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Linus Torvalds1da177e2005-04-16 15:20:36 -07001
2Porting Drivers to the New Driver Model
3
4Patrick Mochel
5
67 January 2003
7
8
9Overview
10
11Please refer to Documentation/driver-model/*.txt for definitions of
12various driver types and concepts.
13
14Most of the work of porting devices drivers to the new model happens
15at the bus driver layer. This was intentional, to minimize the
16negative effect on kernel drivers, and to allow a gradual transition
17of bus drivers.
18
19In a nutshell, the driver model consists of a set of objects that can
20be embedded in larger, bus-specific objects. Fields in these generic
21objects can replace fields in the bus-specific objects.
22
23The generic objects must be registered with the driver model core. By
24doing so, they will exported via the sysfs filesystem. sysfs can be
25mounted by doing
26
27 # mount -t sysfs sysfs /sys
28
29
30
31The Process
32
33Step 0: Read include/linux/device.h for object and function definitions.
34
35Step 1: Registering the bus driver.
36
37
38- Define a struct bus_type for the bus driver.
39
40struct bus_type pci_bus_type = {
41 .name = "pci",
42};
43
44
45- Register the bus type.
46 This should be done in the initialization function for the bus type,
47 which is usually the module_init(), or equivalent, function.
48
49static int __init pci_driver_init(void)
50{
51 return bus_register(&pci_bus_type);
52}
53
54subsys_initcall(pci_driver_init);
55
56
57 The bus type may be unregistered (if the bus driver may be compiled
58 as a module) by doing:
59
60 bus_unregister(&pci_bus_type);
61
62
63- Export the bus type for others to use.
64
65 Other code may wish to reference the bus type, so declare it in a
66 shared header file and export the symbol.
67
68From include/linux/pci.h:
69
70extern struct bus_type pci_bus_type;
71
72
73From file the above code appears in:
74
75EXPORT_SYMBOL(pci_bus_type);
76
77
78
79- This will cause the bus to show up in /sys/bus/pci/ with two
80 subdirectories: 'devices' and 'drivers'.
81
82# tree -d /sys/bus/pci/
83/sys/bus/pci/
84|-- devices
85`-- drivers
86
87
88
89Step 2: Registering Devices.
90
91struct device represents a single device. It mainly contains metadata
92describing the relationship the device has to other entities.
93
94
Matt LaPlante5d3f0832006-11-30 05:21:10 +010095- Embed a struct device in the bus-specific device type.
Linus Torvalds1da177e2005-04-16 15:20:36 -070096
97
98struct pci_dev {
99 ...
100 struct device dev; /* Generic device interface */
101 ...
102};
103
104 It is recommended that the generic device not be the first item in
105 the struct to discourage programmers from doing mindless casts
106 between the object types. Instead macros, or inline functions,
107 should be created to convert from the generic object type.
108
109
110#define to_pci_dev(n) container_of(n, struct pci_dev, dev)
111
112or
113
114static inline struct pci_dev * to_pci_dev(struct kobject * kobj)
115{
116 return container_of(n, struct pci_dev, dev);
117}
118
119 This allows the compiler to verify type-safety of the operations
120 that are performed (which is Good).
121
122
123- Initialize the device on registration.
124
125 When devices are discovered or registered with the bus type, the
126 bus driver should initialize the generic device. The most important
127 things to initialize are the bus_id, parent, and bus fields.
128
129 The bus_id is an ASCII string that contains the device's address on
130 the bus. The format of this string is bus-specific. This is
131 necessary for representing devices in sysfs.
132
133 parent is the physical parent of the device. It is important that
134 the bus driver sets this field correctly.
135
136 The driver model maintains an ordered list of devices that it uses
137 for power management. This list must be in order to guarantee that
138 devices are shutdown before their physical parents, and vice versa.
139 The order of this list is determined by the parent of registered
140 devices.
141
142 Also, the location of the device's sysfs directory depends on a
143 device's parent. sysfs exports a directory structure that mirrors
144 the device hierarchy. Accurately setting the parent guarantees that
145 sysfs will accurately represent the hierarchy.
146
147 The device's bus field is a pointer to the bus type the device
148 belongs to. This should be set to the bus_type that was declared
149 and initialized before.
150
151 Optionally, the bus driver may set the device's name and release
152 fields.
153
154 The name field is an ASCII string describing the device, like
155
156 "ATI Technologies Inc Radeon QD"
157
158 The release field is a callback that the driver model core calls
159 when the device has been removed, and all references to it have
160 been released. More on this in a moment.
161
162
163- Register the device.
164
165 Once the generic device has been initialized, it can be registered
166 with the driver model core by doing:
167
168 device_register(&dev->dev);
169
170 It can later be unregistered by doing:
171
172 device_unregister(&dev->dev);
173
174 This should happen on buses that support hotpluggable devices.
175 If a bus driver unregisters a device, it should not immediately free
176 it. It should instead wait for the driver model core to call the
177 device's release method, then free the bus-specific object.
178 (There may be other code that is currently referencing the device
179 structure, and it would be rude to free the device while that is
180 happening).
181
182
183 When the device is registered, a directory in sysfs is created.
184 The PCI tree in sysfs looks like:
185
186/sys/devices/pci0/
187|-- 00:00.0
188|-- 00:01.0
189| `-- 01:00.0
190|-- 00:02.0
191| `-- 02:1f.0
192| `-- 03:00.0
193|-- 00:1e.0
194| `-- 04:04.0
195|-- 00:1f.0
196|-- 00:1f.1
197| |-- ide0
198| | |-- 0.0
199| | `-- 0.1
200| `-- ide1
201| `-- 1.0
202|-- 00:1f.2
203|-- 00:1f.3
204`-- 00:1f.5
205
206 Also, symlinks are created in the bus's 'devices' directory
207 that point to the device's directory in the physical hierarchy.
208
209/sys/bus/pci/devices/
210|-- 00:00.0 -> ../../../devices/pci0/00:00.0
211|-- 00:01.0 -> ../../../devices/pci0/00:01.0
212|-- 00:02.0 -> ../../../devices/pci0/00:02.0
213|-- 00:1e.0 -> ../../../devices/pci0/00:1e.0
214|-- 00:1f.0 -> ../../../devices/pci0/00:1f.0
215|-- 00:1f.1 -> ../../../devices/pci0/00:1f.1
216|-- 00:1f.2 -> ../../../devices/pci0/00:1f.2
217|-- 00:1f.3 -> ../../../devices/pci0/00:1f.3
218|-- 00:1f.5 -> ../../../devices/pci0/00:1f.5
219|-- 01:00.0 -> ../../../devices/pci0/00:01.0/01:00.0
220|-- 02:1f.0 -> ../../../devices/pci0/00:02.0/02:1f.0
221|-- 03:00.0 -> ../../../devices/pci0/00:02.0/02:1f.0/03:00.0
222`-- 04:04.0 -> ../../../devices/pci0/00:1e.0/04:04.0
223
224
225
226Step 3: Registering Drivers.
227
228struct device_driver is a simple driver structure that contains a set
229of operations that the driver model core may call.
230
231
232- Embed a struct device_driver in the bus-specific driver.
233
234 Just like with devices, do something like:
235
236struct pci_driver {
237 ...
238 struct device_driver driver;
239};
240
241
242- Initialize the generic driver structure.
243
244 When the driver registers with the bus (e.g. doing pci_register_driver()),
245 initialize the necessary fields of the driver: the name and bus
246 fields.
247
248
249- Register the driver.
250
251 After the generic driver has been initialized, call
252
253 driver_register(&drv->driver);
254
255 to register the driver with the core.
256
257 When the driver is unregistered from the bus, unregister it from the
258 core by doing:
259
260 driver_unregister(&drv->driver);
261
262 Note that this will block until all references to the driver have
263 gone away. Normally, there will not be any.
264
265
266- Sysfs representation.
267
268 Drivers are exported via sysfs in their bus's 'driver's directory.
269 For example:
270
271/sys/bus/pci/drivers/
272|-- 3c59x
273|-- Ensoniq AudioPCI
274|-- agpgart-amdk7
275|-- e100
276`-- serial
277
278
279Step 4: Define Generic Methods for Drivers.
280
281struct device_driver defines a set of operations that the driver model
282core calls. Most of these operations are probably similar to
283operations the bus already defines for drivers, but taking different
284parameters.
285
286It would be difficult and tedious to force every driver on a bus to
287simultaneously convert their drivers to generic format. Instead, the
288bus driver should define single instances of the generic methods that
289forward call to the bus-specific drivers. For instance:
290
291
292static int pci_device_remove(struct device * dev)
293{
294 struct pci_dev * pci_dev = to_pci_dev(dev);
295 struct pci_driver * drv = pci_dev->driver;
296
297 if (drv) {
298 if (drv->remove)
299 drv->remove(pci_dev);
300 pci_dev->driver = NULL;
301 }
302 return 0;
303}
304
305
306The generic driver should be initialized with these methods before it
307is registered.
308
309 /* initialize common driver fields */
310 drv->driver.name = drv->name;
311 drv->driver.bus = &pci_bus_type;
312 drv->driver.probe = pci_device_probe;
313 drv->driver.resume = pci_device_resume;
314 drv->driver.suspend = pci_device_suspend;
315 drv->driver.remove = pci_device_remove;
316
317 /* register with core */
318 driver_register(&drv->driver);
319
320
321Ideally, the bus should only initialize the fields if they are not
322already set. This allows the drivers to implement their own generic
323methods.
324
325
326Step 5: Support generic driver binding.
327
328The model assumes that a device or driver can be dynamically
329registered with the bus at any time. When registration happens,
330devices must be bound to a driver, or drivers must be bound to all
331devices that it supports.
332
333A driver typically contains a list of device IDs that it supports. The
334bus driver compares these IDs to the IDs of devices registered with it.
335The format of the device IDs, and the semantics for comparing them are
336bus-specific, so the generic model does attempt to generalize them.
337
338Instead, a bus may supply a method in struct bus_type that does the
339comparison:
340
341 int (*match)(struct device * dev, struct device_driver * drv);
342
Tomeu Vizoso656b8032016-02-15 09:25:06 +0100343match should return positive value if the driver supports the device,
344and zero otherwise. It may also return error code (for example
345-EPROBE_DEFER) if determining that given driver supports the device is
346not possible.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700347
348When a device is registered, the bus's list of drivers is iterated
349over. bus->match() is called for each one until a match is found.
350
351When a driver is registered, the bus's list of devices is iterated
352over. bus->match() is called for each device that is not already
353claimed by a driver.
354
Erik Hovland2a7ff1f2005-10-06 10:47:49 -0700355When a device is successfully bound to a driver, device->driver is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700356set, the device is added to a per-driver list of devices, and a
357symlink is created in the driver's sysfs directory that points to the
358device's physical directory:
359
360/sys/bus/pci/drivers/
361|-- 3c59x
362| `-- 00:0b.0 -> ../../../../devices/pci0/00:0b.0
363|-- Ensoniq AudioPCI
364|-- agpgart-amdk7
365| `-- 00:00.0 -> ../../../../devices/pci0/00:00.0
366|-- e100
367| `-- 00:0c.0 -> ../../../../devices/pci0/00:0c.0
368`-- serial
369
370
371This driver binding should replace the existing driver binding
372mechanism the bus currently uses.
373
374
375Step 6: Supply a hotplug callback.
376
377Whenever a device is registered with the driver model core, the
378userspace program /sbin/hotplug is called to notify userspace.
379Users can define actions to perform when a device is inserted or
380removed.
381
382The driver model core passes several arguments to userspace via
383environment variables, including
384
385- ACTION: set to 'add' or 'remove'
386- DEVPATH: set to the device's physical path in sysfs.
387
388A bus driver may also supply additional parameters for userspace to
389consume. To do this, a bus must implement the 'hotplug' method in
390struct bus_type:
391
392 int (*hotplug) (struct device *dev, char **envp,
393 int num_envp, char *buffer, int buffer_size);
394
395This is called immediately before /sbin/hotplug is executed.
396
397
398Step 7: Cleaning up the bus driver.
399
400The generic bus, device, and driver structures provide several fields
401that can replace those defined privately to the bus driver.
402
403- Device list.
404
405struct bus_type contains a list of all devices registered with the bus
406type. This includes all devices on all instances of that bus type.
407An internal list that the bus uses may be removed, in favor of using
408this one.
409
410The core provides an iterator to access these devices.
411
412int bus_for_each_dev(struct bus_type * bus, struct device * start,
413 void * data, int (*fn)(struct device *, void *));
414
415
416- Driver list.
417
418struct bus_type also contains a list of all drivers registered with
419it. An internal list of drivers that the bus driver maintains may
420be removed in favor of using the generic one.
421
422The drivers may be iterated over, like devices:
423
424int bus_for_each_drv(struct bus_type * bus, struct device_driver * start,
425 void * data, int (*fn)(struct device_driver *, void *));
426
427
428Please see drivers/base/bus.c for more information.
429
430
431- rwsem
432
433struct bus_type contains an rwsem that protects all core accesses to
434the device and driver lists. This can be used by the bus driver
435internally, and should be used when accessing the device or driver
436lists the bus maintains.
437
438
439- Device and driver fields.
440
441Some of the fields in struct device and struct device_driver duplicate
442fields in the bus-specific representations of these objects. Feel free
443to remove the bus-specific ones and favor the generic ones. Note
444though, that this will likely mean fixing up all the drivers that
445reference the bus-specific fields (though those should all be 1-line
446changes).
447