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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 | ||
f44026db HV |
47 | 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX and /dev/radioX) |
48 | and keeping track of device-node specific data. | |
2a1fcdf0 | 49 | |
44061c05 MCC |
50 | 4) Filehandle-specific structs containing per-filehandle data; |
51 | ||
52 | 5) video buffer handling. | |
2a1fcdf0 HV |
53 | |
54 | This is a rough schematic of how it all relates: | |
55 | ||
56 | device instances | |
57 | | | |
58 | +-sub-device instances | |
59 | | | |
60 | \-V4L2 device nodes | |
61 | | | |
62 | \-filehandle instances | |
63 | ||
64 | ||
65 | Structure of the framework | |
66 | -------------------------- | |
67 | ||
68 | The framework closely resembles the driver structure: it has a v4l2_device | |
69 | struct for the device instance data, a v4l2_subdev struct to refer to | |
70 | sub-device instances, the video_device struct stores V4L2 device node data | |
71 | and in the future a v4l2_fh struct will keep track of filehandle instances | |
72 | (this is not yet implemented). | |
73 | ||
2c0ab67b LP |
74 | The V4L2 framework also optionally integrates with the media framework. If a |
75 | driver sets the struct v4l2_device mdev field, sub-devices and video nodes | |
76 | will automatically appear in the media framework as entities. | |
77 | ||
2a1fcdf0 HV |
78 | |
79 | struct v4l2_device | |
80 | ------------------ | |
81 | ||
82 | Each device instance is represented by a struct v4l2_device (v4l2-device.h). | |
83 | Very simple devices can just allocate this struct, but most of the time you | |
84 | would embed this struct inside a larger struct. | |
85 | ||
86 | You must register the device instance: | |
87 | ||
88 | v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev); | |
89 | ||
95db3a60 | 90 | Registration will initialize the v4l2_device struct. If the dev->driver_data |
2c0ab67b LP |
91 | field is NULL, it will be linked to v4l2_dev. |
92 | ||
93 | Drivers that want integration with the media device framework need to set | |
95db3a60 LP |
94 | dev->driver_data manually to point to the driver-specific device structure |
95 | that embed the struct v4l2_device instance. This is achieved by a | |
2c0ab67b LP |
96 | dev_set_drvdata() call before registering the V4L2 device instance. They must |
97 | also set the struct v4l2_device mdev field to point to a properly initialized | |
98 | and registered media_device instance. | |
95db3a60 LP |
99 | |
100 | If v4l2_dev->name is empty then it will be set to a value derived from dev | |
101 | (driver name followed by the bus_id, to be precise). If you set it up before | |
102 | calling v4l2_device_register then it will be untouched. If dev is NULL, then | |
103 | you *must* setup v4l2_dev->name before calling v4l2_device_register. | |
2a1fcdf0 | 104 | |
102e7813 HV |
105 | You can use v4l2_device_set_name() to set the name based on a driver name and |
106 | a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1, | |
107 | etc. If the name ends with a digit, then it will insert a dash: cx18-0, | |
108 | cx18-1, etc. This function returns the instance number. | |
109 | ||
a47ddf14 | 110 | The first 'dev' argument is normally the struct device pointer of a pci_dev, |
073d696d | 111 | usb_interface or platform_device. It is rare for dev to be NULL, but it happens |
00575961 HV |
112 | with ISA devices or when one device creates multiple PCI devices, thus making |
113 | it impossible to associate v4l2_dev with a particular parent. | |
a47ddf14 | 114 | |
98ec6339 HV |
115 | You can also supply a notify() callback that can be called by sub-devices to |
116 | notify you of events. Whether you need to set this depends on the sub-device. | |
117 | Any notifications a sub-device supports must be defined in a header in | |
118 | include/media/<subdevice>.h. | |
119 | ||
2a1fcdf0 HV |
120 | You unregister with: |
121 | ||
122 | v4l2_device_unregister(struct v4l2_device *v4l2_dev); | |
123 | ||
95db3a60 | 124 | If the dev->driver_data field points to v4l2_dev, it will be reset to NULL. |
2a1fcdf0 HV |
125 | Unregistering will also automatically unregister all subdevs from the device. |
126 | ||
ae6cfaac HV |
127 | If you have a hotpluggable device (e.g. a USB device), then when a disconnect |
128 | happens the parent device becomes invalid. Since v4l2_device has a pointer to | |
129 | that parent device it has to be cleared as well to mark that the parent is | |
130 | gone. To do this call: | |
131 | ||
132 | v4l2_device_disconnect(struct v4l2_device *v4l2_dev); | |
133 | ||
134 | This does *not* unregister the subdevs, so you still need to call the | |
135 | v4l2_device_unregister() function for that. If your driver is not hotpluggable, | |
136 | then there is no need to call v4l2_device_disconnect(). | |
137 | ||
2a1fcdf0 HV |
138 | Sometimes you need to iterate over all devices registered by a specific |
139 | driver. This is usually the case if multiple device drivers use the same | |
140 | hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv | |
141 | hardware. The same is true for alsa drivers for example. | |
142 | ||
143 | You can iterate over all registered devices as follows: | |
144 | ||
145 | static int callback(struct device *dev, void *p) | |
146 | { | |
147 | struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); | |
148 | ||
149 | /* test if this device was inited */ | |
150 | if (v4l2_dev == NULL) | |
151 | return 0; | |
152 | ... | |
153 | return 0; | |
154 | } | |
155 | ||
156 | int iterate(void *p) | |
157 | { | |
158 | struct device_driver *drv; | |
159 | int err; | |
160 | ||
161 | /* Find driver 'ivtv' on the PCI bus. | |
162 | pci_bus_type is a global. For USB busses use usb_bus_type. */ | |
163 | drv = driver_find("ivtv", &pci_bus_type); | |
164 | /* iterate over all ivtv device instances */ | |
165 | err = driver_for_each_device(drv, NULL, p, callback); | |
166 | put_driver(drv); | |
167 | return err; | |
168 | } | |
169 | ||
170 | Sometimes you need to keep a running counter of the device instance. This is | |
171 | commonly used to map a device instance to an index of a module option array. | |
172 | ||
173 | The recommended approach is as follows: | |
174 | ||
175 | static atomic_t drv_instance = ATOMIC_INIT(0); | |
176 | ||
89aec3e1 | 177 | static int __devinit drv_probe(struct pci_dev *pdev, |
2a1fcdf0 HV |
178 | const struct pci_device_id *pci_id) |
179 | { | |
180 | ... | |
181 | state->instance = atomic_inc_return(&drv_instance) - 1; | |
182 | } | |
183 | ||
2335e2b8 HV |
184 | If you have multiple device nodes then it can be difficult to know when it is |
185 | safe to unregister v4l2_device. For this purpose v4l2_device has refcounting | |
186 | support. The refcount is increased whenever video_register_device is called and | |
187 | it is decreased whenever that device node is released. When the refcount reaches | |
188 | zero, then the v4l2_device release() callback is called. You can do your final | |
189 | cleanup there. | |
190 | ||
191 | If other device nodes (e.g. ALSA) are created, then you can increase and | |
192 | decrease the refcount manually as well by calling: | |
193 | ||
194 | void v4l2_device_get(struct v4l2_device *v4l2_dev); | |
195 | ||
196 | or: | |
197 | ||
198 | int v4l2_device_put(struct v4l2_device *v4l2_dev); | |
2a1fcdf0 HV |
199 | |
200 | struct v4l2_subdev | |
201 | ------------------ | |
202 | ||
203 | Many drivers need to communicate with sub-devices. These devices can do all | |
204 | sort of tasks, but most commonly they handle audio and/or video muxing, | |
205 | encoding or decoding. For webcams common sub-devices are sensors and camera | |
206 | controllers. | |
207 | ||
208 | Usually these are I2C devices, but not necessarily. In order to provide the | |
209 | driver with a consistent interface to these sub-devices the v4l2_subdev struct | |
210 | (v4l2-subdev.h) was created. | |
211 | ||
212 | Each sub-device driver must have a v4l2_subdev struct. This struct can be | |
213 | stand-alone for simple sub-devices or it might be embedded in a larger struct | |
214 | if more state information needs to be stored. Usually there is a low-level | |
215 | device struct (e.g. i2c_client) that contains the device data as setup | |
216 | by the kernel. It is recommended to store that pointer in the private | |
217 | data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go | |
218 | from a v4l2_subdev to the actual low-level bus-specific device data. | |
219 | ||
220 | You also need a way to go from the low-level struct to v4l2_subdev. For the | |
221 | common i2c_client struct the i2c_set_clientdata() call is used to store a | |
222 | v4l2_subdev pointer, for other busses you may have to use other methods. | |
223 | ||
692d5522 LP |
224 | Bridges might also need to store per-subdev private data, such as a pointer to |
225 | bridge-specific per-subdev private data. The v4l2_subdev structure provides | |
226 | host private data for that purpose that can be accessed with | |
227 | v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata(). | |
228 | ||
2a1fcdf0 HV |
229 | From the bridge driver perspective you load the sub-device module and somehow |
230 | obtain the v4l2_subdev pointer. For i2c devices this is easy: you call | |
231 | i2c_get_clientdata(). For other busses something similar needs to be done. | |
232 | Helper functions exists for sub-devices on an I2C bus that do most of this | |
233 | tricky work for you. | |
234 | ||
235 | Each v4l2_subdev contains function pointers that sub-device drivers can | |
236 | implement (or leave NULL if it is not applicable). Since sub-devices can do | |
237 | so many different things and you do not want to end up with a huge ops struct | |
238 | of which only a handful of ops are commonly implemented, the function pointers | |
239 | are sorted according to category and each category has its own ops struct. | |
240 | ||
241 | The top-level ops struct contains pointers to the category ops structs, which | |
242 | may be NULL if the subdev driver does not support anything from that category. | |
243 | ||
244 | It looks like this: | |
245 | ||
246 | struct v4l2_subdev_core_ops { | |
aecde8b5 | 247 | int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); |
2a1fcdf0 HV |
248 | int (*log_status)(struct v4l2_subdev *sd); |
249 | int (*init)(struct v4l2_subdev *sd, u32 val); | |
250 | ... | |
251 | }; | |
252 | ||
253 | struct v4l2_subdev_tuner_ops { | |
254 | ... | |
255 | }; | |
256 | ||
257 | struct v4l2_subdev_audio_ops { | |
258 | ... | |
259 | }; | |
260 | ||
261 | struct v4l2_subdev_video_ops { | |
262 | ... | |
263 | }; | |
264 | ||
265 | struct v4l2_subdev_ops { | |
266 | const struct v4l2_subdev_core_ops *core; | |
267 | const struct v4l2_subdev_tuner_ops *tuner; | |
268 | const struct v4l2_subdev_audio_ops *audio; | |
269 | const struct v4l2_subdev_video_ops *video; | |
270 | }; | |
271 | ||
272 | The core ops are common to all subdevs, the other categories are implemented | |
273 | depending on the sub-device. E.g. a video device is unlikely to support the | |
274 | audio ops and vice versa. | |
275 | ||
276 | This setup limits the number of function pointers while still making it easy | |
277 | to add new ops and categories. | |
278 | ||
279 | A sub-device driver initializes the v4l2_subdev struct using: | |
280 | ||
89aec3e1 | 281 | v4l2_subdev_init(sd, &ops); |
2a1fcdf0 HV |
282 | |
283 | Afterwards you need to initialize subdev->name with a unique name and set the | |
284 | module owner. This is done for you if you use the i2c helper functions. | |
285 | ||
61f5db54 LP |
286 | If integration with the media framework is needed, you must initialize the |
287 | media_entity struct embedded in the v4l2_subdev struct (entity field) by | |
288 | calling media_entity_init(): | |
289 | ||
290 | struct media_pad *pads = &my_sd->pads; | |
291 | int err; | |
292 | ||
293 | err = media_entity_init(&sd->entity, npads, pads, 0); | |
294 | ||
295 | The pads array must have been previously initialized. There is no need to | |
296 | manually set the struct media_entity type and name fields, but the revision | |
297 | field must be initialized if needed. | |
298 | ||
299 | A reference to the entity will be automatically acquired/released when the | |
300 | subdev device node (if any) is opened/closed. | |
301 | ||
302 | Don't forget to cleanup the media entity before the sub-device is destroyed: | |
303 | ||
304 | media_entity_cleanup(&sd->entity); | |
305 | ||
2a1fcdf0 HV |
306 | A device (bridge) driver needs to register the v4l2_subdev with the |
307 | v4l2_device: | |
308 | ||
89aec3e1 | 309 | int err = v4l2_device_register_subdev(v4l2_dev, sd); |
2a1fcdf0 HV |
310 | |
311 | This can fail if the subdev module disappeared before it could be registered. | |
312 | After this function was called successfully the subdev->dev field points to | |
313 | the v4l2_device. | |
314 | ||
61f5db54 LP |
315 | If the v4l2_device parent device has a non-NULL mdev field, the sub-device |
316 | entity will be automatically registered with the media device. | |
317 | ||
2a1fcdf0 HV |
318 | You can unregister a sub-device using: |
319 | ||
89aec3e1 | 320 | v4l2_device_unregister_subdev(sd); |
2a1fcdf0 | 321 | |
89aec3e1 | 322 | Afterwards the subdev module can be unloaded and sd->dev == NULL. |
2a1fcdf0 HV |
323 | |
324 | You can call an ops function either directly: | |
325 | ||
89aec3e1 | 326 | err = sd->ops->core->g_chip_ident(sd, &chip); |
2a1fcdf0 HV |
327 | |
328 | but it is better and easier to use this macro: | |
329 | ||
89aec3e1 | 330 | err = v4l2_subdev_call(sd, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
331 | |
332 | The macro will to the right NULL pointer checks and returns -ENODEV if subdev | |
333 | is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is | |
334 | NULL, or the actual result of the subdev->ops->core->g_chip_ident ops. | |
335 | ||
336 | It is also possible to call all or a subset of the sub-devices: | |
337 | ||
89aec3e1 | 338 | v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
339 | |
340 | Any subdev that does not support this ops is skipped and error results are | |
341 | ignored. If you want to check for errors use this: | |
342 | ||
89aec3e1 | 343 | err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
344 | |
345 | Any error except -ENOIOCTLCMD will exit the loop with that error. If no | |
25985edc | 346 | errors (except -ENOIOCTLCMD) occurred, then 0 is returned. |
2a1fcdf0 HV |
347 | |
348 | The second argument to both calls is a group ID. If 0, then all subdevs are | |
349 | called. If non-zero, then only those whose group ID match that value will | |
b0167600 | 350 | be called. Before a bridge driver registers a subdev it can set sd->grp_id |
2a1fcdf0 HV |
351 | to whatever value it wants (it's 0 by default). This value is owned by the |
352 | bridge driver and the sub-device driver will never modify or use it. | |
353 | ||
354 | The group ID gives the bridge driver more control how callbacks are called. | |
355 | For example, there may be multiple audio chips on a board, each capable of | |
356 | changing the volume. But usually only one will actually be used when the | |
357 | user want to change the volume. You can set the group ID for that subdev to | |
358 | e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling | |
359 | v4l2_device_call_all(). That ensures that it will only go to the subdev | |
360 | that needs it. | |
361 | ||
98ec6339 HV |
362 | If the sub-device needs to notify its v4l2_device parent of an event, then |
363 | it can call v4l2_subdev_notify(sd, notification, arg). This macro checks | |
364 | whether there is a notify() callback defined and returns -ENODEV if not. | |
365 | Otherwise the result of the notify() call is returned. | |
366 | ||
2a1fcdf0 HV |
367 | The advantage of using v4l2_subdev is that it is a generic struct and does |
368 | not contain any knowledge about the underlying hardware. So a driver might | |
369 | contain several subdevs that use an I2C bus, but also a subdev that is | |
370 | controlled through GPIO pins. This distinction is only relevant when setting | |
371 | up the device, but once the subdev is registered it is completely transparent. | |
372 | ||
373 | ||
2096a5dc LP |
374 | V4L2 sub-device userspace API |
375 | ----------------------------- | |
376 | ||
377 | Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2 | |
378 | sub-devices can also be controlled directly by userspace applications. | |
379 | ||
380 | Device nodes named v4l-subdevX can be created in /dev to access sub-devices | |
381 | directly. If a sub-device supports direct userspace configuration it must set | |
382 | the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered. | |
383 | ||
384 | After registering sub-devices, the v4l2_device driver can create device nodes | |
385 | for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling | |
386 | v4l2_device_register_subdev_nodes(). Those device nodes will be automatically | |
387 | removed when sub-devices are unregistered. | |
388 | ||
ea8aa434 LP |
389 | The device node handles a subset of the V4L2 API. |
390 | ||
391 | VIDIOC_QUERYCTRL | |
392 | VIDIOC_QUERYMENU | |
393 | VIDIOC_G_CTRL | |
394 | VIDIOC_S_CTRL | |
395 | VIDIOC_G_EXT_CTRLS | |
396 | VIDIOC_S_EXT_CTRLS | |
397 | VIDIOC_TRY_EXT_CTRLS | |
398 | ||
399 | The controls ioctls are identical to the ones defined in V4L2. They | |
400 | behave identically, with the only exception that they deal only with | |
401 | controls implemented in the sub-device. Depending on the driver, those | |
402 | controls can be also be accessed through one (or several) V4L2 device | |
403 | nodes. | |
404 | ||
02adb1cc SA |
405 | VIDIOC_DQEVENT |
406 | VIDIOC_SUBSCRIBE_EVENT | |
407 | VIDIOC_UNSUBSCRIBE_EVENT | |
408 | ||
409 | The events ioctls are identical to the ones defined in V4L2. They | |
410 | behave identically, with the only exception that they deal only with | |
411 | events generated by the sub-device. Depending on the driver, those | |
412 | events can also be reported by one (or several) V4L2 device nodes. | |
413 | ||
414 | Sub-device drivers that want to use events need to set the | |
415 | V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize | |
416 | v4l2_subdev::nevents to events queue depth before registering the | |
417 | sub-device. After registration events can be queued as usual on the | |
418 | v4l2_subdev::devnode device node. | |
419 | ||
420 | To properly support events, the poll() file operation is also | |
421 | implemented. | |
422 | ||
c30b46e5 LP |
423 | Private ioctls |
424 | ||
425 | All ioctls not in the above list are passed directly to the sub-device | |
426 | driver through the core::ioctl operation. | |
427 | ||
2096a5dc | 428 | |
2a1fcdf0 HV |
429 | I2C sub-device drivers |
430 | ---------------------- | |
431 | ||
432 | Since these drivers are so common, special helper functions are available to | |
433 | ease the use of these drivers (v4l2-common.h). | |
434 | ||
435 | The recommended method of adding v4l2_subdev support to an I2C driver is to | |
436 | embed the v4l2_subdev struct into the state struct that is created for each | |
437 | I2C device instance. Very simple devices have no state struct and in that case | |
438 | you can just create a v4l2_subdev directly. | |
439 | ||
440 | A typical state struct would look like this (where 'chipname' is replaced by | |
441 | the name of the chip): | |
442 | ||
443 | struct chipname_state { | |
444 | struct v4l2_subdev sd; | |
445 | ... /* additional state fields */ | |
446 | }; | |
447 | ||
448 | Initialize the v4l2_subdev struct as follows: | |
449 | ||
450 | v4l2_i2c_subdev_init(&state->sd, client, subdev_ops); | |
451 | ||
452 | This function will fill in all the fields of v4l2_subdev and ensure that the | |
453 | v4l2_subdev and i2c_client both point to one another. | |
454 | ||
455 | You should also add a helper inline function to go from a v4l2_subdev pointer | |
456 | to a chipname_state struct: | |
457 | ||
458 | static inline struct chipname_state *to_state(struct v4l2_subdev *sd) | |
459 | { | |
460 | return container_of(sd, struct chipname_state, sd); | |
461 | } | |
462 | ||
463 | Use this to go from the v4l2_subdev struct to the i2c_client struct: | |
464 | ||
465 | struct i2c_client *client = v4l2_get_subdevdata(sd); | |
466 | ||
467 | And this to go from an i2c_client to a v4l2_subdev struct: | |
468 | ||
469 | struct v4l2_subdev *sd = i2c_get_clientdata(client); | |
470 | ||
2a1fcdf0 HV |
471 | Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback |
472 | is called. This will unregister the sub-device from the bridge driver. It is | |
473 | safe to call this even if the sub-device was never registered. | |
474 | ||
f5360bdc HV |
475 | You need to do this because when the bridge driver destroys the i2c adapter |
476 | the remove() callbacks are called of the i2c devices on that adapter. | |
477 | After that the corresponding v4l2_subdev structures are invalid, so they | |
478 | have to be unregistered first. Calling v4l2_device_unregister_subdev(sd) | |
479 | from the remove() callback ensures that this is always done correctly. | |
480 | ||
2a1fcdf0 HV |
481 | |
482 | The bridge driver also has some helper functions it can use: | |
483 | ||
e6574f2f | 484 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter, |
53dacb15 | 485 | "module_foo", "chipid", 0x36, NULL); |
2a1fcdf0 HV |
486 | |
487 | This loads the given module (can be NULL if no module needs to be loaded) and | |
488 | calls i2c_new_device() with the given i2c_adapter and chip/address arguments. | |
e6574f2f | 489 | If all goes well, then it registers the subdev with the v4l2_device. |
2a1fcdf0 | 490 | |
53dacb15 HV |
491 | You can also use the last argument of v4l2_i2c_new_subdev() to pass an array |
492 | of possible I2C addresses that it should probe. These probe addresses are | |
493 | only used if the previous argument is 0. A non-zero argument means that you | |
494 | know the exact i2c address so in that case no probing will take place. | |
2a1fcdf0 HV |
495 | |
496 | Both functions return NULL if something went wrong. | |
497 | ||
53dacb15 | 498 | Note that the chipid you pass to v4l2_i2c_new_subdev() is usually |
2c792523 HV |
499 | the same as the module name. It allows you to specify a chip variant, e.g. |
500 | "saa7114" or "saa7115". In general though the i2c driver autodetects this. | |
501 | The use of chipid is something that needs to be looked at more closely at a | |
502 | later date. It differs between i2c drivers and as such can be confusing. | |
503 | To see which chip variants are supported you can look in the i2c driver code | |
504 | for the i2c_device_id table. This lists all the possibilities. | |
505 | ||
2c0b19ac HV |
506 | There are two more helper functions: |
507 | ||
508 | v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data | |
509 | arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not | |
510 | 0 then that will be used (non-probing variant), otherwise the probed_addrs | |
511 | are probed. | |
512 | ||
513 | For example: this will probe for address 0x10: | |
514 | ||
515 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter, | |
516 | "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10)); | |
517 | ||
518 | v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed | |
519 | to the i2c driver and replaces the irq, platform_data and addr arguments. | |
520 | ||
521 | If the subdev supports the s_config core ops, then that op is called with | |
522 | the irq and platform_data arguments after the subdev was setup. The older | |
523 | v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with | |
524 | irq set to 0 and platform_data set to NULL. | |
525 | ||
2a1fcdf0 HV |
526 | struct video_device |
527 | ------------------- | |
528 | ||
a47ddf14 HV |
529 | The actual device nodes in the /dev directory are created using the |
530 | video_device struct (v4l2-dev.h). This struct can either be allocated | |
531 | dynamically or embedded in a larger struct. | |
532 | ||
533 | To allocate it dynamically use: | |
534 | ||
535 | struct video_device *vdev = video_device_alloc(); | |
536 | ||
537 | if (vdev == NULL) | |
538 | return -ENOMEM; | |
539 | ||
540 | vdev->release = video_device_release; | |
541 | ||
542 | If you embed it in a larger struct, then you must set the release() | |
543 | callback to your own function: | |
544 | ||
545 | struct video_device *vdev = &my_vdev->vdev; | |
546 | ||
547 | vdev->release = my_vdev_release; | |
548 | ||
549 | The release callback must be set and it is called when the last user | |
550 | of the video device exits. | |
551 | ||
552 | The default video_device_release() callback just calls kfree to free the | |
553 | allocated memory. | |
554 | ||
555 | You should also set these fields: | |
556 | ||
dfa9a5ae | 557 | - v4l2_dev: set to the v4l2_device parent device. |
a47ddf14 | 558 | - name: set to something descriptive and unique. |
c7dd09da | 559 | - fops: set to the v4l2_file_operations struct. |
a47ddf14 HV |
560 | - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance |
561 | (highly recommended to use this and it might become compulsory in the | |
562 | future!), then set this to your v4l2_ioctl_ops struct. | |
ee6869af HV |
563 | - lock: leave to NULL if you want to do all the locking in the driver. |
564 | Otherwise you give it a pointer to a struct mutex_lock and before any | |
565 | of the v4l2_file_operations is called this lock will be taken by the | |
566 | core and released afterwards. | |
6e29ad50 HV |
567 | - prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY. |
568 | If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device. | |
569 | If you want to have a separate priority state per (group of) device node(s), | |
570 | then you can point it to your own struct v4l2_prio_state. | |
00575961 HV |
571 | - parent: you only set this if v4l2_device was registered with NULL as |
572 | the parent device struct. This only happens in cases where one hardware | |
573 | device has multiple PCI devices that all share the same v4l2_device core. | |
574 | ||
575 | The cx88 driver is an example of this: one core v4l2_device struct, but | |
576 | it is used by both an raw video PCI device (cx8800) and a MPEG PCI device | |
577 | (cx8802). Since the v4l2_device cannot be associated with a particular | |
578 | PCI device it is setup without a parent device. But when the struct | |
579 | video_device is setup you do know which parent PCI device to use. | |
b1a873a3 HV |
580 | - flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework |
581 | handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct | |
582 | v4l2_fh. Eventually this flag will disappear once all drivers use the core | |
583 | priority handling. But for now it has to be set explicitly. | |
a47ddf14 | 584 | |
6e29ad50 HV |
585 | If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2 |
586 | in your v4l2_file_operations struct. | |
587 | ||
588 | Do not use .ioctl! This is deprecated and will go away in the future. | |
c7dd09da HV |
589 | |
590 | The v4l2_file_operations struct is a subset of file_operations. The main | |
591 | difference is that the inode argument is omitted since it is never used. | |
a47ddf14 | 592 | |
2c0ab67b LP |
593 | If integration with the media framework is needed, you must initialize the |
594 | media_entity struct embedded in the video_device struct (entity field) by | |
595 | calling media_entity_init(): | |
596 | ||
597 | struct media_pad *pad = &my_vdev->pad; | |
598 | int err; | |
599 | ||
600 | err = media_entity_init(&vdev->entity, 1, pad, 0); | |
601 | ||
602 | The pads array must have been previously initialized. There is no need to | |
603 | manually set the struct media_entity type and name fields. | |
604 | ||
605 | A reference to the entity will be automatically acquired/released when the | |
606 | video device is opened/closed. | |
607 | ||
ee6869af HV |
608 | v4l2_file_operations and locking |
609 | -------------------------------- | |
610 | ||
611 | You can set a pointer to a mutex_lock in struct video_device. Usually this | |
612 | will be either a top-level mutex or a mutex per device node. If you want | |
613 | finer-grained locking then you have to set it to NULL and do you own locking. | |
614 | ||
615 | If a lock is specified then all file operations will be serialized on that | |
616 | lock. If you use videobuf then you must pass the same lock to the videobuf | |
617 | queue initialize function: if videobuf has to wait for a frame to arrive, then | |
618 | it will temporarily unlock the lock and relock it afterwards. If your driver | |
619 | also waits in the code, then you should do the same to allow other processes | |
620 | to access the device node while the first process is waiting for something. | |
621 | ||
622 | The implementation of a hotplug disconnect should also take the lock before | |
9c84d89b | 623 | calling v4l2_device_disconnect. |
a47ddf14 HV |
624 | |
625 | video_device registration | |
626 | ------------------------- | |
627 | ||
628 | Next you register the video device: this will create the character device | |
629 | for you. | |
630 | ||
631 | err = video_register_device(vdev, VFL_TYPE_GRABBER, -1); | |
632 | if (err) { | |
50a2a8b3 | 633 | video_device_release(vdev); /* or kfree(my_vdev); */ |
a47ddf14 HV |
634 | return err; |
635 | } | |
636 | ||
2c0ab67b LP |
637 | If the v4l2_device parent device has a non-NULL mdev field, the video device |
638 | entity will be automatically registered with the media device. | |
639 | ||
a47ddf14 HV |
640 | Which device is registered depends on the type argument. The following |
641 | types exist: | |
642 | ||
643 | VFL_TYPE_GRABBER: videoX for video input/output devices | |
644 | VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) | |
645 | VFL_TYPE_RADIO: radioX for radio tuners | |
a47ddf14 HV |
646 | |
647 | The last argument gives you a certain amount of control over the device | |
6b5270d2 HV |
648 | device node number used (i.e. the X in videoX). Normally you will pass -1 |
649 | to let the v4l2 framework pick the first free number. But sometimes users | |
650 | want to select a specific node number. It is common that drivers allow | |
651 | the user to select a specific device node number through a driver module | |
652 | option. That number is then passed to this function and video_register_device | |
653 | will attempt to select that device node number. If that number was already | |
654 | in use, then the next free device node number will be selected and it | |
655 | will send a warning to the kernel log. | |
656 | ||
657 | Another use-case is if a driver creates many devices. In that case it can | |
658 | be useful to place different video devices in separate ranges. For example, | |
659 | video capture devices start at 0, video output devices start at 16. | |
22e22125 HV |
660 | So you can use the last argument to specify a minimum device node number |
661 | and the v4l2 framework will try to pick the first free number that is equal | |
a47ddf14 HV |
662 | or higher to what you passed. If that fails, then it will just pick the |
663 | first free number. | |
664 | ||
6b5270d2 HV |
665 | Since in this case you do not care about a warning about not being able |
666 | to select the specified device node number, you can call the function | |
667 | video_register_device_no_warn() instead. | |
668 | ||
a47ddf14 HV |
669 | Whenever a device node is created some attributes are also created for you. |
670 | If you look in /sys/class/video4linux you see the devices. Go into e.g. | |
671 | video0 and you will see 'name' and 'index' attributes. The 'name' attribute | |
7ae0cd9b | 672 | is the 'name' field of the video_device struct. |
a47ddf14 | 673 | |
7ae0cd9b HV |
674 | The 'index' attribute is the index of the device node: for each call to |
675 | video_register_device() the index is just increased by 1. The first video | |
676 | device node you register always starts with index 0. | |
a47ddf14 HV |
677 | |
678 | Users can setup udev rules that utilize the index attribute to make fancy | |
679 | device names (e.g. 'mpegX' for MPEG video capture device nodes). | |
680 | ||
681 | After the device was successfully registered, then you can use these fields: | |
682 | ||
683 | - vfl_type: the device type passed to video_register_device. | |
684 | - minor: the assigned device minor number. | |
22e22125 | 685 | - num: the device node number (i.e. the X in videoX). |
7ae0cd9b | 686 | - index: the device index number. |
a47ddf14 HV |
687 | |
688 | If the registration failed, then you need to call video_device_release() | |
689 | to free the allocated video_device struct, or free your own struct if the | |
690 | video_device was embedded in it. The vdev->release() callback will never | |
691 | be called if the registration failed, nor should you ever attempt to | |
692 | unregister the device if the registration failed. | |
693 | ||
694 | ||
695 | video_device cleanup | |
696 | -------------------- | |
697 | ||
698 | When the video device nodes have to be removed, either during the unload | |
699 | of the driver or because the USB device was disconnected, then you should | |
700 | unregister them: | |
701 | ||
702 | video_unregister_device(vdev); | |
703 | ||
704 | This will remove the device nodes from sysfs (causing udev to remove them | |
705 | from /dev). | |
706 | ||
dd1ad942 HV |
707 | After video_unregister_device() returns no new opens can be done. However, |
708 | in the case of USB devices some application might still have one of these | |
d69f2718 HV |
709 | device nodes open. So after the unregister all file operations (except |
710 | release, of course) will return an error as well. | |
a47ddf14 HV |
711 | |
712 | When the last user of the video device node exits, then the vdev->release() | |
713 | callback is called and you can do the final cleanup there. | |
714 | ||
2c0ab67b LP |
715 | Don't forget to cleanup the media entity associated with the video device if |
716 | it has been initialized: | |
717 | ||
718 | media_entity_cleanup(&vdev->entity); | |
719 | ||
720 | This can be done from the release callback. | |
721 | ||
a47ddf14 HV |
722 | |
723 | video_device helper functions | |
724 | ----------------------------- | |
725 | ||
726 | There are a few useful helper functions: | |
727 | ||
eac8ea53 LP |
728 | - file/video_device private data |
729 | ||
a47ddf14 HV |
730 | You can set/get driver private data in the video_device struct using: |
731 | ||
89aec3e1 HV |
732 | void *video_get_drvdata(struct video_device *vdev); |
733 | void video_set_drvdata(struct video_device *vdev, void *data); | |
a47ddf14 HV |
734 | |
735 | Note that you can safely call video_set_drvdata() before calling | |
736 | video_register_device(). | |
737 | ||
738 | And this function: | |
739 | ||
740 | struct video_device *video_devdata(struct file *file); | |
741 | ||
742 | returns the video_device belonging to the file struct. | |
743 | ||
eac8ea53 | 744 | The video_drvdata function combines video_get_drvdata with video_devdata: |
a47ddf14 HV |
745 | |
746 | void *video_drvdata(struct file *file); | |
747 | ||
748 | You can go from a video_device struct to the v4l2_device struct using: | |
749 | ||
dfa9a5ae | 750 | struct v4l2_device *v4l2_dev = vdev->v4l2_dev; |
44061c05 | 751 | |
eac8ea53 LP |
752 | - Device node name |
753 | ||
754 | The video_device node kernel name can be retrieved using | |
755 | ||
756 | const char *video_device_node_name(struct video_device *vdev); | |
757 | ||
758 | The name is used as a hint by userspace tools such as udev. The function | |
759 | should be used where possible instead of accessing the video_device::num and | |
760 | video_device::minor fields. | |
761 | ||
762 | ||
44061c05 MCC |
763 | video buffer helper functions |
764 | ----------------------------- | |
765 | ||
4b586a38 JC |
766 | The v4l2 core API provides a set of standard methods (called "videobuf") |
767 | for dealing with video buffers. Those methods allow a driver to implement | |
768 | read(), mmap() and overlay() in a consistent way. There are currently | |
769 | methods for using video buffers on devices that supports DMA with | |
770 | scatter/gather method (videobuf-dma-sg), DMA with linear access | |
771 | (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers | |
772 | (videobuf-vmalloc). | |
773 | ||
774 | Please see Documentation/video4linux/videobuf for more information on how | |
775 | to use the videobuf layer. | |
6cd84b78 SA |
776 | |
777 | struct v4l2_fh | |
778 | -------------- | |
779 | ||
780 | struct v4l2_fh provides a way to easily keep file handle specific data | |
6e29ad50 | 781 | that is used by the V4L2 framework. New drivers must use struct v4l2_fh |
b1a873a3 HV |
782 | since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY) |
783 | if the video_device flag V4L2_FL_USE_FH_PRIO is also set. | |
6cd84b78 SA |
784 | |
785 | The users of v4l2_fh (in the V4L2 framework, not the driver) know | |
786 | whether a driver uses v4l2_fh as its file->private_data pointer by | |
6e29ad50 HV |
787 | testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is |
788 | set whenever v4l2_fh_init() is called. | |
6cd84b78 | 789 | |
6e29ad50 HV |
790 | struct v4l2_fh is allocated as a part of the driver's own file handle |
791 | structure and file->private_data is set to it in the driver's open | |
792 | function by the driver. | |
6cd84b78 | 793 | |
6e29ad50 HV |
794 | In many cases the struct v4l2_fh will be embedded in a larger structure. |
795 | In that case you should call v4l2_fh_init+v4l2_fh_add in open() and | |
796 | v4l2_fh_del+v4l2_fh_exit in release(). | |
6cd84b78 | 797 | |
6e29ad50 HV |
798 | Drivers can extract their own file handle structure by using the container_of |
799 | macro. Example: | |
6cd84b78 SA |
800 | |
801 | struct my_fh { | |
802 | int blah; | |
803 | struct v4l2_fh fh; | |
804 | }; | |
805 | ||
806 | ... | |
807 | ||
808 | int my_open(struct file *file) | |
809 | { | |
810 | struct my_fh *my_fh; | |
811 | struct video_device *vfd; | |
812 | int ret; | |
813 | ||
814 | ... | |
815 | ||
6e29ad50 HV |
816 | my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL); |
817 | ||
818 | ... | |
819 | ||
98019f5e | 820 | v4l2_fh_init(&my_fh->fh, vfd); |
6cd84b78 | 821 | |
6e29ad50 | 822 | ... |
6cd84b78 SA |
823 | |
824 | file->private_data = &my_fh->fh; | |
6e29ad50 HV |
825 | v4l2_fh_add(&my_fh->fh); |
826 | return 0; | |
6cd84b78 SA |
827 | } |
828 | ||
829 | int my_release(struct file *file) | |
830 | { | |
831 | struct v4l2_fh *fh = file->private_data; | |
832 | struct my_fh *my_fh = container_of(fh, struct my_fh, fh); | |
833 | ||
834 | ... | |
6e29ad50 HV |
835 | v4l2_fh_del(&my_fh->fh); |
836 | v4l2_fh_exit(&my_fh->fh); | |
837 | kfree(my_fh); | |
838 | return 0; | |
6cd84b78 | 839 | } |
dd966083 | 840 | |
6e29ad50 HV |
841 | Below is a short description of the v4l2_fh functions used: |
842 | ||
98019f5e | 843 | void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev) |
6e29ad50 HV |
844 | |
845 | Initialise the file handle. This *MUST* be performed in the driver's | |
846 | v4l2_file_operations->open() handler. | |
847 | ||
848 | void v4l2_fh_add(struct v4l2_fh *fh) | |
849 | ||
850 | Add a v4l2_fh to video_device file handle list. Must be called once the | |
851 | file handle is completely initialized. | |
852 | ||
853 | void v4l2_fh_del(struct v4l2_fh *fh) | |
854 | ||
855 | Unassociate the file handle from video_device(). The file handle | |
856 | exit function may now be called. | |
857 | ||
858 | void v4l2_fh_exit(struct v4l2_fh *fh) | |
859 | ||
860 | Uninitialise the file handle. After uninitialisation the v4l2_fh | |
861 | memory can be freed. | |
862 | ||
863 | ||
864 | If struct v4l2_fh is not embedded, then you can use these helper functions: | |
865 | ||
866 | int v4l2_fh_open(struct file *filp) | |
867 | ||
868 | This allocates a struct v4l2_fh, initializes it and adds it to the struct | |
869 | video_device associated with the file struct. | |
870 | ||
871 | int v4l2_fh_release(struct file *filp) | |
872 | ||
873 | This deletes it from the struct video_device associated with the file | |
874 | struct, uninitialised the v4l2_fh and frees it. | |
875 | ||
876 | These two functions can be plugged into the v4l2_file_operation's open() and | |
877 | release() ops. | |
878 | ||
879 | ||
880 | Several drivers need to do something when the first file handle is opened and | |
881 | when the last file handle closes. Two helper functions were added to check | |
882 | whether the v4l2_fh struct is the only open filehandle of the associated | |
883 | device node: | |
884 | ||
885 | int v4l2_fh_is_singular(struct v4l2_fh *fh) | |
886 | ||
887 | Returns 1 if the file handle is the only open file handle, else 0. | |
888 | ||
889 | int v4l2_fh_is_singular_file(struct file *filp) | |
890 | ||
891 | Same, but it calls v4l2_fh_is_singular with filp->private_data. | |
892 | ||
893 | ||
dd966083 SA |
894 | V4L2 events |
895 | ----------- | |
896 | ||
897 | The V4L2 events provide a generic way to pass events to user space. | |
898 | The driver must use v4l2_fh to be able to support V4L2 events. | |
899 | ||
900 | Useful functions: | |
901 | ||
902 | - v4l2_event_alloc() | |
903 | ||
904 | To use events, the driver must allocate events for the file handle. By | |
905 | calling the function more than once, the driver may assure that at least n | |
906 | events in total have been allocated. The function may not be called in | |
907 | atomic context. | |
908 | ||
909 | - v4l2_event_queue() | |
910 | ||
911 | Queue events to video device. The driver's only responsibility is to fill | |
912 | in the type and the data fields. The other fields will be filled in by | |
913 | V4L2. | |
914 | ||
915 | - v4l2_event_subscribe() | |
916 | ||
917 | The video_device->ioctl_ops->vidioc_subscribe_event must check the driver | |
918 | is able to produce events with specified event id. Then it calls | |
919 | v4l2_event_subscribe() to subscribe the event. | |
920 | ||
921 | - v4l2_event_unsubscribe() | |
922 | ||
923 | vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use | |
924 | v4l2_event_unsubscribe() directly unless it wants to be involved in | |
925 | unsubscription process. | |
926 | ||
927 | The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The | |
928 | drivers may want to handle this in a special way. | |
929 | ||
930 | - v4l2_event_pending() | |
931 | ||
932 | Returns the number of pending events. Useful when implementing poll. | |
933 | ||
934 | Drivers do not initialise events directly. The events are initialised | |
935 | through v4l2_fh_init() if video_device->ioctl_ops->vidioc_subscribe_event is | |
936 | non-NULL. This *MUST* be performed in the driver's | |
937 | v4l2_file_operations->open() handler. | |
938 | ||
939 | Events are delivered to user space through the poll system call. The driver | |
940 | can use v4l2_fh->events->wait wait_queue_head_t as the argument for | |
941 | poll_wait(). | |
942 | ||
943 | There are standard and private events. New standard events must use the | |
944 | smallest available event type. The drivers must allocate their events from | |
945 | their own class starting from class base. Class base is | |
946 | V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number. | |
947 | The first event type in the class is reserved for future use, so the first | |
948 | available event type is 'class base + 1'. | |
949 | ||
950 | An example on how the V4L2 events may be used can be found in the OMAP | |
951 | 3 ISP driver available at <URL:http://gitorious.org/omap3camera> as of | |
952 | writing this. |