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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 | |
f818b358 | 71 | and the v4l2_fh struct keeps track of filehandle instances. |
2a1fcdf0 | 72 | |
2c0ab67b LP |
73 | The V4L2 framework also optionally integrates with the media framework. If a |
74 | driver sets the struct v4l2_device mdev field, sub-devices and video nodes | |
75 | will automatically appear in the media framework as entities. | |
76 | ||
2a1fcdf0 HV |
77 | |
78 | struct v4l2_device | |
79 | ------------------ | |
80 | ||
81 | Each device instance is represented by a struct v4l2_device (v4l2-device.h). | |
82 | Very simple devices can just allocate this struct, but most of the time you | |
83 | would embed this struct inside a larger struct. | |
84 | ||
85 | You must register the device instance: | |
86 | ||
87 | v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev); | |
88 | ||
95db3a60 | 89 | Registration will initialize the v4l2_device struct. If the dev->driver_data |
2c0ab67b LP |
90 | field is NULL, it will be linked to v4l2_dev. |
91 | ||
92 | Drivers that want integration with the media device framework need to set | |
95db3a60 LP |
93 | dev->driver_data manually to point to the driver-specific device structure |
94 | that embed the struct v4l2_device instance. This is achieved by a | |
2c0ab67b LP |
95 | dev_set_drvdata() call before registering the V4L2 device instance. They must |
96 | also set the struct v4l2_device mdev field to point to a properly initialized | |
97 | and registered media_device instance. | |
95db3a60 LP |
98 | |
99 | If v4l2_dev->name is empty then it will be set to a value derived from dev | |
100 | (driver name followed by the bus_id, to be precise). If you set it up before | |
101 | calling v4l2_device_register then it will be untouched. If dev is NULL, then | |
102 | you *must* setup v4l2_dev->name before calling v4l2_device_register. | |
2a1fcdf0 | 103 | |
102e7813 HV |
104 | You can use v4l2_device_set_name() to set the name based on a driver name and |
105 | a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1, | |
106 | etc. If the name ends with a digit, then it will insert a dash: cx18-0, | |
107 | cx18-1, etc. This function returns the instance number. | |
108 | ||
a47ddf14 | 109 | The first 'dev' argument is normally the struct device pointer of a pci_dev, |
073d696d | 110 | usb_interface or platform_device. It is rare for dev to be NULL, but it happens |
00575961 HV |
111 | with ISA devices or when one device creates multiple PCI devices, thus making |
112 | it impossible to associate v4l2_dev with a particular parent. | |
a47ddf14 | 113 | |
98ec6339 HV |
114 | You can also supply a notify() callback that can be called by sub-devices to |
115 | notify you of events. Whether you need to set this depends on the sub-device. | |
116 | Any notifications a sub-device supports must be defined in a header in | |
117 | include/media/<subdevice>.h. | |
118 | ||
2a1fcdf0 HV |
119 | You unregister with: |
120 | ||
121 | v4l2_device_unregister(struct v4l2_device *v4l2_dev); | |
122 | ||
95db3a60 | 123 | If the dev->driver_data field points to v4l2_dev, it will be reset to NULL. |
2a1fcdf0 HV |
124 | Unregistering will also automatically unregister all subdevs from the device. |
125 | ||
ae6cfaac HV |
126 | If you have a hotpluggable device (e.g. a USB device), then when a disconnect |
127 | happens the parent device becomes invalid. Since v4l2_device has a pointer to | |
128 | that parent device it has to be cleared as well to mark that the parent is | |
129 | gone. To do this call: | |
130 | ||
131 | v4l2_device_disconnect(struct v4l2_device *v4l2_dev); | |
132 | ||
133 | This does *not* unregister the subdevs, so you still need to call the | |
134 | v4l2_device_unregister() function for that. If your driver is not hotpluggable, | |
135 | then there is no need to call v4l2_device_disconnect(). | |
136 | ||
2a1fcdf0 HV |
137 | Sometimes you need to iterate over all devices registered by a specific |
138 | driver. This is usually the case if multiple device drivers use the same | |
139 | hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv | |
140 | hardware. The same is true for alsa drivers for example. | |
141 | ||
142 | You can iterate over all registered devices as follows: | |
143 | ||
144 | static int callback(struct device *dev, void *p) | |
145 | { | |
146 | struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); | |
147 | ||
148 | /* test if this device was inited */ | |
149 | if (v4l2_dev == NULL) | |
150 | return 0; | |
151 | ... | |
152 | return 0; | |
153 | } | |
154 | ||
155 | int iterate(void *p) | |
156 | { | |
157 | struct device_driver *drv; | |
158 | int err; | |
159 | ||
160 | /* Find driver 'ivtv' on the PCI bus. | |
161 | pci_bus_type is a global. For USB busses use usb_bus_type. */ | |
162 | drv = driver_find("ivtv", &pci_bus_type); | |
163 | /* iterate over all ivtv device instances */ | |
164 | err = driver_for_each_device(drv, NULL, p, callback); | |
165 | put_driver(drv); | |
166 | return err; | |
167 | } | |
168 | ||
169 | Sometimes you need to keep a running counter of the device instance. This is | |
170 | commonly used to map a device instance to an index of a module option array. | |
171 | ||
172 | The recommended approach is as follows: | |
173 | ||
174 | static atomic_t drv_instance = ATOMIC_INIT(0); | |
175 | ||
63a29f74 | 176 | static int drv_probe(struct pci_dev *pdev, const struct pci_device_id *pci_id) |
2a1fcdf0 HV |
177 | { |
178 | ... | |
179 | state->instance = atomic_inc_return(&drv_instance) - 1; | |
180 | } | |
181 | ||
2335e2b8 | 182 | If you have multiple device nodes then it can be difficult to know when it is |
ee71e7b3 HV |
183 | safe to unregister v4l2_device for hotpluggable devices. For this purpose |
184 | v4l2_device has refcounting support. The refcount is increased whenever | |
185 | video_register_device is called and it is decreased whenever that device node | |
186 | is released. When the refcount reaches zero, then the v4l2_device release() | |
187 | callback is called. You can do your final cleanup there. | |
2335e2b8 HV |
188 | |
189 | If other device nodes (e.g. ALSA) are created, then you can increase and | |
190 | decrease the refcount manually as well by calling: | |
191 | ||
192 | void v4l2_device_get(struct v4l2_device *v4l2_dev); | |
193 | ||
194 | or: | |
195 | ||
196 | int v4l2_device_put(struct v4l2_device *v4l2_dev); | |
2a1fcdf0 | 197 | |
ee71e7b3 HV |
198 | Since the initial refcount is 1 you also need to call v4l2_device_put in the |
199 | disconnect() callback (for USB devices) or in the remove() callback (for e.g. | |
200 | PCI devices), otherwise the refcount will never reach 0. | |
201 | ||
2a1fcdf0 HV |
202 | struct v4l2_subdev |
203 | ------------------ | |
204 | ||
205 | Many drivers need to communicate with sub-devices. These devices can do all | |
206 | sort of tasks, but most commonly they handle audio and/or video muxing, | |
207 | encoding or decoding. For webcams common sub-devices are sensors and camera | |
208 | controllers. | |
209 | ||
210 | Usually these are I2C devices, but not necessarily. In order to provide the | |
211 | driver with a consistent interface to these sub-devices the v4l2_subdev struct | |
212 | (v4l2-subdev.h) was created. | |
213 | ||
214 | Each sub-device driver must have a v4l2_subdev struct. This struct can be | |
215 | stand-alone for simple sub-devices or it might be embedded in a larger struct | |
216 | if more state information needs to be stored. Usually there is a low-level | |
217 | device struct (e.g. i2c_client) that contains the device data as setup | |
218 | by the kernel. It is recommended to store that pointer in the private | |
219 | data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go | |
220 | from a v4l2_subdev to the actual low-level bus-specific device data. | |
221 | ||
222 | You also need a way to go from the low-level struct to v4l2_subdev. For the | |
223 | common i2c_client struct the i2c_set_clientdata() call is used to store a | |
224 | v4l2_subdev pointer, for other busses you may have to use other methods. | |
225 | ||
692d5522 LP |
226 | Bridges might also need to store per-subdev private data, such as a pointer to |
227 | bridge-specific per-subdev private data. The v4l2_subdev structure provides | |
228 | host private data for that purpose that can be accessed with | |
229 | v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata(). | |
230 | ||
2a1fcdf0 HV |
231 | From the bridge driver perspective you load the sub-device module and somehow |
232 | obtain the v4l2_subdev pointer. For i2c devices this is easy: you call | |
233 | i2c_get_clientdata(). For other busses something similar needs to be done. | |
234 | Helper functions exists for sub-devices on an I2C bus that do most of this | |
235 | tricky work for you. | |
236 | ||
237 | Each v4l2_subdev contains function pointers that sub-device drivers can | |
238 | implement (or leave NULL if it is not applicable). Since sub-devices can do | |
239 | so many different things and you do not want to end up with a huge ops struct | |
240 | of which only a handful of ops are commonly implemented, the function pointers | |
241 | are sorted according to category and each category has its own ops struct. | |
242 | ||
243 | The top-level ops struct contains pointers to the category ops structs, which | |
244 | may be NULL if the subdev driver does not support anything from that category. | |
245 | ||
246 | It looks like this: | |
247 | ||
248 | struct v4l2_subdev_core_ops { | |
2a1fcdf0 HV |
249 | int (*log_status)(struct v4l2_subdev *sd); |
250 | int (*init)(struct v4l2_subdev *sd, u32 val); | |
251 | ... | |
252 | }; | |
253 | ||
254 | struct v4l2_subdev_tuner_ops { | |
255 | ... | |
256 | }; | |
257 | ||
258 | struct v4l2_subdev_audio_ops { | |
259 | ... | |
260 | }; | |
261 | ||
262 | struct v4l2_subdev_video_ops { | |
263 | ... | |
264 | }; | |
265 | ||
48398f93 SA |
266 | struct v4l2_subdev_pad_ops { |
267 | ... | |
268 | }; | |
269 | ||
2a1fcdf0 HV |
270 | struct v4l2_subdev_ops { |
271 | const struct v4l2_subdev_core_ops *core; | |
272 | const struct v4l2_subdev_tuner_ops *tuner; | |
273 | const struct v4l2_subdev_audio_ops *audio; | |
274 | const struct v4l2_subdev_video_ops *video; | |
48398f93 | 275 | const struct v4l2_subdev_pad_ops *video; |
2a1fcdf0 HV |
276 | }; |
277 | ||
278 | The core ops are common to all subdevs, the other categories are implemented | |
279 | depending on the sub-device. E.g. a video device is unlikely to support the | |
280 | audio ops and vice versa. | |
281 | ||
282 | This setup limits the number of function pointers while still making it easy | |
283 | to add new ops and categories. | |
284 | ||
285 | A sub-device driver initializes the v4l2_subdev struct using: | |
286 | ||
89aec3e1 | 287 | v4l2_subdev_init(sd, &ops); |
2a1fcdf0 HV |
288 | |
289 | Afterwards you need to initialize subdev->name with a unique name and set the | |
290 | module owner. This is done for you if you use the i2c helper functions. | |
291 | ||
61f5db54 LP |
292 | If integration with the media framework is needed, you must initialize the |
293 | media_entity struct embedded in the v4l2_subdev struct (entity field) by | |
294 | calling media_entity_init(): | |
295 | ||
296 | struct media_pad *pads = &my_sd->pads; | |
297 | int err; | |
298 | ||
299 | err = media_entity_init(&sd->entity, npads, pads, 0); | |
300 | ||
301 | The pads array must have been previously initialized. There is no need to | |
302 | manually set the struct media_entity type and name fields, but the revision | |
303 | field must be initialized if needed. | |
304 | ||
305 | A reference to the entity will be automatically acquired/released when the | |
306 | subdev device node (if any) is opened/closed. | |
307 | ||
308 | Don't forget to cleanup the media entity before the sub-device is destroyed: | |
309 | ||
310 | media_entity_cleanup(&sd->entity); | |
311 | ||
48398f93 SA |
312 | If the subdev driver intends to process video and integrate with the media |
313 | framework, it must implement format related functionality using | |
314 | v4l2_subdev_pad_ops instead of v4l2_subdev_video_ops. | |
315 | ||
8227c92b SA |
316 | In that case, the subdev driver may set the link_validate field to provide |
317 | its own link validation function. The link validation function is called for | |
318 | every link in the pipeline where both of the ends of the links are V4L2 | |
319 | sub-devices. The driver is still responsible for validating the correctness | |
320 | of the format configuration between sub-devices and video nodes. | |
321 | ||
322 | If link_validate op is not set, the default function | |
323 | v4l2_subdev_link_validate_default() is used instead. This function ensures | |
324 | that width, height and the media bus pixel code are equal on both source and | |
325 | sink of the link. Subdev drivers are also free to use this function to | |
326 | perform the checks mentioned above in addition to their own checks. | |
327 | ||
2a1fcdf0 HV |
328 | A device (bridge) driver needs to register the v4l2_subdev with the |
329 | v4l2_device: | |
330 | ||
89aec3e1 | 331 | int err = v4l2_device_register_subdev(v4l2_dev, sd); |
2a1fcdf0 HV |
332 | |
333 | This can fail if the subdev module disappeared before it could be registered. | |
334 | After this function was called successfully the subdev->dev field points to | |
335 | the v4l2_device. | |
336 | ||
61f5db54 LP |
337 | If the v4l2_device parent device has a non-NULL mdev field, the sub-device |
338 | entity will be automatically registered with the media device. | |
339 | ||
2a1fcdf0 HV |
340 | You can unregister a sub-device using: |
341 | ||
89aec3e1 | 342 | v4l2_device_unregister_subdev(sd); |
2a1fcdf0 | 343 | |
89aec3e1 | 344 | Afterwards the subdev module can be unloaded and sd->dev == NULL. |
2a1fcdf0 HV |
345 | |
346 | You can call an ops function either directly: | |
347 | ||
2249aa5c | 348 | err = sd->ops->core->g_std(sd, &norm); |
2a1fcdf0 HV |
349 | |
350 | but it is better and easier to use this macro: | |
351 | ||
2249aa5c | 352 | err = v4l2_subdev_call(sd, core, g_std, &norm); |
2a1fcdf0 HV |
353 | |
354 | The macro will to the right NULL pointer checks and returns -ENODEV if subdev | |
2249aa5c HV |
355 | is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_std is |
356 | NULL, or the actual result of the subdev->ops->core->g_std ops. | |
2a1fcdf0 HV |
357 | |
358 | It is also possible to call all or a subset of the sub-devices: | |
359 | ||
2249aa5c | 360 | v4l2_device_call_all(v4l2_dev, 0, core, g_std, &norm); |
2a1fcdf0 HV |
361 | |
362 | Any subdev that does not support this ops is skipped and error results are | |
363 | ignored. If you want to check for errors use this: | |
364 | ||
2249aa5c | 365 | err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_std, &norm); |
2a1fcdf0 HV |
366 | |
367 | Any error except -ENOIOCTLCMD will exit the loop with that error. If no | |
25985edc | 368 | errors (except -ENOIOCTLCMD) occurred, then 0 is returned. |
2a1fcdf0 HV |
369 | |
370 | The second argument to both calls is a group ID. If 0, then all subdevs are | |
371 | called. If non-zero, then only those whose group ID match that value will | |
b0167600 | 372 | be called. Before a bridge driver registers a subdev it can set sd->grp_id |
2a1fcdf0 HV |
373 | to whatever value it wants (it's 0 by default). This value is owned by the |
374 | bridge driver and the sub-device driver will never modify or use it. | |
375 | ||
376 | The group ID gives the bridge driver more control how callbacks are called. | |
377 | For example, there may be multiple audio chips on a board, each capable of | |
378 | changing the volume. But usually only one will actually be used when the | |
379 | user want to change the volume. You can set the group ID for that subdev to | |
380 | e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling | |
381 | v4l2_device_call_all(). That ensures that it will only go to the subdev | |
382 | that needs it. | |
383 | ||
98ec6339 HV |
384 | If the sub-device needs to notify its v4l2_device parent of an event, then |
385 | it can call v4l2_subdev_notify(sd, notification, arg). This macro checks | |
386 | whether there is a notify() callback defined and returns -ENODEV if not. | |
387 | Otherwise the result of the notify() call is returned. | |
388 | ||
2a1fcdf0 HV |
389 | The advantage of using v4l2_subdev is that it is a generic struct and does |
390 | not contain any knowledge about the underlying hardware. So a driver might | |
391 | contain several subdevs that use an I2C bus, but also a subdev that is | |
392 | controlled through GPIO pins. This distinction is only relevant when setting | |
393 | up the device, but once the subdev is registered it is completely transparent. | |
394 | ||
395 | ||
2096a5dc LP |
396 | V4L2 sub-device userspace API |
397 | ----------------------------- | |
398 | ||
399 | Beside exposing a kernel API through the v4l2_subdev_ops structure, V4L2 | |
400 | sub-devices can also be controlled directly by userspace applications. | |
401 | ||
402 | Device nodes named v4l-subdevX can be created in /dev to access sub-devices | |
403 | directly. If a sub-device supports direct userspace configuration it must set | |
404 | the V4L2_SUBDEV_FL_HAS_DEVNODE flag before being registered. | |
405 | ||
406 | After registering sub-devices, the v4l2_device driver can create device nodes | |
407 | for all registered sub-devices marked with V4L2_SUBDEV_FL_HAS_DEVNODE by calling | |
408 | v4l2_device_register_subdev_nodes(). Those device nodes will be automatically | |
409 | removed when sub-devices are unregistered. | |
410 | ||
ea8aa434 LP |
411 | The device node handles a subset of the V4L2 API. |
412 | ||
413 | VIDIOC_QUERYCTRL | |
414 | VIDIOC_QUERYMENU | |
415 | VIDIOC_G_CTRL | |
416 | VIDIOC_S_CTRL | |
417 | VIDIOC_G_EXT_CTRLS | |
418 | VIDIOC_S_EXT_CTRLS | |
419 | VIDIOC_TRY_EXT_CTRLS | |
420 | ||
421 | The controls ioctls are identical to the ones defined in V4L2. They | |
422 | behave identically, with the only exception that they deal only with | |
423 | controls implemented in the sub-device. Depending on the driver, those | |
424 | controls can be also be accessed through one (or several) V4L2 device | |
425 | nodes. | |
426 | ||
02adb1cc SA |
427 | VIDIOC_DQEVENT |
428 | VIDIOC_SUBSCRIBE_EVENT | |
429 | VIDIOC_UNSUBSCRIBE_EVENT | |
430 | ||
431 | The events ioctls are identical to the ones defined in V4L2. They | |
432 | behave identically, with the only exception that they deal only with | |
433 | events generated by the sub-device. Depending on the driver, those | |
434 | events can also be reported by one (or several) V4L2 device nodes. | |
435 | ||
436 | Sub-device drivers that want to use events need to set the | |
437 | V4L2_SUBDEV_USES_EVENTS v4l2_subdev::flags and initialize | |
438 | v4l2_subdev::nevents to events queue depth before registering the | |
439 | sub-device. After registration events can be queued as usual on the | |
440 | v4l2_subdev::devnode device node. | |
441 | ||
442 | To properly support events, the poll() file operation is also | |
443 | implemented. | |
444 | ||
c30b46e5 LP |
445 | Private ioctls |
446 | ||
447 | All ioctls not in the above list are passed directly to the sub-device | |
448 | driver through the core::ioctl operation. | |
449 | ||
2096a5dc | 450 | |
2a1fcdf0 HV |
451 | I2C sub-device drivers |
452 | ---------------------- | |
453 | ||
454 | Since these drivers are so common, special helper functions are available to | |
455 | ease the use of these drivers (v4l2-common.h). | |
456 | ||
457 | The recommended method of adding v4l2_subdev support to an I2C driver is to | |
458 | embed the v4l2_subdev struct into the state struct that is created for each | |
459 | I2C device instance. Very simple devices have no state struct and in that case | |
460 | you can just create a v4l2_subdev directly. | |
461 | ||
462 | A typical state struct would look like this (where 'chipname' is replaced by | |
463 | the name of the chip): | |
464 | ||
465 | struct chipname_state { | |
466 | struct v4l2_subdev sd; | |
467 | ... /* additional state fields */ | |
468 | }; | |
469 | ||
470 | Initialize the v4l2_subdev struct as follows: | |
471 | ||
472 | v4l2_i2c_subdev_init(&state->sd, client, subdev_ops); | |
473 | ||
474 | This function will fill in all the fields of v4l2_subdev and ensure that the | |
475 | v4l2_subdev and i2c_client both point to one another. | |
476 | ||
477 | You should also add a helper inline function to go from a v4l2_subdev pointer | |
478 | to a chipname_state struct: | |
479 | ||
480 | static inline struct chipname_state *to_state(struct v4l2_subdev *sd) | |
481 | { | |
482 | return container_of(sd, struct chipname_state, sd); | |
483 | } | |
484 | ||
485 | Use this to go from the v4l2_subdev struct to the i2c_client struct: | |
486 | ||
487 | struct i2c_client *client = v4l2_get_subdevdata(sd); | |
488 | ||
489 | And this to go from an i2c_client to a v4l2_subdev struct: | |
490 | ||
491 | struct v4l2_subdev *sd = i2c_get_clientdata(client); | |
492 | ||
2a1fcdf0 HV |
493 | Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback |
494 | is called. This will unregister the sub-device from the bridge driver. It is | |
495 | safe to call this even if the sub-device was never registered. | |
496 | ||
f5360bdc HV |
497 | You need to do this because when the bridge driver destroys the i2c adapter |
498 | the remove() callbacks are called of the i2c devices on that adapter. | |
499 | After that the corresponding v4l2_subdev structures are invalid, so they | |
500 | have to be unregistered first. Calling v4l2_device_unregister_subdev(sd) | |
501 | from the remove() callback ensures that this is always done correctly. | |
502 | ||
2a1fcdf0 HV |
503 | |
504 | The bridge driver also has some helper functions it can use: | |
505 | ||
e6574f2f | 506 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter, |
53dacb15 | 507 | "module_foo", "chipid", 0x36, NULL); |
2a1fcdf0 HV |
508 | |
509 | This loads the given module (can be NULL if no module needs to be loaded) and | |
510 | calls i2c_new_device() with the given i2c_adapter and chip/address arguments. | |
e6574f2f | 511 | If all goes well, then it registers the subdev with the v4l2_device. |
2a1fcdf0 | 512 | |
53dacb15 HV |
513 | You can also use the last argument of v4l2_i2c_new_subdev() to pass an array |
514 | of possible I2C addresses that it should probe. These probe addresses are | |
515 | only used if the previous argument is 0. A non-zero argument means that you | |
516 | know the exact i2c address so in that case no probing will take place. | |
2a1fcdf0 HV |
517 | |
518 | Both functions return NULL if something went wrong. | |
519 | ||
53dacb15 | 520 | Note that the chipid you pass to v4l2_i2c_new_subdev() is usually |
2c792523 HV |
521 | the same as the module name. It allows you to specify a chip variant, e.g. |
522 | "saa7114" or "saa7115". In general though the i2c driver autodetects this. | |
523 | The use of chipid is something that needs to be looked at more closely at a | |
524 | later date. It differs between i2c drivers and as such can be confusing. | |
525 | To see which chip variants are supported you can look in the i2c driver code | |
526 | for the i2c_device_id table. This lists all the possibilities. | |
527 | ||
2c0b19ac HV |
528 | There are two more helper functions: |
529 | ||
530 | v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data | |
531 | arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not | |
532 | 0 then that will be used (non-probing variant), otherwise the probed_addrs | |
533 | are probed. | |
534 | ||
535 | For example: this will probe for address 0x10: | |
536 | ||
537 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter, | |
538 | "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10)); | |
539 | ||
540 | v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed | |
541 | to the i2c driver and replaces the irq, platform_data and addr arguments. | |
542 | ||
543 | If the subdev supports the s_config core ops, then that op is called with | |
544 | the irq and platform_data arguments after the subdev was setup. The older | |
545 | v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with | |
546 | irq set to 0 and platform_data set to NULL. | |
547 | ||
2a1fcdf0 HV |
548 | struct video_device |
549 | ------------------- | |
550 | ||
a47ddf14 HV |
551 | The actual device nodes in the /dev directory are created using the |
552 | video_device struct (v4l2-dev.h). This struct can either be allocated | |
553 | dynamically or embedded in a larger struct. | |
554 | ||
555 | To allocate it dynamically use: | |
556 | ||
557 | struct video_device *vdev = video_device_alloc(); | |
558 | ||
559 | if (vdev == NULL) | |
560 | return -ENOMEM; | |
561 | ||
562 | vdev->release = video_device_release; | |
563 | ||
564 | If you embed it in a larger struct, then you must set the release() | |
565 | callback to your own function: | |
566 | ||
567 | struct video_device *vdev = &my_vdev->vdev; | |
568 | ||
569 | vdev->release = my_vdev_release; | |
570 | ||
571 | The release callback must be set and it is called when the last user | |
572 | of the video device exits. | |
573 | ||
574 | The default video_device_release() callback just calls kfree to free the | |
575 | allocated memory. | |
576 | ||
577 | You should also set these fields: | |
578 | ||
dfa9a5ae | 579 | - v4l2_dev: set to the v4l2_device parent device. |
8ab75e3e | 580 | |
a47ddf14 | 581 | - name: set to something descriptive and unique. |
8ab75e3e | 582 | |
d2210f9e HV |
583 | - vfl_dir: set this to VFL_DIR_RX for capture devices (VFL_DIR_RX has value 0, |
584 | so this is normally already the default), set to VFL_DIR_TX for output | |
585 | devices and VFL_DIR_M2M for mem2mem (codec) devices. | |
586 | ||
c7dd09da | 587 | - fops: set to the v4l2_file_operations struct. |
8ab75e3e | 588 | |
a47ddf14 HV |
589 | - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance |
590 | (highly recommended to use this and it might become compulsory in the | |
d2210f9e HV |
591 | future!), then set this to your v4l2_ioctl_ops struct. The vfl_type and |
592 | vfl_dir fields are used to disable ops that do not match the type/dir | |
593 | combination. E.g. VBI ops are disabled for non-VBI nodes, and output ops | |
594 | are disabled for a capture device. This makes it possible to provide | |
595 | just one v4l2_ioctl_ops struct for both vbi and video nodes. | |
8ab75e3e | 596 | |
ee6869af | 597 | - lock: leave to NULL if you want to do all the locking in the driver. |
74f22c48 HV |
598 | Otherwise you give it a pointer to a struct mutex_lock and before the |
599 | unlocked_ioctl file operation is called this lock will be taken by the | |
8ab75e3e HV |
600 | core and released afterwards. See the next section for more details. |
601 | ||
4a77a836 HV |
602 | - queue: a pointer to the struct vb2_queue associated with this device node. |
603 | If queue is non-NULL, and queue->lock is non-NULL, then queue->lock is | |
604 | used for the queuing ioctls (VIDIOC_REQBUFS, CREATE_BUFS, QBUF, DQBUF, | |
605 | QUERYBUF, PREPARE_BUF, STREAMON and STREAMOFF) instead of the lock above. | |
606 | That way the vb2 queuing framework does not have to wait for other ioctls. | |
607 | This queue pointer is also used by the vb2 helper functions to check for | |
608 | queuing ownership (i.e. is the filehandle calling it allowed to do the | |
609 | operation). | |
610 | ||
6e29ad50 HV |
611 | - prio: keeps track of the priorities. Used to implement VIDIOC_G/S_PRIORITY. |
612 | If left to NULL, then it will use the struct v4l2_prio_state in v4l2_device. | |
613 | If you want to have a separate priority state per (group of) device node(s), | |
614 | then you can point it to your own struct v4l2_prio_state. | |
8ab75e3e | 615 | |
00575961 HV |
616 | - parent: you only set this if v4l2_device was registered with NULL as |
617 | the parent device struct. This only happens in cases where one hardware | |
618 | device has multiple PCI devices that all share the same v4l2_device core. | |
619 | ||
620 | The cx88 driver is an example of this: one core v4l2_device struct, but | |
621 | it is used by both an raw video PCI device (cx8800) and a MPEG PCI device | |
622 | (cx8802). Since the v4l2_device cannot be associated with a particular | |
623 | PCI device it is setup without a parent device. But when the struct | |
624 | video_device is setup you do know which parent PCI device to use. | |
8ab75e3e | 625 | |
b1a873a3 HV |
626 | - flags: optional. Set to V4L2_FL_USE_FH_PRIO if you want to let the framework |
627 | handle the VIDIOC_G/S_PRIORITY ioctls. This requires that you use struct | |
628 | v4l2_fh. Eventually this flag will disappear once all drivers use the core | |
629 | priority handling. But for now it has to be set explicitly. | |
a47ddf14 | 630 | |
6e29ad50 HV |
631 | If you use v4l2_ioctl_ops, then you should set .unlocked_ioctl to video_ioctl2 |
632 | in your v4l2_file_operations struct. | |
633 | ||
634 | Do not use .ioctl! This is deprecated and will go away in the future. | |
c7dd09da | 635 | |
1dd8728e HV |
636 | In some cases you want to tell the core that a function you had specified in |
637 | your v4l2_ioctl_ops should be ignored. You can mark such ioctls by calling this | |
638 | function before video_device_register is called: | |
639 | ||
152a3a73 | 640 | void v4l2_disable_ioctl(struct video_device *vdev, unsigned int cmd); |
1dd8728e HV |
641 | |
642 | This tends to be needed if based on external factors (e.g. which card is | |
643 | being used) you want to turns off certain features in v4l2_ioctl_ops without | |
644 | having to make a new struct. | |
645 | ||
c7dd09da HV |
646 | The v4l2_file_operations struct is a subset of file_operations. The main |
647 | difference is that the inode argument is omitted since it is never used. | |
a47ddf14 | 648 | |
2c0ab67b LP |
649 | If integration with the media framework is needed, you must initialize the |
650 | media_entity struct embedded in the video_device struct (entity field) by | |
651 | calling media_entity_init(): | |
652 | ||
653 | struct media_pad *pad = &my_vdev->pad; | |
654 | int err; | |
655 | ||
656 | err = media_entity_init(&vdev->entity, 1, pad, 0); | |
657 | ||
658 | The pads array must have been previously initialized. There is no need to | |
659 | manually set the struct media_entity type and name fields. | |
660 | ||
661 | A reference to the entity will be automatically acquired/released when the | |
662 | video device is opened/closed. | |
663 | ||
4a77a836 HV |
664 | ioctls and locking |
665 | ------------------ | |
8ab75e3e | 666 | |
4a77a836 HV |
667 | The V4L core provides optional locking services. The main service is the |
668 | lock field in struct video_device, which is a pointer to a mutex. If you set | |
669 | this pointer, then that will be used by unlocked_ioctl to serialize all ioctls. | |
8ab75e3e | 670 | |
4a77a836 HV |
671 | If you are using the videobuf2 framework, then there is a second lock that you |
672 | can set: video_device->queue->lock. If set, then this lock will be used instead | |
673 | of video_device->lock to serialize all queuing ioctls (see the previous section | |
674 | for the full list of those ioctls). | |
8ab75e3e | 675 | |
4a77a836 HV |
676 | The advantage of using a different lock for the queuing ioctls is that for some |
677 | drivers (particularly USB drivers) certain commands such as setting controls | |
678 | can take a long time, so you want to use a separate lock for the buffer queuing | |
679 | ioctls. That way your VIDIOC_DQBUF doesn't stall because the driver is busy | |
680 | changing the e.g. exposure of the webcam. | |
ee6869af | 681 | |
4a77a836 HV |
682 | Of course, you can always do all the locking yourself by leaving both lock |
683 | pointers at NULL. | |
43599f31 | 684 | |
4a77a836 HV |
685 | If you use the old videobuf then you must pass the video_device lock to the |
686 | videobuf queue initialize function: if videobuf has to wait for a frame to | |
687 | arrive, then it will temporarily unlock the lock and relock it afterwards. If | |
688 | your driver also waits in the code, then you should do the same to allow other | |
689 | processes to access the device node while the first process is waiting for | |
690 | something. | |
ee6869af | 691 | |
43599f31 | 692 | In the case of videobuf2 you will need to implement the wait_prepare and |
4a77a836 HV |
693 | wait_finish callbacks to unlock/lock if applicable. If you use the queue->lock |
694 | pointer, then you can use the helper functions vb2_ops_wait_prepare/finish. | |
695 | ||
696 | The implementation of a hotplug disconnect should also take the lock from | |
697 | video_device before calling v4l2_device_disconnect. If you are also using | |
698 | video_device->queue->lock, then you have to first lock video_device->queue->lock | |
699 | followed by video_device->lock. That way you can be sure no ioctl is running | |
700 | when you call v4l2_device_disconnect. | |
a47ddf14 HV |
701 | |
702 | video_device registration | |
703 | ------------------------- | |
704 | ||
705 | Next you register the video device: this will create the character device | |
706 | for you. | |
707 | ||
708 | err = video_register_device(vdev, VFL_TYPE_GRABBER, -1); | |
709 | if (err) { | |
50a2a8b3 | 710 | video_device_release(vdev); /* or kfree(my_vdev); */ |
a47ddf14 HV |
711 | return err; |
712 | } | |
713 | ||
2c0ab67b LP |
714 | If the v4l2_device parent device has a non-NULL mdev field, the video device |
715 | entity will be automatically registered with the media device. | |
716 | ||
a47ddf14 HV |
717 | Which device is registered depends on the type argument. The following |
718 | types exist: | |
719 | ||
720 | VFL_TYPE_GRABBER: videoX for video input/output devices | |
721 | VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) | |
722 | VFL_TYPE_RADIO: radioX for radio tuners | |
a47ddf14 HV |
723 | |
724 | The last argument gives you a certain amount of control over the device | |
6b5270d2 HV |
725 | device node number used (i.e. the X in videoX). Normally you will pass -1 |
726 | to let the v4l2 framework pick the first free number. But sometimes users | |
727 | want to select a specific node number. It is common that drivers allow | |
728 | the user to select a specific device node number through a driver module | |
729 | option. That number is then passed to this function and video_register_device | |
730 | will attempt to select that device node number. If that number was already | |
731 | in use, then the next free device node number will be selected and it | |
732 | will send a warning to the kernel log. | |
733 | ||
734 | Another use-case is if a driver creates many devices. In that case it can | |
735 | be useful to place different video devices in separate ranges. For example, | |
736 | video capture devices start at 0, video output devices start at 16. | |
22e22125 HV |
737 | So you can use the last argument to specify a minimum device node number |
738 | and the v4l2 framework will try to pick the first free number that is equal | |
a47ddf14 HV |
739 | or higher to what you passed. If that fails, then it will just pick the |
740 | first free number. | |
741 | ||
6b5270d2 HV |
742 | Since in this case you do not care about a warning about not being able |
743 | to select the specified device node number, you can call the function | |
744 | video_register_device_no_warn() instead. | |
745 | ||
a47ddf14 HV |
746 | Whenever a device node is created some attributes are also created for you. |
747 | If you look in /sys/class/video4linux you see the devices. Go into e.g. | |
748 | video0 and you will see 'name' and 'index' attributes. The 'name' attribute | |
7ae0cd9b | 749 | is the 'name' field of the video_device struct. |
a47ddf14 | 750 | |
7ae0cd9b HV |
751 | The 'index' attribute is the index of the device node: for each call to |
752 | video_register_device() the index is just increased by 1. The first video | |
753 | device node you register always starts with index 0. | |
a47ddf14 HV |
754 | |
755 | Users can setup udev rules that utilize the index attribute to make fancy | |
756 | device names (e.g. 'mpegX' for MPEG video capture device nodes). | |
757 | ||
758 | After the device was successfully registered, then you can use these fields: | |
759 | ||
760 | - vfl_type: the device type passed to video_register_device. | |
761 | - minor: the assigned device minor number. | |
22e22125 | 762 | - num: the device node number (i.e. the X in videoX). |
7ae0cd9b | 763 | - index: the device index number. |
a47ddf14 HV |
764 | |
765 | If the registration failed, then you need to call video_device_release() | |
766 | to free the allocated video_device struct, or free your own struct if the | |
767 | video_device was embedded in it. The vdev->release() callback will never | |
768 | be called if the registration failed, nor should you ever attempt to | |
769 | unregister the device if the registration failed. | |
770 | ||
771 | ||
772 | video_device cleanup | |
773 | -------------------- | |
774 | ||
775 | When the video device nodes have to be removed, either during the unload | |
776 | of the driver or because the USB device was disconnected, then you should | |
777 | unregister them: | |
778 | ||
779 | video_unregister_device(vdev); | |
780 | ||
781 | This will remove the device nodes from sysfs (causing udev to remove them | |
782 | from /dev). | |
783 | ||
dd1ad942 HV |
784 | After video_unregister_device() returns no new opens can be done. However, |
785 | in the case of USB devices some application might still have one of these | |
d69f2718 HV |
786 | device nodes open. So after the unregister all file operations (except |
787 | release, of course) will return an error as well. | |
a47ddf14 HV |
788 | |
789 | When the last user of the video device node exits, then the vdev->release() | |
790 | callback is called and you can do the final cleanup there. | |
791 | ||
2c0ab67b LP |
792 | Don't forget to cleanup the media entity associated with the video device if |
793 | it has been initialized: | |
794 | ||
795 | media_entity_cleanup(&vdev->entity); | |
796 | ||
797 | This can be done from the release callback. | |
798 | ||
a47ddf14 HV |
799 | |
800 | video_device helper functions | |
801 | ----------------------------- | |
802 | ||
803 | There are a few useful helper functions: | |
804 | ||
eac8ea53 LP |
805 | - file/video_device private data |
806 | ||
a47ddf14 HV |
807 | You can set/get driver private data in the video_device struct using: |
808 | ||
89aec3e1 HV |
809 | void *video_get_drvdata(struct video_device *vdev); |
810 | void video_set_drvdata(struct video_device *vdev, void *data); | |
a47ddf14 HV |
811 | |
812 | Note that you can safely call video_set_drvdata() before calling | |
813 | video_register_device(). | |
814 | ||
815 | And this function: | |
816 | ||
817 | struct video_device *video_devdata(struct file *file); | |
818 | ||
819 | returns the video_device belonging to the file struct. | |
820 | ||
eac8ea53 | 821 | The video_drvdata function combines video_get_drvdata with video_devdata: |
a47ddf14 HV |
822 | |
823 | void *video_drvdata(struct file *file); | |
824 | ||
825 | You can go from a video_device struct to the v4l2_device struct using: | |
826 | ||
dfa9a5ae | 827 | struct v4l2_device *v4l2_dev = vdev->v4l2_dev; |
44061c05 | 828 | |
eac8ea53 LP |
829 | - Device node name |
830 | ||
831 | The video_device node kernel name can be retrieved using | |
832 | ||
833 | const char *video_device_node_name(struct video_device *vdev); | |
834 | ||
835 | The name is used as a hint by userspace tools such as udev. The function | |
836 | should be used where possible instead of accessing the video_device::num and | |
837 | video_device::minor fields. | |
838 | ||
839 | ||
44061c05 MCC |
840 | video buffer helper functions |
841 | ----------------------------- | |
842 | ||
4b586a38 JC |
843 | The v4l2 core API provides a set of standard methods (called "videobuf") |
844 | for dealing with video buffers. Those methods allow a driver to implement | |
845 | read(), mmap() and overlay() in a consistent way. There are currently | |
846 | methods for using video buffers on devices that supports DMA with | |
847 | scatter/gather method (videobuf-dma-sg), DMA with linear access | |
848 | (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers | |
849 | (videobuf-vmalloc). | |
850 | ||
851 | Please see Documentation/video4linux/videobuf for more information on how | |
852 | to use the videobuf layer. | |
6cd84b78 SA |
853 | |
854 | struct v4l2_fh | |
855 | -------------- | |
856 | ||
857 | struct v4l2_fh provides a way to easily keep file handle specific data | |
6e29ad50 | 858 | that is used by the V4L2 framework. New drivers must use struct v4l2_fh |
b1a873a3 HV |
859 | since it is also used to implement priority handling (VIDIOC_G/S_PRIORITY) |
860 | if the video_device flag V4L2_FL_USE_FH_PRIO is also set. | |
6cd84b78 SA |
861 | |
862 | The users of v4l2_fh (in the V4L2 framework, not the driver) know | |
863 | whether a driver uses v4l2_fh as its file->private_data pointer by | |
6e29ad50 HV |
864 | testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. This bit is |
865 | set whenever v4l2_fh_init() is called. | |
6cd84b78 | 866 | |
6e29ad50 HV |
867 | struct v4l2_fh is allocated as a part of the driver's own file handle |
868 | structure and file->private_data is set to it in the driver's open | |
869 | function by the driver. | |
6cd84b78 | 870 | |
6e29ad50 HV |
871 | In many cases the struct v4l2_fh will be embedded in a larger structure. |
872 | In that case you should call v4l2_fh_init+v4l2_fh_add in open() and | |
873 | v4l2_fh_del+v4l2_fh_exit in release(). | |
6cd84b78 | 874 | |
6e29ad50 HV |
875 | Drivers can extract their own file handle structure by using the container_of |
876 | macro. Example: | |
6cd84b78 SA |
877 | |
878 | struct my_fh { | |
879 | int blah; | |
880 | struct v4l2_fh fh; | |
881 | }; | |
882 | ||
883 | ... | |
884 | ||
885 | int my_open(struct file *file) | |
886 | { | |
887 | struct my_fh *my_fh; | |
888 | struct video_device *vfd; | |
889 | int ret; | |
890 | ||
891 | ... | |
892 | ||
6e29ad50 HV |
893 | my_fh = kzalloc(sizeof(*my_fh), GFP_KERNEL); |
894 | ||
895 | ... | |
896 | ||
98019f5e | 897 | v4l2_fh_init(&my_fh->fh, vfd); |
6cd84b78 | 898 | |
6e29ad50 | 899 | ... |
6cd84b78 SA |
900 | |
901 | file->private_data = &my_fh->fh; | |
6e29ad50 HV |
902 | v4l2_fh_add(&my_fh->fh); |
903 | return 0; | |
6cd84b78 SA |
904 | } |
905 | ||
906 | int my_release(struct file *file) | |
907 | { | |
908 | struct v4l2_fh *fh = file->private_data; | |
909 | struct my_fh *my_fh = container_of(fh, struct my_fh, fh); | |
910 | ||
911 | ... | |
6e29ad50 HV |
912 | v4l2_fh_del(&my_fh->fh); |
913 | v4l2_fh_exit(&my_fh->fh); | |
914 | kfree(my_fh); | |
915 | return 0; | |
6cd84b78 | 916 | } |
dd966083 | 917 | |
6e29ad50 HV |
918 | Below is a short description of the v4l2_fh functions used: |
919 | ||
98019f5e | 920 | void v4l2_fh_init(struct v4l2_fh *fh, struct video_device *vdev) |
6e29ad50 HV |
921 | |
922 | Initialise the file handle. This *MUST* be performed in the driver's | |
923 | v4l2_file_operations->open() handler. | |
924 | ||
925 | void v4l2_fh_add(struct v4l2_fh *fh) | |
926 | ||
927 | Add a v4l2_fh to video_device file handle list. Must be called once the | |
928 | file handle is completely initialized. | |
929 | ||
930 | void v4l2_fh_del(struct v4l2_fh *fh) | |
931 | ||
932 | Unassociate the file handle from video_device(). The file handle | |
933 | exit function may now be called. | |
934 | ||
935 | void v4l2_fh_exit(struct v4l2_fh *fh) | |
936 | ||
937 | Uninitialise the file handle. After uninitialisation the v4l2_fh | |
938 | memory can be freed. | |
939 | ||
940 | ||
941 | If struct v4l2_fh is not embedded, then you can use these helper functions: | |
942 | ||
943 | int v4l2_fh_open(struct file *filp) | |
944 | ||
945 | This allocates a struct v4l2_fh, initializes it and adds it to the struct | |
946 | video_device associated with the file struct. | |
947 | ||
948 | int v4l2_fh_release(struct file *filp) | |
949 | ||
950 | This deletes it from the struct video_device associated with the file | |
951 | struct, uninitialised the v4l2_fh and frees it. | |
952 | ||
953 | These two functions can be plugged into the v4l2_file_operation's open() and | |
954 | release() ops. | |
955 | ||
956 | ||
957 | Several drivers need to do something when the first file handle is opened and | |
958 | when the last file handle closes. Two helper functions were added to check | |
959 | whether the v4l2_fh struct is the only open filehandle of the associated | |
960 | device node: | |
961 | ||
962 | int v4l2_fh_is_singular(struct v4l2_fh *fh) | |
963 | ||
964 | Returns 1 if the file handle is the only open file handle, else 0. | |
965 | ||
966 | int v4l2_fh_is_singular_file(struct file *filp) | |
967 | ||
968 | Same, but it calls v4l2_fh_is_singular with filp->private_data. | |
969 | ||
970 | ||
dd966083 SA |
971 | V4L2 events |
972 | ----------- | |
973 | ||
974 | The V4L2 events provide a generic way to pass events to user space. | |
975 | The driver must use v4l2_fh to be able to support V4L2 events. | |
976 | ||
1de7310a HV |
977 | Events are defined by a type and an optional ID. The ID may refer to a V4L2 |
978 | object such as a control ID. If unused, then the ID is 0. | |
979 | ||
980 | When the user subscribes to an event the driver will allocate a number of | |
981 | kevent structs for that event. So every (type, ID) event tuple will have | |
982 | its own set of kevent structs. This guarantees that if a driver is generating | |
983 | lots of events of one type in a short time, then that will not overwrite | |
984 | events of another type. | |
985 | ||
986 | But if you get more events of one type than the number of kevents that were | |
987 | reserved, then the oldest event will be dropped and the new one added. | |
988 | ||
989 | Furthermore, the internal struct v4l2_subscribed_event has merge() and | |
990 | replace() callbacks which drivers can set. These callbacks are called when | |
991 | a new event is raised and there is no more room. The replace() callback | |
992 | allows you to replace the payload of the old event with that of the new event, | |
993 | merging any relevant data from the old payload into the new payload that | |
994 | replaces it. It is called when this event type has only one kevent struct | |
995 | allocated. The merge() callback allows you to merge the oldest event payload | |
996 | into that of the second-oldest event payload. It is called when there are two | |
997 | or more kevent structs allocated. | |
dd966083 | 998 | |
1de7310a HV |
999 | This way no status information is lost, just the intermediate steps leading |
1000 | up to that state. | |
dd966083 | 1001 | |
1de7310a HV |
1002 | A good example of these replace/merge callbacks is in v4l2-event.c: |
1003 | ctrls_replace() and ctrls_merge() callbacks for the control event. | |
1004 | ||
1005 | Note: these callbacks can be called from interrupt context, so they must be | |
1006 | fast. | |
1007 | ||
1008 | Useful functions: | |
dd966083 | 1009 | |
c53c2549 | 1010 | void v4l2_event_queue(struct video_device *vdev, const struct v4l2_event *ev) |
dd966083 SA |
1011 | |
1012 | Queue events to video device. The driver's only responsibility is to fill | |
1013 | in the type and the data fields. The other fields will be filled in by | |
1014 | V4L2. | |
1015 | ||
c53c2549 HG |
1016 | int v4l2_event_subscribe(struct v4l2_fh *fh, |
1017 | struct v4l2_event_subscription *sub, unsigned elems, | |
1018 | const struct v4l2_subscribed_event_ops *ops) | |
dd966083 SA |
1019 | |
1020 | The video_device->ioctl_ops->vidioc_subscribe_event must check the driver | |
1021 | is able to produce events with specified event id. Then it calls | |
c53c2549 | 1022 | v4l2_event_subscribe() to subscribe the event. |
dd966083 | 1023 | |
c53c2549 HG |
1024 | The elems argument is the size of the event queue for this event. If it is 0, |
1025 | then the framework will fill in a default value (this depends on the event | |
1026 | type). | |
1027 | ||
1028 | The ops argument allows the driver to specify a number of callbacks: | |
1029 | * add: called when a new listener gets added (subscribing to the same | |
1030 | event twice will only cause this callback to get called once) | |
1031 | * del: called when a listener stops listening | |
1032 | * replace: replace event 'old' with event 'new'. | |
1033 | * merge: merge event 'old' into event 'new'. | |
1034 | All 4 callbacks are optional, if you don't want to specify any callbacks | |
1035 | the ops argument itself maybe NULL. | |
1036 | ||
1037 | int v4l2_event_unsubscribe(struct v4l2_fh *fh, | |
1038 | struct v4l2_event_subscription *sub) | |
dd966083 SA |
1039 | |
1040 | vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use | |
1041 | v4l2_event_unsubscribe() directly unless it wants to be involved in | |
1042 | unsubscription process. | |
1043 | ||
1044 | The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The | |
1045 | drivers may want to handle this in a special way. | |
1046 | ||
c53c2549 | 1047 | int v4l2_event_pending(struct v4l2_fh *fh) |
dd966083 SA |
1048 | |
1049 | Returns the number of pending events. Useful when implementing poll. | |
1050 | ||
dd966083 | 1051 | Events are delivered to user space through the poll system call. The driver |
1de7310a | 1052 | can use v4l2_fh->wait (a wait_queue_head_t) as the argument for poll_wait(). |
dd966083 SA |
1053 | |
1054 | There are standard and private events. New standard events must use the | |
1055 | smallest available event type. The drivers must allocate their events from | |
1056 | their own class starting from class base. Class base is | |
1057 | V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number. | |
1058 | The first event type in the class is reserved for future use, so the first | |
1059 | available event type is 'class base + 1'. | |
1060 | ||
1061 | An example on how the V4L2 events may be used can be found in the OMAP | |
83c73537 | 1062 | 3 ISP driver (drivers/media/platform/omap3isp). |