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2a1fcdf0 HV |
1 | Overview of the V4L2 driver framework |
2 | ===================================== | |
3 | ||
4 | This text documents the various structures provided by the V4L2 framework and | |
5 | their relationships. | |
6 | ||
7 | ||
8 | Introduction | |
9 | ------------ | |
10 | ||
11 | The V4L2 drivers tend to be very complex due to the complexity of the | |
12 | hardware: most devices have multiple ICs, export multiple device nodes in | |
13 | /dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input | |
14 | (IR) devices. | |
15 | ||
16 | Especially the fact that V4L2 drivers have to setup supporting ICs to | |
17 | do audio/video muxing/encoding/decoding makes it more complex than most. | |
18 | Usually these ICs are connected to the main bridge driver through one or | |
19 | more I2C busses, but other busses can also be used. Such devices are | |
20 | called 'sub-devices'. | |
21 | ||
22 | For a long time the framework was limited to the video_device struct for | |
23 | creating V4L device nodes and video_buf for handling the video buffers | |
24 | (note that this document does not discuss the video_buf framework). | |
25 | ||
26 | This meant that all drivers had to do the setup of device instances and | |
27 | connecting to sub-devices themselves. Some of this is quite complicated | |
28 | to do right and many drivers never did do it correctly. | |
29 | ||
30 | There is also a lot of common code that could never be refactored due to | |
31 | the lack of a framework. | |
32 | ||
33 | So this framework sets up the basic building blocks that all drivers | |
34 | need and this same framework should make it much easier to refactor | |
35 | common code into utility functions shared by all drivers. | |
36 | ||
37 | ||
38 | Structure of a driver | |
39 | --------------------- | |
40 | ||
41 | All drivers have the following structure: | |
42 | ||
43 | 1) A struct for each device instance containing the device state. | |
44 | ||
45 | 2) A way of initializing and commanding sub-devices (if any). | |
46 | ||
47 | 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and | |
48 | /dev/vtxX) and keeping track of device-node specific data. | |
49 | ||
50 | 4) Filehandle-specific structs containing per-filehandle data. | |
51 | ||
52 | This is a rough schematic of how it all relates: | |
53 | ||
54 | device instances | |
55 | | | |
56 | +-sub-device instances | |
57 | | | |
58 | \-V4L2 device nodes | |
59 | | | |
60 | \-filehandle instances | |
61 | ||
62 | ||
63 | Structure of the framework | |
64 | -------------------------- | |
65 | ||
66 | The framework closely resembles the driver structure: it has a v4l2_device | |
67 | struct for the device instance data, a v4l2_subdev struct to refer to | |
68 | sub-device instances, the video_device struct stores V4L2 device node data | |
69 | and in the future a v4l2_fh struct will keep track of filehandle instances | |
70 | (this is not yet implemented). | |
71 | ||
72 | ||
73 | struct v4l2_device | |
74 | ------------------ | |
75 | ||
76 | Each device instance is represented by a struct v4l2_device (v4l2-device.h). | |
77 | Very simple devices can just allocate this struct, but most of the time you | |
78 | would embed this struct inside a larger struct. | |
79 | ||
80 | You must register the device instance: | |
81 | ||
82 | v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev); | |
83 | ||
84 | Registration will initialize the v4l2_device struct and link dev->driver_data | |
85 | to v4l2_dev. Registration will also set v4l2_dev->name to a value derived from | |
86 | dev (driver name followed by the bus_id, to be precise). You may change the | |
87 | name after registration if you want. | |
88 | ||
a47ddf14 HV |
89 | The first 'dev' argument is normally the struct device pointer of a pci_dev, |
90 | usb_device or platform_device. | |
91 | ||
2a1fcdf0 HV |
92 | You unregister with: |
93 | ||
94 | v4l2_device_unregister(struct v4l2_device *v4l2_dev); | |
95 | ||
96 | Unregistering will also automatically unregister all subdevs from the device. | |
97 | ||
98 | Sometimes you need to iterate over all devices registered by a specific | |
99 | driver. This is usually the case if multiple device drivers use the same | |
100 | hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv | |
101 | hardware. The same is true for alsa drivers for example. | |
102 | ||
103 | You can iterate over all registered devices as follows: | |
104 | ||
105 | static int callback(struct device *dev, void *p) | |
106 | { | |
107 | struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); | |
108 | ||
109 | /* test if this device was inited */ | |
110 | if (v4l2_dev == NULL) | |
111 | return 0; | |
112 | ... | |
113 | return 0; | |
114 | } | |
115 | ||
116 | int iterate(void *p) | |
117 | { | |
118 | struct device_driver *drv; | |
119 | int err; | |
120 | ||
121 | /* Find driver 'ivtv' on the PCI bus. | |
122 | pci_bus_type is a global. For USB busses use usb_bus_type. */ | |
123 | drv = driver_find("ivtv", &pci_bus_type); | |
124 | /* iterate over all ivtv device instances */ | |
125 | err = driver_for_each_device(drv, NULL, p, callback); | |
126 | put_driver(drv); | |
127 | return err; | |
128 | } | |
129 | ||
130 | Sometimes you need to keep a running counter of the device instance. This is | |
131 | commonly used to map a device instance to an index of a module option array. | |
132 | ||
133 | The recommended approach is as follows: | |
134 | ||
135 | static atomic_t drv_instance = ATOMIC_INIT(0); | |
136 | ||
137 | static int __devinit drv_probe(struct pci_dev *dev, | |
138 | const struct pci_device_id *pci_id) | |
139 | { | |
140 | ... | |
141 | state->instance = atomic_inc_return(&drv_instance) - 1; | |
142 | } | |
143 | ||
144 | ||
145 | struct v4l2_subdev | |
146 | ------------------ | |
147 | ||
148 | Many drivers need to communicate with sub-devices. These devices can do all | |
149 | sort of tasks, but most commonly they handle audio and/or video muxing, | |
150 | encoding or decoding. For webcams common sub-devices are sensors and camera | |
151 | controllers. | |
152 | ||
153 | Usually these are I2C devices, but not necessarily. In order to provide the | |
154 | driver with a consistent interface to these sub-devices the v4l2_subdev struct | |
155 | (v4l2-subdev.h) was created. | |
156 | ||
157 | Each sub-device driver must have a v4l2_subdev struct. This struct can be | |
158 | stand-alone for simple sub-devices or it might be embedded in a larger struct | |
159 | if more state information needs to be stored. Usually there is a low-level | |
160 | device struct (e.g. i2c_client) that contains the device data as setup | |
161 | by the kernel. It is recommended to store that pointer in the private | |
162 | data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go | |
163 | from a v4l2_subdev to the actual low-level bus-specific device data. | |
164 | ||
165 | You also need a way to go from the low-level struct to v4l2_subdev. For the | |
166 | common i2c_client struct the i2c_set_clientdata() call is used to store a | |
167 | v4l2_subdev pointer, for other busses you may have to use other methods. | |
168 | ||
169 | From the bridge driver perspective you load the sub-device module and somehow | |
170 | obtain the v4l2_subdev pointer. For i2c devices this is easy: you call | |
171 | i2c_get_clientdata(). For other busses something similar needs to be done. | |
172 | Helper functions exists for sub-devices on an I2C bus that do most of this | |
173 | tricky work for you. | |
174 | ||
175 | Each v4l2_subdev contains function pointers that sub-device drivers can | |
176 | implement (or leave NULL if it is not applicable). Since sub-devices can do | |
177 | so many different things and you do not want to end up with a huge ops struct | |
178 | of which only a handful of ops are commonly implemented, the function pointers | |
179 | are sorted according to category and each category has its own ops struct. | |
180 | ||
181 | The top-level ops struct contains pointers to the category ops structs, which | |
182 | may be NULL if the subdev driver does not support anything from that category. | |
183 | ||
184 | It looks like this: | |
185 | ||
186 | struct v4l2_subdev_core_ops { | |
aecde8b5 | 187 | int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); |
2a1fcdf0 HV |
188 | int (*log_status)(struct v4l2_subdev *sd); |
189 | int (*init)(struct v4l2_subdev *sd, u32 val); | |
190 | ... | |
191 | }; | |
192 | ||
193 | struct v4l2_subdev_tuner_ops { | |
194 | ... | |
195 | }; | |
196 | ||
197 | struct v4l2_subdev_audio_ops { | |
198 | ... | |
199 | }; | |
200 | ||
201 | struct v4l2_subdev_video_ops { | |
202 | ... | |
203 | }; | |
204 | ||
205 | struct v4l2_subdev_ops { | |
206 | const struct v4l2_subdev_core_ops *core; | |
207 | const struct v4l2_subdev_tuner_ops *tuner; | |
208 | const struct v4l2_subdev_audio_ops *audio; | |
209 | const struct v4l2_subdev_video_ops *video; | |
210 | }; | |
211 | ||
212 | The core ops are common to all subdevs, the other categories are implemented | |
213 | depending on the sub-device. E.g. a video device is unlikely to support the | |
214 | audio ops and vice versa. | |
215 | ||
216 | This setup limits the number of function pointers while still making it easy | |
217 | to add new ops and categories. | |
218 | ||
219 | A sub-device driver initializes the v4l2_subdev struct using: | |
220 | ||
221 | v4l2_subdev_init(subdev, &ops); | |
222 | ||
223 | Afterwards you need to initialize subdev->name with a unique name and set the | |
224 | module owner. This is done for you if you use the i2c helper functions. | |
225 | ||
226 | A device (bridge) driver needs to register the v4l2_subdev with the | |
227 | v4l2_device: | |
228 | ||
229 | int err = v4l2_device_register_subdev(device, subdev); | |
230 | ||
231 | This can fail if the subdev module disappeared before it could be registered. | |
232 | After this function was called successfully the subdev->dev field points to | |
233 | the v4l2_device. | |
234 | ||
235 | You can unregister a sub-device using: | |
236 | ||
237 | v4l2_device_unregister_subdev(subdev); | |
238 | ||
239 | Afterwards the subdev module can be unloaded and subdev->dev == NULL. | |
240 | ||
241 | You can call an ops function either directly: | |
242 | ||
243 | err = subdev->ops->core->g_chip_ident(subdev, &chip); | |
244 | ||
245 | but it is better and easier to use this macro: | |
246 | ||
247 | err = v4l2_subdev_call(subdev, core, g_chip_ident, &chip); | |
248 | ||
249 | The macro will to the right NULL pointer checks and returns -ENODEV if subdev | |
250 | is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is | |
251 | NULL, or the actual result of the subdev->ops->core->g_chip_ident ops. | |
252 | ||
253 | It is also possible to call all or a subset of the sub-devices: | |
254 | ||
255 | v4l2_device_call_all(dev, 0, core, g_chip_ident, &chip); | |
256 | ||
257 | Any subdev that does not support this ops is skipped and error results are | |
258 | ignored. If you want to check for errors use this: | |
259 | ||
260 | err = v4l2_device_call_until_err(dev, 0, core, g_chip_ident, &chip); | |
261 | ||
262 | Any error except -ENOIOCTLCMD will exit the loop with that error. If no | |
263 | errors (except -ENOIOCTLCMD) occured, then 0 is returned. | |
264 | ||
265 | The second argument to both calls is a group ID. If 0, then all subdevs are | |
266 | called. If non-zero, then only those whose group ID match that value will | |
267 | be called. Before a bridge driver registers a subdev it can set subdev->grp_id | |
268 | to whatever value it wants (it's 0 by default). This value is owned by the | |
269 | bridge driver and the sub-device driver will never modify or use it. | |
270 | ||
271 | The group ID gives the bridge driver more control how callbacks are called. | |
272 | For example, there may be multiple audio chips on a board, each capable of | |
273 | changing the volume. But usually only one will actually be used when the | |
274 | user want to change the volume. You can set the group ID for that subdev to | |
275 | e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling | |
276 | v4l2_device_call_all(). That ensures that it will only go to the subdev | |
277 | that needs it. | |
278 | ||
279 | The advantage of using v4l2_subdev is that it is a generic struct and does | |
280 | not contain any knowledge about the underlying hardware. So a driver might | |
281 | contain several subdevs that use an I2C bus, but also a subdev that is | |
282 | controlled through GPIO pins. This distinction is only relevant when setting | |
283 | up the device, but once the subdev is registered it is completely transparent. | |
284 | ||
285 | ||
286 | I2C sub-device drivers | |
287 | ---------------------- | |
288 | ||
289 | Since these drivers are so common, special helper functions are available to | |
290 | ease the use of these drivers (v4l2-common.h). | |
291 | ||
292 | The recommended method of adding v4l2_subdev support to an I2C driver is to | |
293 | embed the v4l2_subdev struct into the state struct that is created for each | |
294 | I2C device instance. Very simple devices have no state struct and in that case | |
295 | you can just create a v4l2_subdev directly. | |
296 | ||
297 | A typical state struct would look like this (where 'chipname' is replaced by | |
298 | the name of the chip): | |
299 | ||
300 | struct chipname_state { | |
301 | struct v4l2_subdev sd; | |
302 | ... /* additional state fields */ | |
303 | }; | |
304 | ||
305 | Initialize the v4l2_subdev struct as follows: | |
306 | ||
307 | v4l2_i2c_subdev_init(&state->sd, client, subdev_ops); | |
308 | ||
309 | This function will fill in all the fields of v4l2_subdev and ensure that the | |
310 | v4l2_subdev and i2c_client both point to one another. | |
311 | ||
312 | You should also add a helper inline function to go from a v4l2_subdev pointer | |
313 | to a chipname_state struct: | |
314 | ||
315 | static inline struct chipname_state *to_state(struct v4l2_subdev *sd) | |
316 | { | |
317 | return container_of(sd, struct chipname_state, sd); | |
318 | } | |
319 | ||
320 | Use this to go from the v4l2_subdev struct to the i2c_client struct: | |
321 | ||
322 | struct i2c_client *client = v4l2_get_subdevdata(sd); | |
323 | ||
324 | And this to go from an i2c_client to a v4l2_subdev struct: | |
325 | ||
326 | struct v4l2_subdev *sd = i2c_get_clientdata(client); | |
327 | ||
328 | Finally you need to make a command function to make driver->command() | |
329 | call the right subdev_ops functions: | |
330 | ||
331 | static int subdev_command(struct i2c_client *client, unsigned cmd, void *arg) | |
332 | { | |
333 | return v4l2_subdev_command(i2c_get_clientdata(client), cmd, arg); | |
334 | } | |
335 | ||
336 | If driver->command is never used then you can leave this out. Eventually the | |
337 | driver->command usage should be removed from v4l. | |
338 | ||
339 | Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback | |
340 | is called. This will unregister the sub-device from the bridge driver. It is | |
341 | safe to call this even if the sub-device was never registered. | |
342 | ||
343 | ||
344 | The bridge driver also has some helper functions it can use: | |
345 | ||
346 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev(adapter, "module_foo", "chipid", 0x36); | |
347 | ||
348 | This loads the given module (can be NULL if no module needs to be loaded) and | |
349 | calls i2c_new_device() with the given i2c_adapter and chip/address arguments. | |
350 | If all goes well, then it registers the subdev with the v4l2_device. It gets | |
351 | the v4l2_device by calling i2c_get_adapdata(adapter), so you should make sure | |
352 | that adapdata is set to v4l2_device when you setup the i2c_adapter in your | |
353 | driver. | |
354 | ||
355 | You can also use v4l2_i2c_new_probed_subdev() which is very similar to | |
356 | v4l2_i2c_new_subdev(), except that it has an array of possible I2C addresses | |
357 | that it should probe. Internally it calls i2c_new_probed_device(). | |
358 | ||
359 | Both functions return NULL if something went wrong. | |
360 | ||
361 | ||
362 | struct video_device | |
363 | ------------------- | |
364 | ||
a47ddf14 HV |
365 | The actual device nodes in the /dev directory are created using the |
366 | video_device struct (v4l2-dev.h). This struct can either be allocated | |
367 | dynamically or embedded in a larger struct. | |
368 | ||
369 | To allocate it dynamically use: | |
370 | ||
371 | struct video_device *vdev = video_device_alloc(); | |
372 | ||
373 | if (vdev == NULL) | |
374 | return -ENOMEM; | |
375 | ||
376 | vdev->release = video_device_release; | |
377 | ||
378 | If you embed it in a larger struct, then you must set the release() | |
379 | callback to your own function: | |
380 | ||
381 | struct video_device *vdev = &my_vdev->vdev; | |
382 | ||
383 | vdev->release = my_vdev_release; | |
384 | ||
385 | The release callback must be set and it is called when the last user | |
386 | of the video device exits. | |
387 | ||
388 | The default video_device_release() callback just calls kfree to free the | |
389 | allocated memory. | |
390 | ||
391 | You should also set these fields: | |
392 | ||
dfa9a5ae | 393 | - v4l2_dev: set to the v4l2_device parent device. |
a47ddf14 | 394 | - name: set to something descriptive and unique. |
c7dd09da | 395 | - fops: set to the v4l2_file_operations struct. |
a47ddf14 HV |
396 | - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance |
397 | (highly recommended to use this and it might become compulsory in the | |
398 | future!), then set this to your v4l2_ioctl_ops struct. | |
399 | ||
c7dd09da HV |
400 | If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or |
401 | .ioctl to video_ioctl2 in your v4l2_file_operations struct. | |
402 | ||
403 | The v4l2_file_operations struct is a subset of file_operations. The main | |
404 | difference is that the inode argument is omitted since it is never used. | |
a47ddf14 HV |
405 | |
406 | ||
407 | video_device registration | |
408 | ------------------------- | |
409 | ||
410 | Next you register the video device: this will create the character device | |
411 | for you. | |
412 | ||
413 | err = video_register_device(vdev, VFL_TYPE_GRABBER, -1); | |
414 | if (err) { | |
50a2a8b3 | 415 | video_device_release(vdev); /* or kfree(my_vdev); */ |
a47ddf14 HV |
416 | return err; |
417 | } | |
418 | ||
419 | Which device is registered depends on the type argument. The following | |
420 | types exist: | |
421 | ||
422 | VFL_TYPE_GRABBER: videoX for video input/output devices | |
423 | VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) | |
424 | VFL_TYPE_RADIO: radioX for radio tuners | |
425 | VFL_TYPE_VTX: vtxX for teletext devices (deprecated, don't use) | |
426 | ||
427 | The last argument gives you a certain amount of control over the device | |
428 | kernel number used (i.e. the X in videoX). Normally you will pass -1 to | |
429 | let the v4l2 framework pick the first free number. But if a driver creates | |
430 | many devices, then it can be useful to have different video devices in | |
431 | separate ranges. For example, video capture devices start at 0, video | |
432 | output devices start at 16. | |
433 | ||
434 | So you can use the last argument to specify a minimum kernel number and | |
435 | the v4l2 framework will try to pick the first free number that is equal | |
436 | or higher to what you passed. If that fails, then it will just pick the | |
437 | first free number. | |
438 | ||
439 | Whenever a device node is created some attributes are also created for you. | |
440 | If you look in /sys/class/video4linux you see the devices. Go into e.g. | |
441 | video0 and you will see 'name' and 'index' attributes. The 'name' attribute | |
442 | is the 'name' field of the video_device struct. The 'index' attribute is | |
443 | a device node index that can be assigned by the driver, or that is calculated | |
444 | for you. | |
445 | ||
446 | If you call video_register_device(), then the index is just increased by | |
447 | 1 for each device node you register. The first video device node you register | |
448 | always starts off with 0. | |
449 | ||
450 | Alternatively you can call video_register_device_index() which is identical | |
451 | to video_register_device(), but with an extra index argument. Here you can | |
452 | pass a specific index value (between 0 and 31) that should be used. | |
453 | ||
454 | Users can setup udev rules that utilize the index attribute to make fancy | |
455 | device names (e.g. 'mpegX' for MPEG video capture device nodes). | |
456 | ||
457 | After the device was successfully registered, then you can use these fields: | |
458 | ||
459 | - vfl_type: the device type passed to video_register_device. | |
460 | - minor: the assigned device minor number. | |
461 | - num: the device kernel number (i.e. the X in videoX). | |
462 | - index: the device index number (calculated or set explicitly using | |
463 | video_register_device_index). | |
464 | ||
465 | If the registration failed, then you need to call video_device_release() | |
466 | to free the allocated video_device struct, or free your own struct if the | |
467 | video_device was embedded in it. The vdev->release() callback will never | |
468 | be called if the registration failed, nor should you ever attempt to | |
469 | unregister the device if the registration failed. | |
470 | ||
471 | ||
472 | video_device cleanup | |
473 | -------------------- | |
474 | ||
475 | When the video device nodes have to be removed, either during the unload | |
476 | of the driver or because the USB device was disconnected, then you should | |
477 | unregister them: | |
478 | ||
479 | video_unregister_device(vdev); | |
480 | ||
481 | This will remove the device nodes from sysfs (causing udev to remove them | |
482 | from /dev). | |
483 | ||
484 | After video_unregister_device() returns no new opens can be done. | |
485 | ||
486 | However, in the case of USB devices some application might still have one | |
487 | of these device nodes open. You should block all new accesses to read, | |
488 | write, poll, etc. except possibly for certain ioctl operations like | |
489 | queueing buffers. | |
490 | ||
491 | When the last user of the video device node exits, then the vdev->release() | |
492 | callback is called and you can do the final cleanup there. | |
493 | ||
494 | ||
495 | video_device helper functions | |
496 | ----------------------------- | |
497 | ||
498 | There are a few useful helper functions: | |
499 | ||
500 | You can set/get driver private data in the video_device struct using: | |
501 | ||
502 | void *video_get_drvdata(struct video_device *dev); | |
503 | void video_set_drvdata(struct video_device *dev, void *data); | |
504 | ||
505 | Note that you can safely call video_set_drvdata() before calling | |
506 | video_register_device(). | |
507 | ||
508 | And this function: | |
509 | ||
510 | struct video_device *video_devdata(struct file *file); | |
511 | ||
512 | returns the video_device belonging to the file struct. | |
513 | ||
514 | The final helper function combines video_get_drvdata with | |
515 | video_devdata: | |
516 | ||
517 | void *video_drvdata(struct file *file); | |
518 | ||
519 | You can go from a video_device struct to the v4l2_device struct using: | |
520 | ||
dfa9a5ae | 521 | struct v4l2_device *v4l2_dev = vdev->v4l2_dev; |