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-
-Porting Drivers to the New Driver Model
-
-Patrick Mochel
-
-7 January 2003
-
-
-Overview
-
-Please refer to Documentation/driver-model/*.txt 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 '1' if the driver supports the device, and '0'
-otherwise.
-
-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).
-