmidi_transmit_ack(substream, 1);
+          else
+                  break; /* hardware FIFO full */
+  }
+
+If you know beforehand that the hardware will accept data, you can use
+the :c:func:`snd_rawmidi_transmit()` function which reads some
+data and removes them from the buffer at once::
+
+  while (snd_mychip_transmit_possible()) {
+          unsigned char data;
+          if (snd_rawmidi_transmit(substream, &data, 1) != 1)
+                  break; /* no more data */
+          snd_mychip_transmit(data);
+  }
+
+If you know beforehand how many bytes you can accept, you can use a
+buffer size greater than one with the ``snd_rawmidi_transmit*()`` functions.
+
+The ``trigger`` callback must not sleep. If the hardware FIFO is full
+before the substream buffer has been emptied, you have to continue
+transmitting data later, either in an interrupt handler, or with a
+timer if the hardware doesn't have a MIDI transmit interrupt.
+
+The ``trigger`` callback is called with a zero ``up`` parameter when
+the transmission of data should be aborted.
+
+RawMIDI trigger callback for input substreams
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+      static void snd_xxx_input_trigger(struct snd_rawmidi_substream *substream, int up);
+
+
+This is called with a nonzero ``up`` parameter to enable receiving data,
+or with a zero ``up`` parameter do disable receiving data.
+
+The ``trigger`` callback must not sleep; the actual reading of data
+from the device is usually done in an interrupt handler.
+
+When data reception is enabled, your interrupt handler should call
+:c:func:`snd_rawmidi_receive()` for all received data::
+
+  void snd_mychip_midi_interrupt(...)
+  {
+          while (mychip_midi_available()) {
+                  unsigned char data;
+                  data = mychip_midi_read();
+                  snd_rawmidi_receive(substream, &data, 1);
+          }
+  }
+
+
+drain callback
+~~~~~~~~~~~~~~
+
+::
+
+      static void snd_xxx_drain(struct snd_rawmidi_substream *substream);
+
+
+This is only used with output substreams. This function should wait
+until all data read from the substream buffer have been transmitted.
+This ensures that the device can be closed and the driver unloaded
+without losing data.
+
+This callback is optional. If you do not set ``drain`` in the struct
+snd_rawmidi_ops structure, ALSA will simply wait for 50 milliseconds
+instead.
+
+Miscellaneous Devices
+=====================
+
+FM OPL3
+-------
+
+The FM OPL3 is still used in many chips (mainly for backward
+compatibility). ALSA has a nice OPL3 FM control layer, too. The OPL3 API
+is defined in ``<sound/opl3.h>``.
+
+FM registers can be directly accessed through the direct-FM API, defined
+in ``<sound/asound_fm.h>``. In ALSA native mode, FM registers are
+accessed through the Hardware-Dependent Device direct-FM extension API,
+whereas in OSS compatible mode, FM registers can be accessed with the
+OSS direct-FM compatible API in ``/dev/dmfmX`` device.
+
+To create the OPL3 component, you have two functions to call. The first
+one is a constructor for the ``opl3_t`` instance::
+
+  struct snd_opl3 *opl3;
+  snd_opl3_create(card, lport, rport, OPL3_HW_OPL3_XXX,
+                  integrated, &opl3);
+
+The first argument is the card pointer, the second one is the left port
+address, and the third is the right port address. In most cases, the
+right port is placed at the left port + 2.
+
+The fourth argument is the hardware type.
+
+When the left and right ports have been already allocated by the card
+driver, pass non-zero to the fifth argument (``integrated``). Otherwise,
+the opl3 module will allocate the specified ports by itself.
+
+When the accessing the hardware requires special method instead of the
+standard I/O access, you can create opl3 instance separately with
+:c:func:`snd_opl3_new()`::
+
+  struct snd_opl3 *opl3;
+  snd_opl3_new(card, OPL3_HW_OPL3_XXX, &opl3);
+
+Then set ``command``, ``private_data`` and ``private_free`` for the
+private access function, the private data and the destructor. The
+``l_port`` and ``r_port`` are not necessarily set. Only the command
+must be set properly. You can retrieve the data from the
+``opl3->private_data`` field. 
+
+After creating the opl3 instance via :c:func:`snd_opl3_new()`,
+call :c:func:`snd_opl3_init()` to initialize the chip to the
+proper state. Note that :c:func:`snd_opl3_create()` always calls
+it internally.
+
+If the opl3 instance is created successfully, then create a hwdep device
+for this opl3::
+
+  struct snd_hwdep *opl3hwdep;
+  snd_opl3_hwdep_new(opl3, 0, 1, &opl3hwdep);
+
+The first argument is the ``opl3_t`` instance you created, and the
+second is the index number, usually 0.
+
+The third argument is the index-offset for the sequencer client assigned
+to the OPL3 port. When there is an MPU401-UART, give 1 for here (UART
+always takes 0).
+
+Hardware-Dependent Devices
+--------------------------
+
+Some chips need user-space access for special controls or for loading
+the micro code. In such a case, you can create a hwdep
+(hardware-dependent) device. The hwdep API is defined in
+``<sound/hwdep.h>``. You can find examples in opl3 driver or
+``isa/sb/sb16_csp.c``.
+
+The creation of the ``hwdep`` instance is done via
+:c:func:`snd_hwdep_new()`::
+
+  struct snd_hwdep *hw;
+  snd_hwdep_new(card, "My HWDEP", 0, &hw);
+
+where the third argument is the index number.
+
+You can then pass any pointer value to the ``private_data``. If you
+assign private data, you should define a destructor, too. The
+destructor function is set in the ``private_free`` field::
+
+  struct mydata *p = kmalloc(sizeof(*p), GFP_KERNEL);
+  hw->private_data = p;
+  hw->private_free = mydata_free;
+
+and the implementation of the destructor would be::
+
+  static void mydata_free(struct snd_hwdep *hw)
+  {
+          struct mydata *p = hw->private_data;
+          kfree(p);
+  }
+
+The arbitrary file operations can be defined for this instance. The file
+operators are defined in the ``ops`` table. For example, assume that
+this chip needs an ioctl::
+
+  hw->ops.open = mydata_open;
+  hw->ops.ioctl = mydata_ioctl;
+  hw->ops.release = mydata_release;
+
+And implement the callback functions as you like.
+
+IEC958 (S/PDIF)
+---------------
+
+Usually the controls for IEC958 devices are implemented via the control
+interface. There is a macro to compose a name string for IEC958
+controls, :c:func:`SNDRV_CTL_NAME_IEC958()` defined in
+``<include/asound.h>``.
+
+There are some standard controls for IEC958 status bits. These controls
+use the type ``SNDRV_CTL_ELEM_TYPE_IEC958``, and the size of element is
+fixed as 4 bytes array (value.iec958.status[x]). For the ``info``
+callback, you don't specify the value field for this type (the count
+field must be set, though).
+
+“IEC958 Playback Con Mask” is used to return the bit-mask for the IEC958
+status bits of consumer mode. Similarly, “IEC958 Playback Pro Mask”
+returns the bitmask for professional mode. They are read-only controls.
+
+Meanwhile, “IEC958 Playback Default” control is defined for getting and
+setting the current default IEC958 bits.
+
+Due to historical reasons, both variants of the Playback Mask and the
+Playback Default controls can be implemented on either a
+``SNDRV_CTL_ELEM_IFACE_PCM`` or a ``SNDRV_CTL_ELEM_IFACE_MIXER`` iface.
+Drivers should expose the mask and default on the same iface though.
+
+In addition, you can define the control switches to enable/disable or to
+set the raw bit mode. The implementation will depend on the chip, but
+the control should be named as “IEC958 xxx”, preferably using the
+:c:func:`SNDRV_CTL_NAME_IEC958()` macro.
+
+You can find several cases, for example, ``pci/emu10k1``,
+``pci/ice1712``, or ``pci/cmipci.c``.
+
+Buffer and Memory Management
+============================
+
+Buffer Types
+------------
+
+ALSA provides several different buffer allocation functions depending on
+the bus and the architecture. All these have a consistent API. The
+allocation of physically-contiguous pages is done via the
+:c:func:`snd_malloc_xxx_pages()` function, where xxx is the bus
+type.
+
+The allocation of pages with fallback is done via
+:c:func:`snd_dma_alloc_pages_fallback()`. This function tries
+to allocate the specified number of pages, but if not enough pages are
+available, it tries to reduce the request size until enough space
+is found, down to one page.
+
+To release the pages, call the :c:func:`snd_dma_free_pages()`
+function.
+
+Usually, ALSA drivers try to allocate and reserve a large contiguous
+physical space at the time the module is loaded for later use. This
+is called “pre-allocation”. As already written, you can call the
+following function at PCM instance construction time (in the case of PCI
+bus)::
+
+  snd_pcm_lib_preallocate_pages_for_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                        &pci->dev, size, max);
+
+where ``size`` is the byte size to be pre-allocated and ``max`` is
+the maximum size settable via the ``prealloc`` proc file. The
+allocator will try to get an area as large as possible within the
+given size.
+
+The second argument (type) and the third argument (device pointer) are
+dependent on the bus. For normal devices, pass the device pointer
+(typically identical as ``card->dev``) to the third argument with
+``SNDRV_DMA_TYPE_DEV`` type.
+
+A continuous buffer unrelated to the
+bus can be pre-allocated with ``SNDRV_DMA_TYPE_CONTINUOUS`` type.
+You can pass NULL to the device pointer in that case, which is the
+default mode implying to allocate with the ``GFP_KERNEL`` flag.
+If you need a restricted (lower) address, set up the coherent DMA mask
+bits for the device, and pass the device pointer, like the normal
+device memory allocations.  For this type, it's still allowed to pass
+NULL to the device pointer, too, if no address restriction is needed.
+
+For the scatter-gather buffers, use ``SNDRV_DMA_TYPE_DEV_SG`` with the
+device pointer (see the `Non-Contiguous Buffers`_ section).
+
+Once the buffer is pre-allocated, you can use the allocator in the
+``hw_params`` callback::
+
+  snd_pcm_lib_malloc_pages(substream, size);
+
+Note that you have to pre-allocate to use this function.
+
+But most drivers use the "managed buffer allocation mode" instead
+of manual allocation and release.
+This is done by calling :c:func:`snd_pcm_set_managed_buffer_all()`
+instead of :c:func:`snd_pcm_lib_preallocate_pages_for_all()`::
+
+  snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV,
+                                 &pci->dev, size, max);
+
+where the passed arguments are identical for both functions.
+The difference in the managed mode is that PCM core will call
+:c:func:`snd_pcm_lib_malloc_pages()` internally already before calling
+the PCM ``hw_params`` callback, and call :c:func:`snd_pcm_lib_free_pages()`
+after the PCM ``hw_free`` callback automatically.  So the driver
+doesn't have to call these functions explicitly in its callback any
+longer.  This allows many drivers to have NULL ``hw_params`` and
+``hw_free`` entries.
+
+External Hardware Buffers
+-------------------------
+
+Some chips have their own hardware buffers and DMA transfer from the
+host memory is not available. In such a case, you need to either 1)
+copy/set the audio data directly to the external hardware buffer, or 2)
+make an intermediate buffer and copy/set the data from it to the
+external hardware buffer in interrupts (or in tasklets, preferably).
+
+The first case works fine if the external hardware buffer is large
+enough. This method doesn't need any extra buffers and thus is more
+efficient. You need to define the ``copy`` callback
+for the data transfer, in addition to the ``fill_silence``
+callback for playback. However, there is a drawback: it cannot be
+mmapped. The examples are GUS's GF1 PCM or emu8000's wavetable PCM.
+
+The second case allows for mmap on the buffer, although you have to
+handle an interrupt or a tasklet to transfer the data from the
+intermediate buffer to the hardware buffer. You can find an example in
+the vxpocket driver.
+
+Another case is when the chip uses a PCI memory-map region for the
+buffer instead of the host memory. In this case, mmap is available only
+on certain architectures like the Intel one. In non-mmap mode, the data
+cannot be transferred as in the normal way. Thus you need to define the
+``copy`` and ``fill_silence`` callbacks as well,
+as in the cases above. Examples are found in ``rme32.c`` and
+``rme96.c``.
+
+The implementation of the ``copy`` and
+``silence`` callbacks depends upon whether the hardware supports
+interleaved or non-interleaved samples. The ``copy`` callback is
+defined like below, a bit differently depending on whether the direction
+is playback or capture::
+
+  static int playback_copy(struct snd_pcm_substream *substream,
+               int channel, unsigned long pos,
+               struct iov_iter *src, unsigned long count);
+  static int capture_copy(struct snd_pcm_substream *substream,
+               int channel, unsigned long pos,
+               struct iov_iter *dst, unsigned long count);
+
+In the case of interleaved samples, the second argument (``channel``) is
+not used. The third argument (``pos``) specifies the position in bytes.
+
+The meaning of the fourth argument is different between playback and
+capture. For playback, it holds the source data pointer, and for
+capture, it's the destination data pointer.
+
+The last argument is the number of bytes to be copied.
+
+What you have to do in this callback is again different between playback
+and capture directions. In the playback case, you copy the given amount
+of data (``count``) at the specified pointer (``src``) to the specified
+offset (``pos``) in the hardware buffer. When coded like memcpy-like
+way, the copy would look like::
+
+  my_memcpy_from_iter(my_buffer + pos, src, count);
+
+For the capture direction, you copy the given amount of data (``count``)
+at the specified offset (``pos``) in the hardware buffer to the
+specified pointer (``dst``)::
+
+  my_memcpy_to_iter(dst, my_buffer + pos, count);
+
+The given ``src`` or ``dst`` a struct iov_iter pointer containing the
+pointer and the size.  Use the existing helpers to copy or access the
+data as defined in ``linux/uio.h``.
+
+Careful readers might notice that these callbacks receive the
+arguments in bytes, not in frames like other callbacks.  It's because
+this makes coding easier like in the examples above, and also it makes
+it easier to unify both the interleaved and non-interleaved cases, as
+explained below.
+
+In the case of non-interleaved samples, the implementation will be a bit
+more complicated.  The callback is called for each channel, passed in
+the second argument, so in total it's called N times per transfer.
+
+The meaning of the other arguments are almost the same as in the
+interleaved case.  The callback is supposed to copy the data from/to
+the given user-space buffer, but only for the given channel. For
+details, please check ``isa/gus/gus_pcm.c`` or ``pci/rme9652/rme9652.c``
+as examples.
+
+Usually for the playback, another callback ``fill_silence`` is
+defined.  It's implemented in a similar way as the copy callbacks
+above::
+
+  static int silence(struct snd_pcm_substream *substream, int channel,
+                     unsigned long pos, unsigned long count);
+
+The meanings of arguments are the same as in the ``copy`` callback,
+although there is no buffer pointer
+argument. In the case of interleaved samples, the channel argument has
+no meaning, as for the ``copy`` callback.
+
+The role of the ``fill_silence`` callback is to set the given amount
+(``count``) of silence data at the specified offset (``pos``) in the
+hardware buffer. Suppose that the data format is signed (that is, the
+silent-data is 0), and the implementation using a memset-like function
+would look like::
+
+  my_memset(my_buffer + pos, 0, count);
+
+In the case of non-interleaved samples, again, the implementation
+becomes a bit more complicated, as it's called N times per transfer
+for each channel. See, for example, ``isa/gus/gus_pcm.c``.
+
+Non-Contiguous Buffers
+----------------------
+
+If your hardware supports a page table as in emu10k1 or buffer
+descriptors as in via82xx, you can use scatter-gather (SG) DMA. ALSA
+provides an interface for handling SG-buffers. The API is provided in
+``<sound/pcm.h>``.
+
+For creating the SG-buffer handler, call
+:c:func:`snd_pcm_set_managed_buffer()` or
+:c:func:`snd_pcm_set_managed_buffer_all()` with
+``SNDRV_DMA_TYPE_DEV_SG`` in the PCM constructor like for other PCI
+pre-allocations. You need to pass ``&pci->dev``, where pci is
+the struct pci_dev pointer of the chip as well::
+
+  snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_DEV_SG,
+                                 &pci->dev, size, max);
+
+The ``struct snd_sg_buf`` instance is created as
+``substream->dma_private`` in turn. You can cast the pointer like::
+
+  struct snd_sg_buf *sgbuf = (struct snd_sg_buf *)substream->dma_private;
+
+Then in the :c:func:`snd_pcm_lib_malloc_pages()` call, the common SG-buffer
+handler will allocate the non-contiguous kernel pages of the given size
+and map them as virtually contiguous memory. The virtual pointer
+is addressed via runtime->dma_area. The physical address
+(``runtime->dma_addr``) is set to zero, because the buffer is
+physically non-contiguous. The physical address table is set up in
+``sgbuf->table``. You can get the physical address at a certain offset
+via :c:func:`snd_pcm_sgbuf_get_addr()`.
+
+If you need to release the SG-buffer data explicitly, call the
+standard API function :c:func:`snd_pcm_lib_free_pages()` as usual.
+
+Vmalloc'ed Buffers
+------------------
+
+It's possible to use a buffer allocated via :c:func:`vmalloc()`, for
+example, for an intermediate buffer.
+You can simply allocate it via the standard
+:c:func:`snd_pcm_lib_malloc_pages()` and co. after setting up the
+buffer preallocation with ``SNDRV_DMA_TYPE_VMALLOC`` type::
+
+  snd_pcm_set_managed_buffer_all(pcm, SNDRV_DMA_TYPE_VMALLOC,
+                                 NULL, 0, 0);
+
+NULL is passed as the device pointer argument, which indicates
+that default pages (GFP_KERNEL and GFP_HIGHMEM) will be
+allocated.
+
+Also, note that zero is passed as both the size and the max size
+argument here.  Since each vmalloc call should succeed at any time,
+we don't need to pre-allocate the buffers like other continuous
+pages.
+
+Proc Interface
+==============
+
+ALSA provides an easy interface for procfs. The proc files are very
+useful for debugging. I recommend you set up proc files if you write a
+driver and want to get a running status or register dumps. The API is
+found in ``<sound/info.h>``.
+
+To create a proc file, call :c:func:`snd_card_proc_new()`::
+
+  struct snd_info_entry *entry;
+  int err = snd_card_proc_new(card, "my-file", &entry);
+
+where the second argument specifies the name of the proc file to be
+created. The above example will create a file ``my-file`` under the
+card directory, e.g. ``/proc/asound/card0/my-file``.
+
+Like other components, the proc entry created via
+:c:func:`snd_card_proc_new()` will be registered and released
+automatically in the card registration and release functions.
+
+When the creation is successful, the function stores a new instance in
+the pointer given in the third argument. It is initialized as a text
+proc file for read only. To use this proc file as a read-only text file
+as-is, set the read callback with private data via
+:c:func:`snd_info_set_text_ops()`::
+
+  snd_info_set_text_ops(entry, chip, my_proc_read);
+
+where the second argument (``chip``) is the private data to be used in
+the callback. The third parameter specifies the read buffer size and
+the fourth (``my_proc_read``) is the callback function, which is
+defined like::
+
+  static void my_proc_read(struct snd_info_entry *entry,
+                           struct snd_info_buffer *buffer);
+
+In the read callback, use :c:func:`snd_iprintf()` for output
+strings, which works just like normal :c:func:`printf()`. For
+example::
+
+  static void my_proc_read(struct snd_info_entry *entry,
+                           struct snd_info_buffer *buffer)
+  {
+          struct my_chip *chip = entry->private_data;
+
+          snd_iprintf(buffer, "This is my chip!\n");
+          snd_iprintf(buffer, "Port = %ld\n", chip->port);
+  }
+
+The file permissions can be changed afterwards. By default, they are
+read only for all users. If you want to add write permission for the
+user (root by default), do as follows::
+
+ entry->mode = S_IFREG | S_IRUGO | S_IWUSR;
+
+and set the write buffer size and the callback::
+
+  entry->c.text.write = my_proc_write;
+
+In the write callback, you can use :c:func:`snd_info_get_line()`
+to get a text line, and :c:func:`snd_info_get_str()` to retrieve
+a string from the line. Some examples are found in
+``core/oss/mixer_oss.c``, core/oss/and ``pcm_oss.c``.
+
+For a raw-data proc-file, set the attributes as follows::
+
+  static const struct snd_info_entry_ops my_file_io_ops = {
+          .read = my_file_io_read,
+  };
+
+  entry->content = SNDRV_INFO_CONTENT_DATA;
+  entry->private_data = chip;
+  entry->c.ops = &my_file_io_ops;
+  entry->size = 4096;
+  entry->mode = S_IFREG | S_IRUGO;
+
+For raw data, ``size`` field must be set properly. This specifies
+the maximum size of the proc file access.
+
+The read/write callbacks of raw mode are more direct than the text mode.
+You need to use a low-level I/O functions such as
+:c:func:`copy_from_user()` and :c:func:`copy_to_user()` to transfer the
+data::
+
+  static ssize_t my_file_io_read(struct snd_info_entry *entry,
+                              void *file_private_data,
+                              struct file *file,
+                              char *buf,
+                              size_t count,
+                              loff_t pos)
+  {
+          if (copy_to_user(buf, local_data + pos, count))
+                  return -EFAULT;
+          return count;
+  }
+
+If the size of the info entry has been set up properly, ``count`` and
+``pos`` are guaranteed to fit within 0 and the given size. You don't
+have to check the range in the callbacks unless any other condition is
+required.
+
+Power Management
+================
+
+If the chip is supposed to work with suspend/resume functions, you need
+to add power-management code to the driver. The additional code for
+power-management should be ifdef-ed with ``CONFIG_PM``, or annotated
+with __maybe_unused attribute; otherwise the compiler will complain.
+
+If the driver *fully* supports suspend/resume that is, the device can be
+properly resumed to its state when suspend was called, you can set the
+``SNDRV_PCM_INFO_RESUME`` flag in the PCM info field. Usually, this is
+possible when the registers of the chip can be safely saved and restored
+to RAM. If this is set, the trigger callback is called with
+``SNDRV_PCM_TRIGGER_RESUME`` after the resume callback completes.
+
+Even if the driver doesn't support PM fully but partial suspend/resume
+is still possible, it's still worthy to implement suspend/resume
+callbacks. In such a case, applications would reset the status by
+calling :c:func:`snd_pcm_prepare()` and restart the stream
+appropriately. Hence, you can define suspend/resume callbacks below but
+don't set the ``SNDRV_PCM_INFO_RESUME`` info flag to the PCM.
+
+Note that the trigger with SUSPEND can always be called when
+:c:func:`snd_pcm_suspend_all()` is called, regardless of the
+``SNDRV_PCM_INFO_RESUME`` flag. The ``RESUME`` flag affects only the
+behavior of :c:func:`snd_pcm_resume()`. (Thus, in theory,
+``SNDRV_PCM_TRIGGER_RESUME`` isn't needed to be handled in the trigger
+callback when no ``SNDRV_PCM_INFO_RESUME`` flag is set. But, it's better
+to keep it for compatibility reasons.)
+
+The driver needs to define the
+suspend/resume hooks according to the bus the device is connected to. In
+the case of PCI drivers, the callbacks look like below::
+
+  static int __maybe_unused snd_my_suspend(struct device *dev)
+  {
+          .... /* do things for suspend */
+          return 0;
+  }
+  static int __maybe_unused snd_my_resume(struct device *dev)
+  {
+          .... /* do things for suspend */
+          return 0;
+  }
+
+The scheme of the real suspend job is as follows:
+
+1. Retrieve the card and the chip data.
+
+2. Call :c:func:`snd_power_change_state()` with
+   ``SNDRV_CTL_POWER_D3hot`` to change the power status.
+
+3. If AC97 codecs are used, call :c:func:`snd_ac97_suspend()` for
+   each codec.
+
+4. Save the register values if necessary.
+
+5. Stop the hardware if necessary.
+
+Typical code would look like::
+
+  static int __maybe_unused mychip_suspend(struct device *dev)
+  {
+          /* (1) */
+          struct snd_card *card = dev_get_drvdata(dev);
+          struct mychip *chip = card->private_data;
+          /* (2) */
+          snd_power_change_state(card, SNDRV_CTL_POWER_D3hot);
+          /* (3) */
+          snd_ac97_suspend(chip->ac97);
+          /* (4) */
+          snd_mychip_save_registers(chip);
+          /* (5) */
+          snd_mychip_stop_hardware(chip);
+          return 0;
+  }
+
+
+The scheme of the real resume job is as follows:
+
+1. Retrieve the card and the chip data.
+
+2. Re-initialize the chip.
+
+3. Restore the saved registers if necessary.
+
+4. Resume the mixer, e.g. by calling :c:func:`snd_ac97_resume()`.
+
+5. Restart the hardware (if any).
+
+6. Call :c:func:`snd_power_change_state()` with
+   ``SNDRV_CTL_POWER_D0`` to notify the processes.
+
+Typical code would look like::
+
+  static int __maybe_unused mychip_resume(struct pci_dev *pci)
+  {
+          /* (1) */
+          struct snd_card *card = dev_get_drvdata(dev);
+          struct mychip *chip = card->private_data;
+          /* (2) */
+          snd_mychip_reinit_chip(chip);
+          /* (3) */
+          snd_mychip_restore_registers(chip);
+          /* (4) */
+          snd_ac97_resume(chip->ac97);
+          /* (5) */
+          snd_mychip_restart_chip(chip);
+          /* (6) */
+          snd_power_change_state(card, SNDRV_CTL_POWER_D0);
+          return 0;
+  }
+
+Note that, at the time this callback gets called, the PCM stream has
+been already suspended via its own PM ops calling
+:c:func:`snd_pcm_suspend_all()` internally.
+
+OK, we have all callbacks now. Let's set them up. In the initialization
+of the card, make sure that you can get the chip data from the card
+instance, typically via ``private_data`` field, in case you created the
+chip data individually::
+
+  static int snd_mychip_probe(struct pci_dev *pci,
+                              const struct pci_device_id *pci_id)
+  {
+          ....
+          struct snd_card *card;
+          struct mychip *chip;
+          int err;
+          ....
+          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                             0, &card);
+          ....
+          chip = kzalloc(sizeof(*chip), GFP_KERNEL);
+          ....
+          card->private_data = chip;
+          ....
+  }
+
+When you created the chip data with :c:func:`snd_card_new()`, it's
+anyway accessible via ``private_data`` field::
+
+  static int snd_mychip_probe(struct pci_dev *pci,
+                              const struct pci_device_id *pci_id)
+  {
+          ....
+          struct snd_card *card;
+          struct mychip *chip;
+          int err;
+          ....
+          err = snd_card_new(&pci->dev, index[dev], id[dev], THIS_MODULE,
+                             sizeof(struct mychip), &card);
+          ....
+          chip = card->private_data;
+          ....
+  }
+
+If you need space to save the registers, allocate the buffer for it
+here, too, since it would be fatal if you cannot allocate a memory in
+the suspend phase. The allocated buffer should be released in the
+corresponding destructor.
+
+And next, set suspend/resume callbacks to the pci_driver::
+
+  static SIMPLE_DEV_PM_OPS(snd_my_pm_ops, mychip_suspend, mychip_resume);
+
+  static struct pci_driver driver = {
+          .name = KBUILD_MODNAME,
+          .id_table = snd_my_ids,
+          .probe = snd_my_probe,
+          .remove = snd_my_remove,
+          .driver.pm = &snd_my_pm_ops,
+  };
+
+Module Parameters
+=================
+
+There are standard module options for ALSA. At least, each module should
+have the ``index``, ``id`` and ``enable`` options.
+
+If the module supports multiple cards (usually up to 8 = ``SNDRV_CARDS``
+cards), they should be arrays. The default initial values are defined
+already as constants for easier programming::
+
+  static int index[SNDRV_CARDS] = SNDRV_DEFAULT_IDX;
+  static char *id[SNDRV_CARDS] = SNDRV_DEFAULT_STR;
+  static int enable[SNDRV_CARDS] = SNDRV_DEFAULT_ENABLE_PNP;
+
+If the module supports only a single card, they could be single
+variables, instead. ``enable`` option is not always necessary in this
+case, but it would be better to have a dummy option for compatibility.
+
+The module parameters must be declared with the standard
+``module_param()``, ``module_param_array()`` and
+:c:func:`MODULE_PARM_DESC()` macros.
+
+Typical code would look as below::
+
+  #define CARD_NAME "My Chip"
+
+  module_param_array(index, int, NULL, 0444);
+  MODULE_PARM_DESC(index, "Index value for " CARD_NAME " soundcard.");
+  module_param_array(id, charp, NULL, 0444);
+  MODULE_PARM_DESC(id, "ID string for " CARD_NAME " soundcard.");
+  module_param_array(enable, bool, NULL, 0444);
+  MODULE_PARM_DESC(enable, "Enable " CARD_NAME " soundcard.");
+
+Also, don't forget to define the module description and the license.
+Especially, the recent modprobe requires to define the
+module license as GPL, etc., otherwise the system is shown as “tainted”::
+
+  MODULE_DESCRIPTION("Sound driver for My Chip");
+  MODULE_LICENSE("GPL");
+
+
+Device-Managed Resources
+========================
+
+In the examples above, all resources are allocated and released
+manually.  But human beings are lazy in nature, especially developers
+are lazier.  So there are some ways to automate the release part; it's
+the (device-)managed resources aka devres or devm family.  For
+example, an object allocated via :c:func:`devm_kmalloc()` will be
+freed automatically at unbinding the device.
+
+ALSA core provides also the device-managed helper, namely,
+:c:func:`snd_devm_card_new()` for creating a card object.
+Call this functions instead of the normal :c:func:`snd_card_new()`,
+and you can forget the explicit :c:func:`snd_card_free()` call, as
+it's called automagically at error and removal paths.
+
+One caveat is that the call of :c:func:`snd_card_free()` would be put
+at the beginning of the call chain only after you call
+:c:func:`snd_card_register()`.
+
+Also, the ``private_free`` callback is always called at the card free,
+so be careful to put the hardware clean-up procedure in
+``private_free`` callback.  It might be called even before you
+actually set up at an earlier error path.  For avoiding such an
+invalid initialization, you can set ``private_free`` callback after
+:c:func:`snd_card_register()` call succeeds.
+
+Another thing to be remarked is that you should use device-managed
+helpers for each component as much as possible once when you manage
+the card in that way.  Mixing up with the normal and the managed
+resources may screw up the release order.
+
+
+How To Put Your Driver Into ALSA Tree
+=====================================
+
+General
+-------
+
+So far, you've learned how to write the driver codes. And you might have
+a question now: how to put my own driver into the ALSA driver tree? Here
+(finally :) the standard procedure is described briefly.
+
+Suppose that you create a new PCI driver for the card “xyz”. The card
+module name would be snd-xyz. The new driver is usually put into the
+alsa-driver tree, ``sound/pci`` directory in the case of PCI
+cards.
+
+In the following sections, the driver code is supposed to be put into
+Linux kernel tree. The two cases are covered: a driver consisting of a
+single source file and one consisting of several source files.
+
+Driver with A Single Source File
+--------------------------------
+
+1. Modify sound/pci/Makefile
+
+   Suppose you have a file xyz.c. Add the following two lines::
+
+     snd-xyz-objs := xyz.o
+     obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+2. Create the Kconfig entry
+
+   Add the new entry of Kconfig for your xyz driver::
+
+     config SND_XYZ
+       tristate "Foobar XYZ"
+       depends on SND
+       select SND_PCM
+       help
+         Say Y here to include support for Foobar XYZ soundcard.
+         To compile this driver as a module, choose M here:
+         the module will be called snd-xyz.
+
+The line ``select SND_PCM`` specifies that the driver xyz supports PCM.
+In addition to SND_PCM, the following components are supported for
+select command: SND_RAWMIDI, SND_TIMER, SND_HWDEP, SND_MPU401_UART,
+SND_OPL3_LIB, SND_OPL4_LIB, SND_VX_LIB, SND_AC97_CODEC.
+Add the select command for each supported component.
+
+Note that some selections imply the lowlevel selections. For example,
+PCM includes TIMER, MPU401_UART includes RAWMIDI, AC97_CODEC
+includes PCM, and OPL3_LIB includes HWDEP. You don't need to give
+the lowlevel selections again.
+
+For the details of Kconfig script, refer to the kbuild documentation.
+
+Drivers with Several Source Files
+---------------------------------
+
+Suppose that the driver snd-xyz have several source files. They are
+located in the new subdirectory, sound/pci/xyz.
+
+1. Add a new directory (``sound/pci/xyz``) in ``sound/pci/Makefile``
+   as below::
+
+     obj-$(CONFIG_SND) += sound/pci/xyz/
+
+
+2. Under the directory ``sound/pci/xyz``, create a Makefile::
+
+         snd-xyz-objs := xyz.o abc.o def.o
+         obj-$(CONFIG_SND_XYZ) += snd-xyz.o
+
+3. Create the Kconfig entry
+
+   This procedure is as same as in the last section.
+
+
+Useful Functions
+================
+
+:c:func:`snd_printk()` and friends
+----------------------------------
+
+.. note:: This subsection describes a few helper functions for
+   decorating a bit more on the standard :c:func:`printk()` & co.
+   However, in general, the use of such helpers is no longer recommended.
+   If possible, try to stick with the standard functions like
+   :c:func:`dev_err()` or :c:func:`pr_err()`.
+
+ALSA provides a verbose version of the :c:func:`printk()` function.
+If a kernel config ``CONFIG_SND_VERBOSE_PRINTK`` is set, this function
+prints the given message together with the file name and the line of the
+caller. The ``KERN_XXX`` prefix is processed as well as the original
+:c:func:`printk()` does, so it's recommended to add this prefix,
+e.g. snd_printk(KERN_ERR "Oh my, sorry, it's extremely bad!\\n");
+
+There are also :c:func:`printk()`'s for debugging.
+:c:func:`snd_printd()` can be used for general debugging purposes.
+If ``CONFIG_SND_DEBUG`` is set, this function is compiled, and works
+just like :c:func:`snd_printk()`. If the ALSA is compiled without
+the debugging flag, it's ignored.
+
+:c:func:`snd_printdd()` is compiled in only when
+``CONFIG_SND_DEBUG_VERBOSE`` is set.
+
+:c:func:`snd_BUG()`
+-------------------
+
+It shows the ``BUG?`` message and stack trace as well as
+:c:func:`snd_BUG_ON()` at the point. It's useful to show that a
+fatal error happens there.
+
+When no debug flag is set, this macro is ignored.
+
+:c:func:`snd_BUG_ON()`
+----------------------
+
+:c:func:`snd_BUG_ON()` macro is similar with
+:c:func:`WARN_ON()` macro. For example, snd_BUG_ON(!pointer); or
+it can be used as the condition, if (snd_BUG_ON(non_zero_is_bug))
+return -EINVAL;
+
+The macro takes an conditional expression to evaluate. When
+``CONFIG_SND_DEBUG``, is set, if the expression is non-zero, it shows
+the warning message such as ``BUG? (xxx)`` normally followed by stack
+trace. In both cases it returns the evaluated value.
+
+Acknowledgments
+===============
+
+I would like to thank Phil Kerr for his help for improvement and
+corrections of this document.
+
+Kevin Conder reformatted the original plain-text to the DocBook format.
+
+Giuliano Pochini corrected typos and contributed the example codes in
+the hardware constraints section.