Porting Drivers to the New Driver ModelPatrick Mochel7 January 2003OverviewPlease refer to Documentation/driver-model/*.txt for definitions ofvarious driver types and concepts. Most of the work of porting devices drivers to the new model happensat the bus driver layer. This was intentional, to minimize thenegative effect on kernel drivers, and to allow a gradual transitionof bus drivers.In a nutshell, the driver model consists of a set of objects that canbe embedded in larger, bus-specific objects. Fields in these genericobjects can replace fields in the bus-specific objects. The generic objects must be registered with the driver model core. Bydoing so, they will exported via the sysfs filesystem. sysfs can bemounted by doing 	# mount -t sysfs sysfs /sysThe ProcessStep 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`-- driversStep 2: Registering Devices. struct device represents a single device. It mainly contains metadatadescribing the relationship the device has to other entities. - Embedd 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.0/04:04.0Step 3: Registering Drivers.struct device_driver is a simple driver structure that contains a setof 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`-- serialStep 4: Define Generic Methods for Drivers.struct device_driver defines a set of operations that the driver modelcore calls. Most of these operations are probably similar tooperations the bus already defines for drivers, but taking differentparameters. It would be difficult and tedious to force every driver on a bus tosimultaneously convert their drivers to generic format. Instead, thebus driver should define single instances of the generic methods thatforward 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 itis 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 notalready set. This allows the drivers to implement their own genericmethods. Step 5: Support generic driver binding. The model assumes that a device or driver can be dynamicallyregistered with the bus at any time. When registration happens,devices must be bound to a driver, or drivers must be bound to alldevices that it supports. A driver typically contains a list of device IDs that it supports. Thebus driver compares these IDs to the IDs of devices registered with it. The format of the device IDs, and the semantics for comparing them arebus-specific, so the generic model does attempt to generalize them. Instead, a bus may supply a method in struct bus_type that does thecomparison:   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 iteratedover. bus->match() is called for each one until a match is found. When a driver is registered, the bus's list of devices is iteratedover. bus->match() is called for each device that is not alreadyclaimed by a driver. When a device is successfully bound to a device, device->driver isset, the device is added to a per-driver list of devices, and asymlink is created in the driver's sysfs directory that points to thedevice'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`-- serialThis driver binding should replace the existing driver bindingmechanism the bus currently uses. Step 6: Supply a hotplug callback.Whenever a device is registered with the driver model core, theuserspace program /sbin/hotplug is called to notify userspace. Users can define actions to perform when a device is inserted orremoved. The driver model core passes several arguments to userspace viaenvironment 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 toconsume. To do this, a bus must implement the 'hotplug' method instruct 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 fieldsthat can replace those defined privately to the bus driver. - Device list.struct bus_type contains a list of all devices registered with the bustype. This includes all devices on all instances of that bus type.An internal list that the bus uses may be removed, in favor of usingthis 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 withit. 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 tothe device and driver lists. This can be used by the bus driverinternally, and should be used when accessing the device or driverlists the bus maintains. - Device and driver fields. Some of the fields in struct device and struct device_driver duplicatefields in the bus-specific representations of these objects. Feel freeto remove the bus-specific ones and favor the generic ones. Notethough, that this will likely mean fixing up all the drivers thatreference the bus-specific fields (though those should all be 1-linechanges).