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1 | |
2 | Porting Drivers to the New Driver Model |
3 | |
4 | Patrick Mochel |
5 | |
6 | 7 January 2003 |
7 | |
8 | |
9 | Overview |
10 | |
11 | Please refer to Documentation/driver-model/*.txt for definitions of |
12 | various driver types and concepts. |
13 | |
14 | Most of the work of porting devices drivers to the new model happens |
15 | at the bus driver layer. This was intentional, to minimize the |
16 | negative effect on kernel drivers, and to allow a gradual transition |
17 | of bus drivers. |
18 | |
19 | In a nutshell, the driver model consists of a set of objects that can |
20 | be embedded in larger, bus-specific objects. Fields in these generic |
21 | objects can replace fields in the bus-specific objects. |
22 | |
23 | The generic objects must be registered with the driver model core. By |
24 | doing so, they will exported via the sysfs filesystem. sysfs can be |
25 | mounted by doing |
26 | |
27 | # mount -t sysfs sysfs /sys |
28 | |
29 | |
30 | |
31 | The Process |
32 | |
33 | Step 0: Read include/linux/device.h for object and function definitions. |
34 | |
35 | Step 1: Registering the bus driver. |
36 | |
37 | |
38 | - Define a struct bus_type for the bus driver. |
39 | |
40 | struct 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 | |
49 | static int __init pci_driver_init(void) |
50 | { |
51 | return bus_register(&pci_bus_type); |
52 | } |
53 | |
54 | subsys_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 | |
68 | From include/linux/pci.h: |
69 | |
70 | extern struct bus_type pci_bus_type; |
71 | |
72 | |
73 | From file the above code appears in: |
74 | |
75 | EXPORT_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 | |
89 | Step 2: Registering Devices. |
90 | |
91 | struct device represents a single device. It mainly contains metadata |
92 | describing the relationship the device has to other entities. |
93 | |
94 | |
95 | - Embed a struct device in the bus-specific device type. |
96 | |
97 | |
98 | struct 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 | |
112 | or |
113 | |
114 | static 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 | |
226 | Step 3: Registering Drivers. |
227 | |
228 | struct device_driver is a simple driver structure that contains a set |
229 | of 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 | |
236 | struct 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 | |
279 | Step 4: Define Generic Methods for Drivers. |
280 | |
281 | struct device_driver defines a set of operations that the driver model |
282 | core calls. Most of these operations are probably similar to |
283 | operations the bus already defines for drivers, but taking different |
284 | parameters. |
285 | |
286 | It would be difficult and tedious to force every driver on a bus to |
287 | simultaneously convert their drivers to generic format. Instead, the |
288 | bus driver should define single instances of the generic methods that |
289 | forward call to the bus-specific drivers. For instance: |
290 | |
291 | |
292 | static 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 | |
306 | The generic driver should be initialized with these methods before it |
307 | is 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 | |
321 | Ideally, the bus should only initialize the fields if they are not |
322 | already set. This allows the drivers to implement their own generic |
323 | methods. |
324 | |
325 | |
326 | Step 5: Support generic driver binding. |
327 | |
328 | The model assumes that a device or driver can be dynamically |
329 | registered with the bus at any time. When registration happens, |
330 | devices must be bound to a driver, or drivers must be bound to all |
331 | devices that it supports. |
332 | |
333 | A driver typically contains a list of device IDs that it supports. The |
334 | bus driver compares these IDs to the IDs of devices registered with it. |
335 | The format of the device IDs, and the semantics for comparing them are |
336 | bus-specific, so the generic model does attempt to generalize them. |
337 | |
338 | Instead, a bus may supply a method in struct bus_type that does the |
339 | comparison: |
340 | |
341 | int (*match)(struct device * dev, struct device_driver * drv); |
342 | |
343 | match should return '1' if the driver supports the device, and '0' |
344 | otherwise. |
345 | |
346 | When a device is registered, the bus's list of drivers is iterated |
347 | over. bus->match() is called for each one until a match is found. |
348 | |
349 | When a driver is registered, the bus's list of devices is iterated |
350 | over. bus->match() is called for each device that is not already |
351 | claimed by a driver. |
352 | |
353 | When a device is successfully bound to a driver, device->driver is |
354 | set, the device is added to a per-driver list of devices, and a |
355 | symlink is created in the driver's sysfs directory that points to the |
356 | device's physical directory: |
357 | |
358 | /sys/bus/pci/drivers/ |
359 | |-- 3c59x |
360 | | `-- 00:0b.0 -> ../../../../devices/pci0/00:0b.0 |
361 | |-- Ensoniq AudioPCI |
362 | |-- agpgart-amdk7 |
363 | | `-- 00:00.0 -> ../../../../devices/pci0/00:00.0 |
364 | |-- e100 |
365 | | `-- 00:0c.0 -> ../../../../devices/pci0/00:0c.0 |
366 | `-- serial |
367 | |
368 | |
369 | This driver binding should replace the existing driver binding |
370 | mechanism the bus currently uses. |
371 | |
372 | |
373 | Step 6: Supply a hotplug callback. |
374 | |
375 | Whenever a device is registered with the driver model core, the |
376 | userspace program /sbin/hotplug is called to notify userspace. |
377 | Users can define actions to perform when a device is inserted or |
378 | removed. |
379 | |
380 | The driver model core passes several arguments to userspace via |
381 | environment variables, including |
382 | |
383 | - ACTION: set to 'add' or 'remove' |
384 | - DEVPATH: set to the device's physical path in sysfs. |
385 | |
386 | A bus driver may also supply additional parameters for userspace to |
387 | consume. To do this, a bus must implement the 'hotplug' method in |
388 | struct bus_type: |
389 | |
390 | int (*hotplug) (struct device *dev, char **envp, |
391 | int num_envp, char *buffer, int buffer_size); |
392 | |
393 | This is called immediately before /sbin/hotplug is executed. |
394 | |
395 | |
396 | Step 7: Cleaning up the bus driver. |
397 | |
398 | The generic bus, device, and driver structures provide several fields |
399 | that can replace those defined privately to the bus driver. |
400 | |
401 | - Device list. |
402 | |
403 | struct bus_type contains a list of all devices registered with the bus |
404 | type. This includes all devices on all instances of that bus type. |
405 | An internal list that the bus uses may be removed, in favor of using |
406 | this one. |
407 | |
408 | The core provides an iterator to access these devices. |
409 | |
410 | int bus_for_each_dev(struct bus_type * bus, struct device * start, |
411 | void * data, int (*fn)(struct device *, void *)); |
412 | |
413 | |
414 | - Driver list. |
415 | |
416 | struct bus_type also contains a list of all drivers registered with |
417 | it. An internal list of drivers that the bus driver maintains may |
418 | be removed in favor of using the generic one. |
419 | |
420 | The drivers may be iterated over, like devices: |
421 | |
422 | int bus_for_each_drv(struct bus_type * bus, struct device_driver * start, |
423 | void * data, int (*fn)(struct device_driver *, void *)); |
424 | |
425 | |
426 | Please see drivers/base/bus.c for more information. |
427 | |
428 | |
429 | - rwsem |
430 | |
431 | struct bus_type contains an rwsem that protects all core accesses to |
432 | the device and driver lists. This can be used by the bus driver |
433 | internally, and should be used when accessing the device or driver |
434 | lists the bus maintains. |
435 | |
436 | |
437 | - Device and driver fields. |
438 | |
439 | Some of the fields in struct device and struct device_driver duplicate |
440 | fields in the bus-specific representations of these objects. Feel free |
441 | to remove the bus-specific ones and favor the generic ones. Note |
442 | though, that this will likely mean fixing up all the drivers that |
443 | reference the bus-specific fields (though those should all be 1-line |
444 | changes). |
445 | |
446 |
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