summaryrefslogtreecommitdiffstats
path: root/Documentation/driver-api/usb/writing_musb_glue_layer.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/driver-api/usb/writing_musb_glue_layer.rst')
-rw-r--r--Documentation/driver-api/usb/writing_musb_glue_layer.rst720
1 files changed, 720 insertions, 0 deletions
diff --git a/Documentation/driver-api/usb/writing_musb_glue_layer.rst b/Documentation/driver-api/usb/writing_musb_glue_layer.rst
new file mode 100644
index 000000000..10416cc11
--- /dev/null
+++ b/Documentation/driver-api/usb/writing_musb_glue_layer.rst
@@ -0,0 +1,720 @@
+=========================
+Writing a MUSB Glue Layer
+=========================
+
+:Author: Apelete Seketeli
+
+Introduction
+============
+
+The Linux MUSB subsystem is part of the larger Linux USB subsystem. It
+provides support for embedded USB Device Controllers (UDC) that do not
+use Universal Host Controller Interface (UHCI) or Open Host Controller
+Interface (OHCI).
+
+Instead, these embedded UDC rely on the USB On-the-Go (OTG)
+specification which they implement at least partially. The silicon
+reference design used in most cases is the Multipoint USB Highspeed
+Dual-Role Controller (MUSB HDRC) found in the Mentor Graphics Inventra™
+design.
+
+As a self-taught exercise I have written an MUSB glue layer for the
+Ingenic JZ4740 SoC, modelled after the many MUSB glue layers in the
+kernel source tree. This layer can be found at
+``drivers/usb/musb/jz4740.c``. In this documentation I will walk through the
+basics of the ``jz4740.c`` glue layer, explaining the different pieces and
+what needs to be done in order to write your own device glue layer.
+
+.. _musb-basics:
+
+Linux MUSB Basics
+=================
+
+To get started on the topic, please read USB On-the-Go Basics (see
+Resources) which provides an introduction of USB OTG operation at the
+hardware level. A couple of wiki pages by Texas Instruments and Analog
+Devices also provide an overview of the Linux kernel MUSB configuration,
+albeit focused on some specific devices provided by these companies.
+Finally, getting acquainted with the USB specification at USB home page
+may come in handy, with practical instance provided through the Writing
+USB Device Drivers documentation (again, see Resources).
+
+Linux USB stack is a layered architecture in which the MUSB controller
+hardware sits at the lowest. The MUSB controller driver abstract the
+MUSB controller hardware to the Linux USB stack::
+
+ ------------------------
+ | | <------- drivers/usb/gadget
+ | Linux USB Core Stack | <------- drivers/usb/host
+ | | <------- drivers/usb/core
+ ------------------------
+ ⬍
+ --------------------------
+ | | <------ drivers/usb/musb/musb_gadget.c
+ | MUSB Controller driver | <------ drivers/usb/musb/musb_host.c
+ | | <------ drivers/usb/musb/musb_core.c
+ --------------------------
+ ⬍
+ ---------------------------------
+ | MUSB Platform Specific Driver |
+ | | <-- drivers/usb/musb/jz4740.c
+ | aka "Glue Layer" |
+ ---------------------------------
+ ⬍
+ ---------------------------------
+ | MUSB Controller Hardware |
+ ---------------------------------
+
+As outlined above, the glue layer is actually the platform specific code
+sitting in between the controller driver and the controller hardware.
+
+Just like a Linux USB driver needs to register itself with the Linux USB
+subsystem, the MUSB glue layer needs first to register itself with the
+MUSB controller driver. This will allow the controller driver to know
+about which device the glue layer supports and which functions to call
+when a supported device is detected or released; remember we are talking
+about an embedded controller chip here, so no insertion or removal at
+run-time.
+
+All of this information is passed to the MUSB controller driver through
+a :c:type:`platform_driver` structure defined in the glue layer as::
+
+ static struct platform_driver jz4740_driver = {
+ .probe = jz4740_probe,
+ .remove = jz4740_remove,
+ .driver = {
+ .name = "musb-jz4740",
+ },
+ };
+
+The probe and remove function pointers are called when a matching device
+is detected and, respectively, released. The name string describes the
+device supported by this glue layer. In the current case it matches a
+platform_device structure declared in ``arch/mips/jz4740/platform.c``. Note
+that we are not using device tree bindings here.
+
+In order to register itself to the controller driver, the glue layer
+goes through a few steps, basically allocating the controller hardware
+resources and initialising a couple of circuits. To do so, it needs to
+keep track of the information used throughout these steps. This is done
+by defining a private ``jz4740_glue`` structure::
+
+ struct jz4740_glue {
+ struct device *dev;
+ struct platform_device *musb;
+ struct clk *clk;
+ };
+
+
+The dev and musb members are both device structure variables. The first
+one holds generic information about the device, since it's the basic
+device structure, and the latter holds information more closely related
+to the subsystem the device is registered to. The clk variable keeps
+information related to the device clock operation.
+
+Let's go through the steps of the probe function that leads the glue
+layer to register itself to the controller driver.
+
+.. note::
+
+ For the sake of readability each function will be split in logical
+ parts, each part being shown as if it was independent from the others.
+
+.. code-block:: c
+ :emphasize-lines: 8,12,18
+
+ static int jz4740_probe(struct platform_device *pdev)
+ {
+ struct platform_device *musb;
+ struct jz4740_glue *glue;
+ struct clk *clk;
+ int ret;
+
+ glue = devm_kzalloc(&pdev->dev, sizeof(*glue), GFP_KERNEL);
+ if (!glue)
+ return -ENOMEM;
+
+ musb = platform_device_alloc("musb-hdrc", PLATFORM_DEVID_AUTO);
+ if (!musb) {
+ dev_err(&pdev->dev, "failed to allocate musb device\n");
+ return -ENOMEM;
+ }
+
+ clk = devm_clk_get(&pdev->dev, "udc");
+ if (IS_ERR(clk)) {
+ dev_err(&pdev->dev, "failed to get clock\n");
+ ret = PTR_ERR(clk);
+ goto err_platform_device_put;
+ }
+
+ ret = clk_prepare_enable(clk);
+ if (ret) {
+ dev_err(&pdev->dev, "failed to enable clock\n");
+ goto err_platform_device_put;
+ }
+
+ musb->dev.parent = &pdev->dev;
+
+ glue->dev = &pdev->dev;
+ glue->musb = musb;
+ glue->clk = clk;
+
+ return 0;
+
+ err_platform_device_put:
+ platform_device_put(musb);
+ return ret;
+ }
+
+The first few lines of the probe function allocate and assign the glue,
+musb and clk variables. The ``GFP_KERNEL`` flag (line 8) allows the
+allocation process to sleep and wait for memory, thus being usable in a
+locking situation. The ``PLATFORM_DEVID_AUTO`` flag (line 12) allows
+automatic allocation and management of device IDs in order to avoid
+device namespace collisions with explicit IDs. With :c:func:`devm_clk_get`
+(line 18) the glue layer allocates the clock -- the ``devm_`` prefix
+indicates that :c:func:`clk_get` is managed: it automatically frees the
+allocated clock resource data when the device is released -- and enable
+it.
+
+
+
+Then comes the registration steps:
+
+.. code-block:: c
+ :emphasize-lines: 3,5,7,9,16
+
+ static int jz4740_probe(struct platform_device *pdev)
+ {
+ struct musb_hdrc_platform_data *pdata = &jz4740_musb_platform_data;
+
+ pdata->platform_ops = &jz4740_musb_ops;
+
+ platform_set_drvdata(pdev, glue);
+
+ ret = platform_device_add_resources(musb, pdev->resource,
+ pdev->num_resources);
+ if (ret) {
+ dev_err(&pdev->dev, "failed to add resources\n");
+ goto err_clk_disable;
+ }
+
+ ret = platform_device_add_data(musb, pdata, sizeof(*pdata));
+ if (ret) {
+ dev_err(&pdev->dev, "failed to add platform_data\n");
+ goto err_clk_disable;
+ }
+
+ return 0;
+
+ err_clk_disable:
+ clk_disable_unprepare(clk);
+ err_platform_device_put:
+ platform_device_put(musb);
+ return ret;
+ }
+
+The first step is to pass the device data privately held by the glue
+layer on to the controller driver through :c:func:`platform_set_drvdata`
+(line 7). Next is passing on the device resources information, also privately
+held at that point, through :c:func:`platform_device_add_resources` (line 9).
+
+Finally comes passing on the platform specific data to the controller
+driver (line 16). Platform data will be discussed in
+:ref:`musb-dev-platform-data`, but here we are looking at the
+``platform_ops`` function pointer (line 5) in ``musb_hdrc_platform_data``
+structure (line 3). This function pointer allows the MUSB controller
+driver to know which function to call for device operation::
+
+ static const struct musb_platform_ops jz4740_musb_ops = {
+ .init = jz4740_musb_init,
+ .exit = jz4740_musb_exit,
+ };
+
+Here we have the minimal case where only init and exit functions are
+called by the controller driver when needed. Fact is the JZ4740 MUSB
+controller is a basic controller, lacking some features found in other
+controllers, otherwise we may also have pointers to a few other
+functions like a power management function or a function to switch
+between OTG and non-OTG modes, for instance.
+
+At that point of the registration process, the controller driver
+actually calls the init function:
+
+ .. code-block:: c
+ :emphasize-lines: 12,14
+
+ static int jz4740_musb_init(struct musb *musb)
+ {
+ musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
+ if (!musb->xceiv) {
+ pr_err("HS UDC: no transceiver configured\n");
+ return -ENODEV;
+ }
+
+ /* Silicon does not implement ConfigData register.
+ * Set dyn_fifo to avoid reading EP config from hardware.
+ */
+ musb->dyn_fifo = true;
+
+ musb->isr = jz4740_musb_interrupt;
+
+ return 0;
+ }
+
+The goal of ``jz4740_musb_init()`` is to get hold of the transceiver
+driver data of the MUSB controller hardware and pass it on to the MUSB
+controller driver, as usual. The transceiver is the circuitry inside the
+controller hardware responsible for sending/receiving the USB data.
+Since it is an implementation of the physical layer of the OSI model,
+the transceiver is also referred to as PHY.
+
+Getting hold of the ``MUSB PHY`` driver data is done with ``usb_get_phy()``
+which returns a pointer to the structure containing the driver instance
+data. The next couple of instructions (line 12 and 14) are used as a
+quirk and to setup IRQ handling respectively. Quirks and IRQ handling
+will be discussed later in :ref:`musb-dev-quirks` and
+:ref:`musb-handling-irqs`\ ::
+
+ static int jz4740_musb_exit(struct musb *musb)
+ {
+ usb_put_phy(musb->xceiv);
+
+ return 0;
+ }
+
+Acting as the counterpart of init, the exit function releases the MUSB
+PHY driver when the controller hardware itself is about to be released.
+
+Again, note that init and exit are fairly simple in this case due to the
+basic set of features of the JZ4740 controller hardware. When writing an
+musb glue layer for a more complex controller hardware, you might need
+to take care of more processing in those two functions.
+
+Returning from the init function, the MUSB controller driver jumps back
+into the probe function::
+
+ static int jz4740_probe(struct platform_device *pdev)
+ {
+ ret = platform_device_add(musb);
+ if (ret) {
+ dev_err(&pdev->dev, "failed to register musb device\n");
+ goto err_clk_disable;
+ }
+
+ return 0;
+
+ err_clk_disable:
+ clk_disable_unprepare(clk);
+ err_platform_device_put:
+ platform_device_put(musb);
+ return ret;
+ }
+
+This is the last part of the device registration process where the glue
+layer adds the controller hardware device to Linux kernel device
+hierarchy: at this stage, all known information about the device is
+passed on to the Linux USB core stack:
+
+ .. code-block:: c
+ :emphasize-lines: 5,6
+
+ static int jz4740_remove(struct platform_device *pdev)
+ {
+ struct jz4740_glue *glue = platform_get_drvdata(pdev);
+
+ platform_device_unregister(glue->musb);
+ clk_disable_unprepare(glue->clk);
+
+ return 0;
+ }
+
+Acting as the counterpart of probe, the remove function unregister the
+MUSB controller hardware (line 5) and disable the clock (line 6),
+allowing it to be gated.
+
+.. _musb-handling-irqs:
+
+Handling IRQs
+=============
+
+Additionally to the MUSB controller hardware basic setup and
+registration, the glue layer is also responsible for handling the IRQs:
+
+ .. code-block:: c
+ :emphasize-lines: 7,9-11,14,24
+
+ static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
+ {
+ unsigned long flags;
+ irqreturn_t retval = IRQ_NONE;
+ struct musb *musb = __hci;
+
+ spin_lock_irqsave(&musb->lock, flags);
+
+ musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
+ musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
+ musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
+
+ /*
+ * The controller is gadget only, the state of the host mode IRQ bits is
+ * undefined. Mask them to make sure that the musb driver core will
+ * never see them set
+ */
+ musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
+ MUSB_INTR_RESET | MUSB_INTR_SOF;
+
+ if (musb->int_usb || musb->int_tx || musb->int_rx)
+ retval = musb_interrupt(musb);
+
+ spin_unlock_irqrestore(&musb->lock, flags);
+
+ return retval;
+ }
+
+Here the glue layer mostly has to read the relevant hardware registers
+and pass their values on to the controller driver which will handle the
+actual event that triggered the IRQ.
+
+The interrupt handler critical section is protected by the
+:c:func:`spin_lock_irqsave` and counterpart :c:func:`spin_unlock_irqrestore`
+functions (line 7 and 24 respectively), which prevent the interrupt
+handler code to be run by two different threads at the same time.
+
+Then the relevant interrupt registers are read (line 9 to 11):
+
+- ``MUSB_INTRUSB``: indicates which USB interrupts are currently active,
+
+- ``MUSB_INTRTX``: indicates which of the interrupts for TX endpoints are
+ currently active,
+
+- ``MUSB_INTRRX``: indicates which of the interrupts for TX endpoints are
+ currently active.
+
+Note that :c:func:`musb_readb` is used to read 8-bit registers at most, while
+:c:func:`musb_readw` allows us to read at most 16-bit registers. There are
+other functions that can be used depending on the size of your device
+registers. See ``musb_io.h`` for more information.
+
+Instruction on line 18 is another quirk specific to the JZ4740 USB
+device controller, which will be discussed later in :ref:`musb-dev-quirks`.
+
+The glue layer still needs to register the IRQ handler though. Remember
+the instruction on line 14 of the init function::
+
+ static int jz4740_musb_init(struct musb *musb)
+ {
+ musb->isr = jz4740_musb_interrupt;
+
+ return 0;
+ }
+
+This instruction sets a pointer to the glue layer IRQ handler function,
+in order for the controller hardware to call the handler back when an
+IRQ comes from the controller hardware. The interrupt handler is now
+implemented and registered.
+
+.. _musb-dev-platform-data:
+
+Device Platform Data
+====================
+
+In order to write an MUSB glue layer, you need to have some data
+describing the hardware capabilities of your controller hardware, which
+is called the platform data.
+
+Platform data is specific to your hardware, though it may cover a broad
+range of devices, and is generally found somewhere in the ``arch/``
+directory, depending on your device architecture.
+
+For instance, platform data for the JZ4740 SoC is found in
+``arch/mips/jz4740/platform.c``. In the ``platform.c`` file each device of the
+JZ4740 SoC is described through a set of structures.
+
+Here is the part of ``arch/mips/jz4740/platform.c`` that covers the USB
+Device Controller (UDC):
+
+ .. code-block:: c
+ :emphasize-lines: 2,7,14-17,21,22,25,26,28,29
+
+ /* USB Device Controller */
+ struct platform_device jz4740_udc_xceiv_device = {
+ .name = "usb_phy_gen_xceiv",
+ .id = 0,
+ };
+
+ static struct resource jz4740_udc_resources[] = {
+ [0] = {
+ .start = JZ4740_UDC_BASE_ADDR,
+ .end = JZ4740_UDC_BASE_ADDR + 0x10000 - 1,
+ .flags = IORESOURCE_MEM,
+ },
+ [1] = {
+ .start = JZ4740_IRQ_UDC,
+ .end = JZ4740_IRQ_UDC,
+ .flags = IORESOURCE_IRQ,
+ .name = "mc",
+ },
+ };
+
+ struct platform_device jz4740_udc_device = {
+ .name = "musb-jz4740",
+ .id = -1,
+ .dev = {
+ .dma_mask = &jz4740_udc_device.dev.coherent_dma_mask,
+ .coherent_dma_mask = DMA_BIT_MASK(32),
+ },
+ .num_resources = ARRAY_SIZE(jz4740_udc_resources),
+ .resource = jz4740_udc_resources,
+ };
+
+The ``jz4740_udc_xceiv_device`` platform device structure (line 2)
+describes the UDC transceiver with a name and id number.
+
+At the time of this writing, note that ``usb_phy_gen_xceiv`` is the
+specific name to be used for all transceivers that are either built-in
+with reference USB IP or autonomous and doesn't require any PHY
+programming. You will need to set ``CONFIG_NOP_USB_XCEIV=y`` in the
+kernel configuration to make use of the corresponding transceiver
+driver. The id field could be set to -1 (equivalent to
+``PLATFORM_DEVID_NONE``), -2 (equivalent to ``PLATFORM_DEVID_AUTO``) or
+start with 0 for the first device of this kind if we want a specific id
+number.
+
+The ``jz4740_udc_resources`` resource structure (line 7) defines the UDC
+registers base addresses.
+
+The first array (line 9 to 11) defines the UDC registers base memory
+addresses: start points to the first register memory address, end points
+to the last register memory address and the flags member defines the
+type of resource we are dealing with. So ``IORESOURCE_MEM`` is used to
+define the registers memory addresses. The second array (line 14 to 17)
+defines the UDC IRQ registers addresses. Since there is only one IRQ
+register available for the JZ4740 UDC, start and end point at the same
+address. The ``IORESOURCE_IRQ`` flag tells that we are dealing with IRQ
+resources, and the name ``mc`` is in fact hard-coded in the MUSB core in
+order for the controller driver to retrieve this IRQ resource by
+querying it by its name.
+
+Finally, the ``jz4740_udc_device`` platform device structure (line 21)
+describes the UDC itself.
+
+The ``musb-jz4740`` name (line 22) defines the MUSB driver that is used
+for this device; remember this is in fact the name that we used in the
+``jz4740_driver`` platform driver structure in :ref:`musb-basics`.
+The id field (line 23) is set to -1 (equivalent to ``PLATFORM_DEVID_NONE``)
+since we do not need an id for the device: the MUSB controller driver was
+already set to allocate an automatic id in :ref:`musb-basics`. In the dev field
+we care for DMA related information here. The ``dma_mask`` field (line 25)
+defines the width of the DMA mask that is going to be used, and
+``coherent_dma_mask`` (line 26) has the same purpose but for the
+``alloc_coherent`` DMA mappings: in both cases we are using a 32 bits mask.
+Then the resource field (line 29) is simply a pointer to the resource
+structure defined before, while the ``num_resources`` field (line 28) keeps
+track of the number of arrays defined in the resource structure (in this
+case there were two resource arrays defined before).
+
+With this quick overview of the UDC platform data at the ``arch/`` level now
+done, let's get back to the MUSB glue layer specific platform data in
+``drivers/usb/musb/jz4740.c``:
+
+ .. code-block:: c
+ :emphasize-lines: 3,5,7-9,11
+
+ static struct musb_hdrc_config jz4740_musb_config = {
+ /* Silicon does not implement USB OTG. */
+ .multipoint = 0,
+ /* Max EPs scanned, driver will decide which EP can be used. */
+ .num_eps = 4,
+ /* RAMbits needed to configure EPs from table */
+ .ram_bits = 9,
+ .fifo_cfg = jz4740_musb_fifo_cfg,
+ .fifo_cfg_size = ARRAY_SIZE(jz4740_musb_fifo_cfg),
+ };
+
+ static struct musb_hdrc_platform_data jz4740_musb_platform_data = {
+ .mode = MUSB_PERIPHERAL,
+ .config = &jz4740_musb_config,
+ };
+
+First the glue layer configures some aspects of the controller driver
+operation related to the controller hardware specifics. This is done
+through the ``jz4740_musb_config`` :c:type:`musb_hdrc_config` structure.
+
+Defining the OTG capability of the controller hardware, the multipoint
+member (line 3) is set to 0 (equivalent to false) since the JZ4740 UDC
+is not OTG compatible. Then ``num_eps`` (line 5) defines the number of USB
+endpoints of the controller hardware, including endpoint 0: here we have
+3 endpoints + endpoint 0. Next is ``ram_bits`` (line 7) which is the width
+of the RAM address bus for the MUSB controller hardware. This
+information is needed when the controller driver cannot automatically
+configure endpoints by reading the relevant controller hardware
+registers. This issue will be discussed when we get to device quirks in
+:ref:`musb-dev-quirks`. Last two fields (line 8 and 9) are also
+about device quirks: ``fifo_cfg`` points to the USB endpoints configuration
+table and ``fifo_cfg_size`` keeps track of the size of the number of
+entries in that configuration table. More on that later in
+:ref:`musb-dev-quirks`.
+
+Then this configuration is embedded inside ``jz4740_musb_platform_data``
+:c:type:`musb_hdrc_platform_data` structure (line 11): config is a pointer to
+the configuration structure itself, and mode tells the controller driver
+if the controller hardware may be used as ``MUSB_HOST`` only,
+``MUSB_PERIPHERAL`` only or ``MUSB_OTG`` which is a dual mode.
+
+Remember that ``jz4740_musb_platform_data`` is then used to convey
+platform data information as we have seen in the probe function in
+:ref:`musb-basics`.
+
+.. _musb-dev-quirks:
+
+Device Quirks
+=============
+
+Completing the platform data specific to your device, you may also need
+to write some code in the glue layer to work around some device specific
+limitations. These quirks may be due to some hardware bugs, or simply be
+the result of an incomplete implementation of the USB On-the-Go
+specification.
+
+The JZ4740 UDC exhibits such quirks, some of which we will discuss here
+for the sake of insight even though these might not be found in the
+controller hardware you are working on.
+
+Let's get back to the init function first:
+
+ .. code-block:: c
+ :emphasize-lines: 12
+
+ static int jz4740_musb_init(struct musb *musb)
+ {
+ musb->xceiv = usb_get_phy(USB_PHY_TYPE_USB2);
+ if (!musb->xceiv) {
+ pr_err("HS UDC: no transceiver configured\n");
+ return -ENODEV;
+ }
+
+ /* Silicon does not implement ConfigData register.
+ * Set dyn_fifo to avoid reading EP config from hardware.
+ */
+ musb->dyn_fifo = true;
+
+ musb->isr = jz4740_musb_interrupt;
+
+ return 0;
+ }
+
+Instruction on line 12 helps the MUSB controller driver to work around
+the fact that the controller hardware is missing registers that are used
+for USB endpoints configuration.
+
+Without these registers, the controller driver is unable to read the
+endpoints configuration from the hardware, so we use line 12 instruction
+to bypass reading the configuration from silicon, and rely on a
+hard-coded table that describes the endpoints configuration instead::
+
+ static struct musb_fifo_cfg jz4740_musb_fifo_cfg[] = {
+ { .hw_ep_num = 1, .style = FIFO_TX, .maxpacket = 512, },
+ { .hw_ep_num = 1, .style = FIFO_RX, .maxpacket = 512, },
+ { .hw_ep_num = 2, .style = FIFO_TX, .maxpacket = 64, },
+ };
+
+Looking at the configuration table above, we see that each endpoints is
+described by three fields: ``hw_ep_num`` is the endpoint number, style is
+its direction (either ``FIFO_TX`` for the controller driver to send packets
+in the controller hardware, or ``FIFO_RX`` to receive packets from
+hardware), and maxpacket defines the maximum size of each data packet
+that can be transmitted over that endpoint. Reading from the table, the
+controller driver knows that endpoint 1 can be used to send and receive
+USB data packets of 512 bytes at once (this is in fact a bulk in/out
+endpoint), and endpoint 2 can be used to send data packets of 64 bytes
+at once (this is in fact an interrupt endpoint).
+
+Note that there is no information about endpoint 0 here: that one is
+implemented by default in every silicon design, with a predefined
+configuration according to the USB specification. For more examples of
+endpoint configuration tables, see ``musb_core.c``.
+
+Let's now get back to the interrupt handler function:
+
+ .. code-block:: c
+ :emphasize-lines: 18-19
+
+ static irqreturn_t jz4740_musb_interrupt(int irq, void *__hci)
+ {
+ unsigned long flags;
+ irqreturn_t retval = IRQ_NONE;
+ struct musb *musb = __hci;
+
+ spin_lock_irqsave(&musb->lock, flags);
+
+ musb->int_usb = musb_readb(musb->mregs, MUSB_INTRUSB);
+ musb->int_tx = musb_readw(musb->mregs, MUSB_INTRTX);
+ musb->int_rx = musb_readw(musb->mregs, MUSB_INTRRX);
+
+ /*
+ * The controller is gadget only, the state of the host mode IRQ bits is
+ * undefined. Mask them to make sure that the musb driver core will
+ * never see them set
+ */
+ musb->int_usb &= MUSB_INTR_SUSPEND | MUSB_INTR_RESUME |
+ MUSB_INTR_RESET | MUSB_INTR_SOF;
+
+ if (musb->int_usb || musb->int_tx || musb->int_rx)
+ retval = musb_interrupt(musb);
+
+ spin_unlock_irqrestore(&musb->lock, flags);
+
+ return retval;
+ }
+
+Instruction on line 18 above is a way for the controller driver to work
+around the fact that some interrupt bits used for USB host mode
+operation are missing in the ``MUSB_INTRUSB`` register, thus left in an
+undefined hardware state, since this MUSB controller hardware is used in
+peripheral mode only. As a consequence, the glue layer masks these
+missing bits out to avoid parasite interrupts by doing a logical AND
+operation between the value read from ``MUSB_INTRUSB`` and the bits that
+are actually implemented in the register.
+
+These are only a couple of the quirks found in the JZ4740 USB device
+controller. Some others were directly addressed in the MUSB core since
+the fixes were generic enough to provide a better handling of the issues
+for others controller hardware eventually.
+
+Conclusion
+==========
+
+Writing a Linux MUSB glue layer should be a more accessible task, as
+this documentation tries to show the ins and outs of this exercise.
+
+The JZ4740 USB device controller being fairly simple, I hope its glue
+layer serves as a good example for the curious mind. Used with the
+current MUSB glue layers, this documentation should provide enough
+guidance to get started; should anything gets out of hand, the linux-usb
+mailing list archive is another helpful resource to browse through.
+
+Acknowledgements
+================
+
+Many thanks to Lars-Peter Clausen and Maarten ter Huurne for answering
+my questions while I was writing the JZ4740 glue layer and for helping
+me out getting the code in good shape.
+
+I would also like to thank the Qi-Hardware community at large for its
+cheerful guidance and support.
+
+Resources
+=========
+
+USB Home Page: https://www.usb.org
+
+linux-usb Mailing List Archives: https://marc.info/?l=linux-usb
+
+USB On-the-Go Basics:
+https://www.maximintegrated.com/app-notes/index.mvp/id/1822
+
+:ref:`Writing USB Device Drivers <writing-usb-driver>`
+
+Texas Instruments USB Configuration Wiki Page:
+http://processors.wiki.ti.com/index.php/Usbgeneralpage