Adding upstream version 6.6.15.upstream/6.6.15

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

1 files changed, 448 insertions, 0 deletions

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+=======================================
+Porting Drivers to the New Driver Model
+=======================================
+
+Patrick Mochel
+
+7 January 2003
+
+
+Overview
+
+Please refer to `Documentation/driver-api/driver-model/*.rst` for definitions of
+various driver types and concepts.
+
+Most of the work of porting devices drivers to the new model happens
+at the bus driver layer. This was intentional, to minimize the
+negative effect on kernel drivers, and to allow a gradual transition
+of bus drivers.
+
+In a nutshell, the driver model consists of a set of objects that can
+be embedded in larger, bus-specific objects. Fields in these generic
+objects can replace fields in the bus-specific objects.
+
+The generic objects must be registered with the driver model core. By
+doing so, they will exported via the sysfs filesystem. sysfs can be
+mounted by doing::
+
+	# mount -t sysfs sysfs /sys
+
+
+
+The Process
+
+Step 0: Read include/linux/device.h for object and function definitions.
+
+Step 1: Registering the bus driver.
+
+
+- Define a struct bus_type for the bus driver::
+
+    struct bus_type pci_bus_type = {
+          .name           = "pci",
+    };
+
+
+- Register the bus type.
+
+  This should be done in the initialization function for the bus type,
+  which is usually the module_init(), or equivalent, function::
+
+    static int __init pci_driver_init(void)
+    {
+            return bus_register(&pci_bus_type);
+    }
+
+    subsys_initcall(pci_driver_init);
+
+
+  The bus type may be unregistered (if the bus driver may be compiled
+  as a module) by doing::
+
+     bus_unregister(&pci_bus_type);
+
+
+- Export the bus type for others to use.
+
+  Other code may wish to reference the bus type, so declare it in a
+  shared header file and export the symbol.
+
+From include/linux/pci.h::
+
+  extern struct bus_type pci_bus_type;
+
+
+From file the above code appears in::
+
+  EXPORT_SYMBOL(pci_bus_type);
+
+
+
+- This will cause the bus to show up in /sys/bus/pci/ with two
+  subdirectories: 'devices' and 'drivers'::
+
+    # tree -d /sys/bus/pci/
+    /sys/bus/pci/
+    |-- devices
+    `-- drivers
+
+
+
+Step 2: Registering Devices.
+
+struct device represents a single device. It mainly contains metadata
+describing the relationship the device has to other entities.
+
+
+- Embed a struct device in the bus-specific device type::
+
+
+    struct pci_dev {
+           ...
+           struct  device  dev;            /* Generic device interface */
+           ...
+    };
+
+  It is recommended that the generic device not be the first item in
+  the struct to discourage programmers from doing mindless casts
+  between the object types. Instead macros, or inline functions,
+  should be created to convert from the generic object type::
+
+
+    #define to_pci_dev(n) container_of(n, struct pci_dev, dev)
+
+    or
+
+    static inline struct pci_dev * to_pci_dev(struct kobject * kobj)
+    {
+	return container_of(n, struct pci_dev, dev);
+    }
+
+  This allows the compiler to verify type-safety of the operations
+  that are performed (which is Good).
+
+
+- Initialize the device on registration.
+
+  When devices are discovered or registered with the bus type, the
+  bus driver should initialize the generic device. The most important
+  things to initialize are the bus_id, parent, and bus fields.
+
+  The bus_id is an ASCII string that contains the device's address on
+  the bus. The format of this string is bus-specific. This is
+  necessary for representing devices in sysfs.
+
+  parent is the physical parent of the device. It is important that
+  the bus driver sets this field correctly.
+
+  The driver model maintains an ordered list of devices that it uses
+  for power management. This list must be in order to guarantee that
+  devices are shutdown before their physical parents, and vice versa.
+  The order of this list is determined by the parent of registered
+  devices.
+
+  Also, the location of the device's sysfs directory depends on a
+  device's parent. sysfs exports a directory structure that mirrors
+  the device hierarchy. Accurately setting the parent guarantees that
+  sysfs will accurately represent the hierarchy.
+
+  The device's bus field is a pointer to the bus type the device
+  belongs to. This should be set to the bus_type that was declared
+  and initialized before.
+
+  Optionally, the bus driver may set the device's name and release
+  fields.
+
+  The name field is an ASCII string describing the device, like
+
+     "ATI Technologies Inc Radeon QD"
+
+  The release field is a callback that the driver model core calls
+  when the device has been removed, and all references to it have
+  been released. More on this in a moment.
+
+
+- Register the device.
+
+  Once the generic device has been initialized, it can be registered
+  with the driver model core by doing::
+
+       device_register(&dev->dev);
+
+  It can later be unregistered by doing::
+
+       device_unregister(&dev->dev);
+
+  This should happen on buses that support hotpluggable devices.
+  If a bus driver unregisters a device, it should not immediately free
+  it. It should instead wait for the driver model core to call the
+  device's release method, then free the bus-specific object.
+  (There may be other code that is currently referencing the device
+  structure, and it would be rude to free the device while that is
+  happening).
+
+
+  When the device is registered, a directory in sysfs is created.
+  The PCI tree in sysfs looks like::
+
+    /sys/devices/pci0/
+    |-- 00:00.0
+    |-- 00:01.0
+    |   `-- 01:00.0
+    |-- 00:02.0
+    |   `-- 02:1f.0
+    |       `-- 03:00.0
+    |-- 00:1e.0
+    |   `-- 04:04.0
+    |-- 00:1f.0
+    |-- 00:1f.1
+    |   |-- ide0
+    |   |   |-- 0.0
+    |   |   `-- 0.1
+    |   `-- ide1
+    |       `-- 1.0
+    |-- 00:1f.2
+    |-- 00:1f.3
+    `-- 00:1f.5
+
+  Also, symlinks are created in the bus's 'devices' directory
+  that point to the device's directory in the physical hierarchy::
+
+    /sys/bus/pci/devices/
+    |-- 00:00.0 -> ../../../devices/pci0/00:00.0
+    |-- 00:01.0 -> ../../../devices/pci0/00:01.0
+    |-- 00:02.0 -> ../../../devices/pci0/00:02.0
+    |-- 00:1e.0 -> ../../../devices/pci0/00:1e.0
+    |-- 00:1f.0 -> ../../../devices/pci0/00:1f.0
+    |-- 00:1f.1 -> ../../../devices/pci0/00:1f.1
+    |-- 00:1f.2 -> ../../../devices/pci0/00:1f.2
+    |-- 00:1f.3 -> ../../../devices/pci0/00:1f.3
+    |-- 00:1f.5 -> ../../../devices/pci0/00:1f.5
+    |-- 01:00.0 -> ../../../devices/pci0/00:01.0/01:00.0
+    |-- 02:1f.0 -> ../../../devices/pci0/00:02.0/02:1f.0
+    |-- 03:00.0 -> ../../../devices/pci0/00:02.0/02:1f.0/03:00.0
+    `-- 04:04.0 -> ../../../devices/pci0/00:1e.0/04:04.0
+
+
+
+Step 3: Registering Drivers.
+
+struct device_driver is a simple driver structure that contains a set
+of operations that the driver model core may call.
+
+
+- Embed a struct device_driver in the bus-specific driver.
+
+  Just like with devices, do something like::
+
+    struct pci_driver {
+           ...
+           struct device_driver    driver;
+    };
+
+
+- Initialize the generic driver structure.
+
+  When the driver registers with the bus (e.g. doing pci_register_driver()),
+  initialize the necessary fields of the driver: the name and bus
+  fields.
+
+
+- Register the driver.
+
+  After the generic driver has been initialized, call::
+
+	driver_register(&drv->driver);
+
+  to register the driver with the core.
+
+  When the driver is unregistered from the bus, unregister it from the
+  core by doing::
+
+        driver_unregister(&drv->driver);
+
+  Note that this will block until all references to the driver have
+  gone away. Normally, there will not be any.
+
+
+- Sysfs representation.
+
+  Drivers are exported via sysfs in their bus's 'driver's directory.
+  For example::
+
+    /sys/bus/pci/drivers/
+    |-- 3c59x
+    |-- Ensoniq AudioPCI
+    |-- agpgart-amdk7
+    |-- e100
+    `-- serial
+
+
+Step 4: Define Generic Methods for Drivers.
+
+struct device_driver defines a set of operations that the driver model
+core calls. Most of these operations are probably similar to
+operations the bus already defines for drivers, but taking different
+parameters.
+
+It would be difficult and tedious to force every driver on a bus to
+simultaneously convert their drivers to generic format. Instead, the
+bus driver should define single instances of the generic methods that
+forward call to the bus-specific drivers. For instance::
+
+
+  static int pci_device_remove(struct device * dev)
+  {
+          struct pci_dev * pci_dev = to_pci_dev(dev);
+          struct pci_driver * drv = pci_dev->driver;
+
+          if (drv) {
+                  if (drv->remove)
+                          drv->remove(pci_dev);
+                  pci_dev->driver = NULL;
+          }
+          return 0;
+  }
+
+
+The generic driver should be initialized with these methods before it
+is registered::
+
+        /* initialize common driver fields */
+        drv->driver.name = drv->name;
+        drv->driver.bus = &pci_bus_type;
+        drv->driver.probe = pci_device_probe;
+        drv->driver.resume = pci_device_resume;
+        drv->driver.suspend = pci_device_suspend;
+        drv->driver.remove = pci_device_remove;
+
+        /* register with core */
+        driver_register(&drv->driver);
+
+
+Ideally, the bus should only initialize the fields if they are not
+already set. This allows the drivers to implement their own generic
+methods.
+
+
+Step 5: Support generic driver binding.
+
+The model assumes that a device or driver can be dynamically
+registered with the bus at any time. When registration happens,
+devices must be bound to a driver, or drivers must be bound to all
+devices that it supports.
+
+A driver typically contains a list of device IDs that it supports. The
+bus driver compares these IDs to the IDs of devices registered with it.
+The format of the device IDs, and the semantics for comparing them are
+bus-specific, so the generic model does attempt to generalize them.
+
+Instead, a bus may supply a method in struct bus_type that does the
+comparison::
+
+  int (*match)(struct device * dev, struct device_driver * drv);
+
+match should return positive value if the driver supports the device,
+and zero otherwise. It may also return error code (for example
+-EPROBE_DEFER) if determining that given driver supports the device is
+not possible.
+
+When a device is registered, the bus's list of drivers is iterated
+over. bus->match() is called for each one until a match is found.
+
+When a driver is registered, the bus's list of devices is iterated
+over. bus->match() is called for each device that is not already
+claimed by a driver.
+
+When a device is successfully bound to a driver, device->driver is
+set, the device is added to a per-driver list of devices, and a
+symlink is created in the driver's sysfs directory that points to the
+device's physical directory::
+
+  /sys/bus/pci/drivers/
+  |-- 3c59x
+  |   `-- 00:0b.0 -> ../../../../devices/pci0/00:0b.0
+  |-- Ensoniq AudioPCI
+  |-- agpgart-amdk7
+  |   `-- 00:00.0 -> ../../../../devices/pci0/00:00.0
+  |-- e100
+  |   `-- 00:0c.0 -> ../../../../devices/pci0/00:0c.0
+  `-- serial
+
+
+This driver binding should replace the existing driver binding
+mechanism the bus currently uses.
+
+
+Step 6: Supply a hotplug callback.
+
+Whenever a device is registered with the driver model core, the
+userspace program /sbin/hotplug is called to notify userspace.
+Users can define actions to perform when a device is inserted or
+removed.
+
+The driver model core passes several arguments to userspace via
+environment variables, including
+
+- ACTION: set to 'add' or 'remove'
+- DEVPATH: set to the device's physical path in sysfs.
+
+A bus driver may also supply additional parameters for userspace to
+consume. To do this, a bus must implement the 'hotplug' method in
+struct bus_type::
+
+     int (*hotplug) (struct device *dev, char **envp,
+                     int num_envp, char *buffer, int buffer_size);
+
+This is called immediately before /sbin/hotplug is executed.
+
+
+Step 7: Cleaning up the bus driver.
+
+The generic bus, device, and driver structures provide several fields
+that can replace those defined privately to the bus driver.
+
+- Device list.
+
+struct bus_type contains a list of all devices registered with the bus
+type. This includes all devices on all instances of that bus type.
+An internal list that the bus uses may be removed, in favor of using
+this one.
+
+The core provides an iterator to access these devices::
+
+  int bus_for_each_dev(struct bus_type * bus, struct device * start,
+                       void * data, int (*fn)(struct device *, void *));
+
+
+- Driver list.
+
+struct bus_type also contains a list of all drivers registered with
+it. An internal list of drivers that the bus driver maintains may
+be removed in favor of using the generic one.
+
+The drivers may be iterated over, like devices::
+
+  int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
+                       void * data, int (*fn)(struct device_driver *, void *));
+
+
+Please see drivers/base/bus.c for more information.
+
+
+- rwsem
+
+struct bus_type contains an rwsem that protects all core accesses to
+the device and driver lists. This can be used by the bus driver
+internally, and should be used when accessing the device or driver
+lists the bus maintains.
+
+
+- Device and driver fields.
+
+Some of the fields in struct device and struct device_driver duplicate
+fields in the bus-specific representations of these objects. Feel free
+to remove the bus-specific ones and favor the generic ones. Note
+though, that this will likely mean fixing up all the drivers that
+reference the bus-specific fields (though those should all be 1-line
+changes).