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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 | ||
74 | ||
75 | struct v4l2_device | |
76 | ------------------ | |
77 | ||
78 | Each device instance is represented by a struct v4l2_device (v4l2-device.h). | |
79 | Very simple devices can just allocate this struct, but most of the time you | |
80 | would embed this struct inside a larger struct. | |
81 | ||
82 | You must register the device instance: | |
83 | ||
84 | v4l2_device_register(struct device *dev, struct v4l2_device *v4l2_dev); | |
85 | ||
86 | Registration will initialize the v4l2_device struct and link dev->driver_data | |
3a63e449 HV |
87 | to v4l2_dev. If v4l2_dev->name is empty then it will be set to a value derived |
88 | from dev (driver name followed by the bus_id, to be precise). If you set it | |
89 | up before calling v4l2_device_register then it will be untouched. If dev is | |
90 | NULL, then you *must* setup v4l2_dev->name before calling v4l2_device_register. | |
2a1fcdf0 | 91 | |
102e7813 HV |
92 | You can use v4l2_device_set_name() to set the name based on a driver name and |
93 | a driver-global atomic_t instance. This will generate names like ivtv0, ivtv1, | |
94 | etc. If the name ends with a digit, then it will insert a dash: cx18-0, | |
95 | cx18-1, etc. This function returns the instance number. | |
96 | ||
a47ddf14 | 97 | The first 'dev' argument is normally the struct device pointer of a pci_dev, |
073d696d | 98 | usb_interface or platform_device. It is rare for dev to be NULL, but it happens |
00575961 HV |
99 | with ISA devices or when one device creates multiple PCI devices, thus making |
100 | it impossible to associate v4l2_dev with a particular parent. | |
a47ddf14 | 101 | |
98ec6339 HV |
102 | You can also supply a notify() callback that can be called by sub-devices to |
103 | notify you of events. Whether you need to set this depends on the sub-device. | |
104 | Any notifications a sub-device supports must be defined in a header in | |
105 | include/media/<subdevice>.h. | |
106 | ||
2a1fcdf0 HV |
107 | You unregister with: |
108 | ||
109 | v4l2_device_unregister(struct v4l2_device *v4l2_dev); | |
110 | ||
111 | Unregistering will also automatically unregister all subdevs from the device. | |
112 | ||
ae6cfaac HV |
113 | If you have a hotpluggable device (e.g. a USB device), then when a disconnect |
114 | happens the parent device becomes invalid. Since v4l2_device has a pointer to | |
115 | that parent device it has to be cleared as well to mark that the parent is | |
116 | gone. To do this call: | |
117 | ||
118 | v4l2_device_disconnect(struct v4l2_device *v4l2_dev); | |
119 | ||
120 | This does *not* unregister the subdevs, so you still need to call the | |
121 | v4l2_device_unregister() function for that. If your driver is not hotpluggable, | |
122 | then there is no need to call v4l2_device_disconnect(). | |
123 | ||
2a1fcdf0 HV |
124 | Sometimes you need to iterate over all devices registered by a specific |
125 | driver. This is usually the case if multiple device drivers use the same | |
126 | hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv | |
127 | hardware. The same is true for alsa drivers for example. | |
128 | ||
129 | You can iterate over all registered devices as follows: | |
130 | ||
131 | static int callback(struct device *dev, void *p) | |
132 | { | |
133 | struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); | |
134 | ||
135 | /* test if this device was inited */ | |
136 | if (v4l2_dev == NULL) | |
137 | return 0; | |
138 | ... | |
139 | return 0; | |
140 | } | |
141 | ||
142 | int iterate(void *p) | |
143 | { | |
144 | struct device_driver *drv; | |
145 | int err; | |
146 | ||
147 | /* Find driver 'ivtv' on the PCI bus. | |
148 | pci_bus_type is a global. For USB busses use usb_bus_type. */ | |
149 | drv = driver_find("ivtv", &pci_bus_type); | |
150 | /* iterate over all ivtv device instances */ | |
151 | err = driver_for_each_device(drv, NULL, p, callback); | |
152 | put_driver(drv); | |
153 | return err; | |
154 | } | |
155 | ||
156 | Sometimes you need to keep a running counter of the device instance. This is | |
157 | commonly used to map a device instance to an index of a module option array. | |
158 | ||
159 | The recommended approach is as follows: | |
160 | ||
161 | static atomic_t drv_instance = ATOMIC_INIT(0); | |
162 | ||
89aec3e1 | 163 | static int __devinit drv_probe(struct pci_dev *pdev, |
2a1fcdf0 HV |
164 | const struct pci_device_id *pci_id) |
165 | { | |
166 | ... | |
167 | state->instance = atomic_inc_return(&drv_instance) - 1; | |
168 | } | |
169 | ||
170 | ||
171 | struct v4l2_subdev | |
172 | ------------------ | |
173 | ||
174 | Many drivers need to communicate with sub-devices. These devices can do all | |
175 | sort of tasks, but most commonly they handle audio and/or video muxing, | |
176 | encoding or decoding. For webcams common sub-devices are sensors and camera | |
177 | controllers. | |
178 | ||
179 | Usually these are I2C devices, but not necessarily. In order to provide the | |
180 | driver with a consistent interface to these sub-devices the v4l2_subdev struct | |
181 | (v4l2-subdev.h) was created. | |
182 | ||
183 | Each sub-device driver must have a v4l2_subdev struct. This struct can be | |
184 | stand-alone for simple sub-devices or it might be embedded in a larger struct | |
185 | if more state information needs to be stored. Usually there is a low-level | |
186 | device struct (e.g. i2c_client) that contains the device data as setup | |
187 | by the kernel. It is recommended to store that pointer in the private | |
188 | data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go | |
189 | from a v4l2_subdev to the actual low-level bus-specific device data. | |
190 | ||
191 | You also need a way to go from the low-level struct to v4l2_subdev. For the | |
192 | common i2c_client struct the i2c_set_clientdata() call is used to store a | |
193 | v4l2_subdev pointer, for other busses you may have to use other methods. | |
194 | ||
692d5522 LP |
195 | Bridges might also need to store per-subdev private data, such as a pointer to |
196 | bridge-specific per-subdev private data. The v4l2_subdev structure provides | |
197 | host private data for that purpose that can be accessed with | |
198 | v4l2_get_subdev_hostdata() and v4l2_set_subdev_hostdata(). | |
199 | ||
2a1fcdf0 HV |
200 | From the bridge driver perspective you load the sub-device module and somehow |
201 | obtain the v4l2_subdev pointer. For i2c devices this is easy: you call | |
202 | i2c_get_clientdata(). For other busses something similar needs to be done. | |
203 | Helper functions exists for sub-devices on an I2C bus that do most of this | |
204 | tricky work for you. | |
205 | ||
206 | Each v4l2_subdev contains function pointers that sub-device drivers can | |
207 | implement (or leave NULL if it is not applicable). Since sub-devices can do | |
208 | so many different things and you do not want to end up with a huge ops struct | |
209 | of which only a handful of ops are commonly implemented, the function pointers | |
210 | are sorted according to category and each category has its own ops struct. | |
211 | ||
212 | The top-level ops struct contains pointers to the category ops structs, which | |
213 | may be NULL if the subdev driver does not support anything from that category. | |
214 | ||
215 | It looks like this: | |
216 | ||
217 | struct v4l2_subdev_core_ops { | |
aecde8b5 | 218 | int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); |
2a1fcdf0 HV |
219 | int (*log_status)(struct v4l2_subdev *sd); |
220 | int (*init)(struct v4l2_subdev *sd, u32 val); | |
221 | ... | |
222 | }; | |
223 | ||
224 | struct v4l2_subdev_tuner_ops { | |
225 | ... | |
226 | }; | |
227 | ||
228 | struct v4l2_subdev_audio_ops { | |
229 | ... | |
230 | }; | |
231 | ||
232 | struct v4l2_subdev_video_ops { | |
233 | ... | |
234 | }; | |
235 | ||
236 | struct v4l2_subdev_ops { | |
237 | const struct v4l2_subdev_core_ops *core; | |
238 | const struct v4l2_subdev_tuner_ops *tuner; | |
239 | const struct v4l2_subdev_audio_ops *audio; | |
240 | const struct v4l2_subdev_video_ops *video; | |
241 | }; | |
242 | ||
243 | The core ops are common to all subdevs, the other categories are implemented | |
244 | depending on the sub-device. E.g. a video device is unlikely to support the | |
245 | audio ops and vice versa. | |
246 | ||
247 | This setup limits the number of function pointers while still making it easy | |
248 | to add new ops and categories. | |
249 | ||
250 | A sub-device driver initializes the v4l2_subdev struct using: | |
251 | ||
89aec3e1 | 252 | v4l2_subdev_init(sd, &ops); |
2a1fcdf0 HV |
253 | |
254 | Afterwards you need to initialize subdev->name with a unique name and set the | |
255 | module owner. This is done for you if you use the i2c helper functions. | |
256 | ||
257 | A device (bridge) driver needs to register the v4l2_subdev with the | |
258 | v4l2_device: | |
259 | ||
89aec3e1 | 260 | int err = v4l2_device_register_subdev(v4l2_dev, sd); |
2a1fcdf0 HV |
261 | |
262 | This can fail if the subdev module disappeared before it could be registered. | |
263 | After this function was called successfully the subdev->dev field points to | |
264 | the v4l2_device. | |
265 | ||
266 | You can unregister a sub-device using: | |
267 | ||
89aec3e1 | 268 | v4l2_device_unregister_subdev(sd); |
2a1fcdf0 | 269 | |
89aec3e1 | 270 | Afterwards the subdev module can be unloaded and sd->dev == NULL. |
2a1fcdf0 HV |
271 | |
272 | You can call an ops function either directly: | |
273 | ||
89aec3e1 | 274 | err = sd->ops->core->g_chip_ident(sd, &chip); |
2a1fcdf0 HV |
275 | |
276 | but it is better and easier to use this macro: | |
277 | ||
89aec3e1 | 278 | err = v4l2_subdev_call(sd, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
279 | |
280 | The macro will to the right NULL pointer checks and returns -ENODEV if subdev | |
281 | is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is | |
282 | NULL, or the actual result of the subdev->ops->core->g_chip_ident ops. | |
283 | ||
284 | It is also possible to call all or a subset of the sub-devices: | |
285 | ||
89aec3e1 | 286 | v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
287 | |
288 | Any subdev that does not support this ops is skipped and error results are | |
289 | ignored. If you want to check for errors use this: | |
290 | ||
89aec3e1 | 291 | err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
292 | |
293 | Any error except -ENOIOCTLCMD will exit the loop with that error. If no | |
294 | errors (except -ENOIOCTLCMD) occured, then 0 is returned. | |
295 | ||
296 | The second argument to both calls is a group ID. If 0, then all subdevs are | |
297 | called. If non-zero, then only those whose group ID match that value will | |
b0167600 | 298 | be called. Before a bridge driver registers a subdev it can set sd->grp_id |
2a1fcdf0 HV |
299 | to whatever value it wants (it's 0 by default). This value is owned by the |
300 | bridge driver and the sub-device driver will never modify or use it. | |
301 | ||
302 | The group ID gives the bridge driver more control how callbacks are called. | |
303 | For example, there may be multiple audio chips on a board, each capable of | |
304 | changing the volume. But usually only one will actually be used when the | |
305 | user want to change the volume. You can set the group ID for that subdev to | |
306 | e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling | |
307 | v4l2_device_call_all(). That ensures that it will only go to the subdev | |
308 | that needs it. | |
309 | ||
98ec6339 HV |
310 | If the sub-device needs to notify its v4l2_device parent of an event, then |
311 | it can call v4l2_subdev_notify(sd, notification, arg). This macro checks | |
312 | whether there is a notify() callback defined and returns -ENODEV if not. | |
313 | Otherwise the result of the notify() call is returned. | |
314 | ||
2a1fcdf0 HV |
315 | The advantage of using v4l2_subdev is that it is a generic struct and does |
316 | not contain any knowledge about the underlying hardware. So a driver might | |
317 | contain several subdevs that use an I2C bus, but also a subdev that is | |
318 | controlled through GPIO pins. This distinction is only relevant when setting | |
319 | up the device, but once the subdev is registered it is completely transparent. | |
320 | ||
321 | ||
322 | I2C sub-device drivers | |
323 | ---------------------- | |
324 | ||
325 | Since these drivers are so common, special helper functions are available to | |
326 | ease the use of these drivers (v4l2-common.h). | |
327 | ||
328 | The recommended method of adding v4l2_subdev support to an I2C driver is to | |
329 | embed the v4l2_subdev struct into the state struct that is created for each | |
330 | I2C device instance. Very simple devices have no state struct and in that case | |
331 | you can just create a v4l2_subdev directly. | |
332 | ||
333 | A typical state struct would look like this (where 'chipname' is replaced by | |
334 | the name of the chip): | |
335 | ||
336 | struct chipname_state { | |
337 | struct v4l2_subdev sd; | |
338 | ... /* additional state fields */ | |
339 | }; | |
340 | ||
341 | Initialize the v4l2_subdev struct as follows: | |
342 | ||
343 | v4l2_i2c_subdev_init(&state->sd, client, subdev_ops); | |
344 | ||
345 | This function will fill in all the fields of v4l2_subdev and ensure that the | |
346 | v4l2_subdev and i2c_client both point to one another. | |
347 | ||
348 | You should also add a helper inline function to go from a v4l2_subdev pointer | |
349 | to a chipname_state struct: | |
350 | ||
351 | static inline struct chipname_state *to_state(struct v4l2_subdev *sd) | |
352 | { | |
353 | return container_of(sd, struct chipname_state, sd); | |
354 | } | |
355 | ||
356 | Use this to go from the v4l2_subdev struct to the i2c_client struct: | |
357 | ||
358 | struct i2c_client *client = v4l2_get_subdevdata(sd); | |
359 | ||
360 | And this to go from an i2c_client to a v4l2_subdev struct: | |
361 | ||
362 | struct v4l2_subdev *sd = i2c_get_clientdata(client); | |
363 | ||
2a1fcdf0 HV |
364 | Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback |
365 | is called. This will unregister the sub-device from the bridge driver. It is | |
366 | safe to call this even if the sub-device was never registered. | |
367 | ||
f5360bdc HV |
368 | You need to do this because when the bridge driver destroys the i2c adapter |
369 | the remove() callbacks are called of the i2c devices on that adapter. | |
370 | After that the corresponding v4l2_subdev structures are invalid, so they | |
371 | have to be unregistered first. Calling v4l2_device_unregister_subdev(sd) | |
372 | from the remove() callback ensures that this is always done correctly. | |
373 | ||
2a1fcdf0 HV |
374 | |
375 | The bridge driver also has some helper functions it can use: | |
376 | ||
e6574f2f | 377 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev(v4l2_dev, adapter, |
53dacb15 | 378 | "module_foo", "chipid", 0x36, NULL); |
2a1fcdf0 HV |
379 | |
380 | This loads the given module (can be NULL if no module needs to be loaded) and | |
381 | calls i2c_new_device() with the given i2c_adapter and chip/address arguments. | |
e6574f2f | 382 | If all goes well, then it registers the subdev with the v4l2_device. |
2a1fcdf0 | 383 | |
53dacb15 HV |
384 | You can also use the last argument of v4l2_i2c_new_subdev() to pass an array |
385 | of possible I2C addresses that it should probe. These probe addresses are | |
386 | only used if the previous argument is 0. A non-zero argument means that you | |
387 | know the exact i2c address so in that case no probing will take place. | |
2a1fcdf0 HV |
388 | |
389 | Both functions return NULL if something went wrong. | |
390 | ||
53dacb15 | 391 | Note that the chipid you pass to v4l2_i2c_new_subdev() is usually |
2c792523 HV |
392 | the same as the module name. It allows you to specify a chip variant, e.g. |
393 | "saa7114" or "saa7115". In general though the i2c driver autodetects this. | |
394 | The use of chipid is something that needs to be looked at more closely at a | |
395 | later date. It differs between i2c drivers and as such can be confusing. | |
396 | To see which chip variants are supported you can look in the i2c driver code | |
397 | for the i2c_device_id table. This lists all the possibilities. | |
398 | ||
2c0b19ac HV |
399 | There are two more helper functions: |
400 | ||
401 | v4l2_i2c_new_subdev_cfg: this function adds new irq and platform_data | |
402 | arguments and has both 'addr' and 'probed_addrs' arguments: if addr is not | |
403 | 0 then that will be used (non-probing variant), otherwise the probed_addrs | |
404 | are probed. | |
405 | ||
406 | For example: this will probe for address 0x10: | |
407 | ||
408 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev_cfg(v4l2_dev, adapter, | |
409 | "module_foo", "chipid", 0, NULL, 0, I2C_ADDRS(0x10)); | |
410 | ||
411 | v4l2_i2c_new_subdev_board uses an i2c_board_info struct which is passed | |
412 | to the i2c driver and replaces the irq, platform_data and addr arguments. | |
413 | ||
414 | If the subdev supports the s_config core ops, then that op is called with | |
415 | the irq and platform_data arguments after the subdev was setup. The older | |
416 | v4l2_i2c_new_(probed_)subdev functions will call s_config as well, but with | |
417 | irq set to 0 and platform_data set to NULL. | |
418 | ||
2a1fcdf0 HV |
419 | struct video_device |
420 | ------------------- | |
421 | ||
a47ddf14 HV |
422 | The actual device nodes in the /dev directory are created using the |
423 | video_device struct (v4l2-dev.h). This struct can either be allocated | |
424 | dynamically or embedded in a larger struct. | |
425 | ||
426 | To allocate it dynamically use: | |
427 | ||
428 | struct video_device *vdev = video_device_alloc(); | |
429 | ||
430 | if (vdev == NULL) | |
431 | return -ENOMEM; | |
432 | ||
433 | vdev->release = video_device_release; | |
434 | ||
435 | If you embed it in a larger struct, then you must set the release() | |
436 | callback to your own function: | |
437 | ||
438 | struct video_device *vdev = &my_vdev->vdev; | |
439 | ||
440 | vdev->release = my_vdev_release; | |
441 | ||
442 | The release callback must be set and it is called when the last user | |
443 | of the video device exits. | |
444 | ||
445 | The default video_device_release() callback just calls kfree to free the | |
446 | allocated memory. | |
447 | ||
448 | You should also set these fields: | |
449 | ||
dfa9a5ae | 450 | - v4l2_dev: set to the v4l2_device parent device. |
a47ddf14 | 451 | - name: set to something descriptive and unique. |
c7dd09da | 452 | - fops: set to the v4l2_file_operations struct. |
a47ddf14 HV |
453 | - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance |
454 | (highly recommended to use this and it might become compulsory in the | |
455 | future!), then set this to your v4l2_ioctl_ops struct. | |
ee6869af HV |
456 | - lock: leave to NULL if you want to do all the locking in the driver. |
457 | Otherwise you give it a pointer to a struct mutex_lock and before any | |
458 | of the v4l2_file_operations is called this lock will be taken by the | |
459 | core and released afterwards. | |
00575961 HV |
460 | - parent: you only set this if v4l2_device was registered with NULL as |
461 | the parent device struct. This only happens in cases where one hardware | |
462 | device has multiple PCI devices that all share the same v4l2_device core. | |
463 | ||
464 | The cx88 driver is an example of this: one core v4l2_device struct, but | |
465 | it is used by both an raw video PCI device (cx8800) and a MPEG PCI device | |
466 | (cx8802). Since the v4l2_device cannot be associated with a particular | |
467 | PCI device it is setup without a parent device. But when the struct | |
468 | video_device is setup you do know which parent PCI device to use. | |
a47ddf14 | 469 | |
c7dd09da HV |
470 | If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or |
471 | .ioctl to video_ioctl2 in your v4l2_file_operations struct. | |
472 | ||
473 | The v4l2_file_operations struct is a subset of file_operations. The main | |
474 | difference is that the inode argument is omitted since it is never used. | |
a47ddf14 | 475 | |
ee6869af HV |
476 | v4l2_file_operations and locking |
477 | -------------------------------- | |
478 | ||
479 | You can set a pointer to a mutex_lock in struct video_device. Usually this | |
480 | will be either a top-level mutex or a mutex per device node. If you want | |
481 | finer-grained locking then you have to set it to NULL and do you own locking. | |
482 | ||
483 | If a lock is specified then all file operations will be serialized on that | |
484 | lock. If you use videobuf then you must pass the same lock to the videobuf | |
485 | queue initialize function: if videobuf has to wait for a frame to arrive, then | |
486 | it will temporarily unlock the lock and relock it afterwards. If your driver | |
487 | also waits in the code, then you should do the same to allow other processes | |
488 | to access the device node while the first process is waiting for something. | |
489 | ||
490 | The implementation of a hotplug disconnect should also take the lock before | |
491 | calling v4l2_device_disconnect and video_unregister_device. | |
a47ddf14 HV |
492 | |
493 | video_device registration | |
494 | ------------------------- | |
495 | ||
496 | Next you register the video device: this will create the character device | |
497 | for you. | |
498 | ||
499 | err = video_register_device(vdev, VFL_TYPE_GRABBER, -1); | |
500 | if (err) { | |
50a2a8b3 | 501 | video_device_release(vdev); /* or kfree(my_vdev); */ |
a47ddf14 HV |
502 | return err; |
503 | } | |
504 | ||
505 | Which device is registered depends on the type argument. The following | |
506 | types exist: | |
507 | ||
508 | VFL_TYPE_GRABBER: videoX for video input/output devices | |
509 | VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) | |
510 | VFL_TYPE_RADIO: radioX for radio tuners | |
a47ddf14 HV |
511 | |
512 | The last argument gives you a certain amount of control over the device | |
6b5270d2 HV |
513 | device node number used (i.e. the X in videoX). Normally you will pass -1 |
514 | to let the v4l2 framework pick the first free number. But sometimes users | |
515 | want to select a specific node number. It is common that drivers allow | |
516 | the user to select a specific device node number through a driver module | |
517 | option. That number is then passed to this function and video_register_device | |
518 | will attempt to select that device node number. If that number was already | |
519 | in use, then the next free device node number will be selected and it | |
520 | will send a warning to the kernel log. | |
521 | ||
522 | Another use-case is if a driver creates many devices. In that case it can | |
523 | be useful to place different video devices in separate ranges. For example, | |
524 | video capture devices start at 0, video output devices start at 16. | |
22e22125 HV |
525 | So you can use the last argument to specify a minimum device node number |
526 | and the v4l2 framework will try to pick the first free number that is equal | |
a47ddf14 HV |
527 | or higher to what you passed. If that fails, then it will just pick the |
528 | first free number. | |
529 | ||
6b5270d2 HV |
530 | Since in this case you do not care about a warning about not being able |
531 | to select the specified device node number, you can call the function | |
532 | video_register_device_no_warn() instead. | |
533 | ||
a47ddf14 HV |
534 | Whenever a device node is created some attributes are also created for you. |
535 | If you look in /sys/class/video4linux you see the devices. Go into e.g. | |
536 | video0 and you will see 'name' and 'index' attributes. The 'name' attribute | |
7ae0cd9b | 537 | is the 'name' field of the video_device struct. |
a47ddf14 | 538 | |
7ae0cd9b HV |
539 | The 'index' attribute is the index of the device node: for each call to |
540 | video_register_device() the index is just increased by 1. The first video | |
541 | device node you register always starts with index 0. | |
a47ddf14 HV |
542 | |
543 | Users can setup udev rules that utilize the index attribute to make fancy | |
544 | device names (e.g. 'mpegX' for MPEG video capture device nodes). | |
545 | ||
546 | After the device was successfully registered, then you can use these fields: | |
547 | ||
548 | - vfl_type: the device type passed to video_register_device. | |
549 | - minor: the assigned device minor number. | |
22e22125 | 550 | - num: the device node number (i.e. the X in videoX). |
7ae0cd9b | 551 | - index: the device index number. |
a47ddf14 HV |
552 | |
553 | If the registration failed, then you need to call video_device_release() | |
554 | to free the allocated video_device struct, or free your own struct if the | |
555 | video_device was embedded in it. The vdev->release() callback will never | |
556 | be called if the registration failed, nor should you ever attempt to | |
557 | unregister the device if the registration failed. | |
558 | ||
559 | ||
560 | video_device cleanup | |
561 | -------------------- | |
562 | ||
563 | When the video device nodes have to be removed, either during the unload | |
564 | of the driver or because the USB device was disconnected, then you should | |
565 | unregister them: | |
566 | ||
567 | video_unregister_device(vdev); | |
568 | ||
569 | This will remove the device nodes from sysfs (causing udev to remove them | |
570 | from /dev). | |
571 | ||
dd1ad942 HV |
572 | After video_unregister_device() returns no new opens can be done. However, |
573 | in the case of USB devices some application might still have one of these | |
d69f2718 HV |
574 | device nodes open. So after the unregister all file operations (except |
575 | release, of course) will return an error as well. | |
a47ddf14 HV |
576 | |
577 | When the last user of the video device node exits, then the vdev->release() | |
578 | callback is called and you can do the final cleanup there. | |
579 | ||
580 | ||
581 | video_device helper functions | |
582 | ----------------------------- | |
583 | ||
584 | There are a few useful helper functions: | |
585 | ||
eac8ea53 LP |
586 | - file/video_device private data |
587 | ||
a47ddf14 HV |
588 | You can set/get driver private data in the video_device struct using: |
589 | ||
89aec3e1 HV |
590 | void *video_get_drvdata(struct video_device *vdev); |
591 | void video_set_drvdata(struct video_device *vdev, void *data); | |
a47ddf14 HV |
592 | |
593 | Note that you can safely call video_set_drvdata() before calling | |
594 | video_register_device(). | |
595 | ||
596 | And this function: | |
597 | ||
598 | struct video_device *video_devdata(struct file *file); | |
599 | ||
600 | returns the video_device belonging to the file struct. | |
601 | ||
eac8ea53 | 602 | The video_drvdata function combines video_get_drvdata with video_devdata: |
a47ddf14 HV |
603 | |
604 | void *video_drvdata(struct file *file); | |
605 | ||
606 | You can go from a video_device struct to the v4l2_device struct using: | |
607 | ||
dfa9a5ae | 608 | struct v4l2_device *v4l2_dev = vdev->v4l2_dev; |
44061c05 | 609 | |
eac8ea53 LP |
610 | - Device node name |
611 | ||
612 | The video_device node kernel name can be retrieved using | |
613 | ||
614 | const char *video_device_node_name(struct video_device *vdev); | |
615 | ||
616 | The name is used as a hint by userspace tools such as udev. The function | |
617 | should be used where possible instead of accessing the video_device::num and | |
618 | video_device::minor fields. | |
619 | ||
620 | ||
44061c05 MCC |
621 | video buffer helper functions |
622 | ----------------------------- | |
623 | ||
4b586a38 JC |
624 | The v4l2 core API provides a set of standard methods (called "videobuf") |
625 | for dealing with video buffers. Those methods allow a driver to implement | |
626 | read(), mmap() and overlay() in a consistent way. There are currently | |
627 | methods for using video buffers on devices that supports DMA with | |
628 | scatter/gather method (videobuf-dma-sg), DMA with linear access | |
629 | (videobuf-dma-contig), and vmalloced buffers, mostly used on USB drivers | |
630 | (videobuf-vmalloc). | |
631 | ||
632 | Please see Documentation/video4linux/videobuf for more information on how | |
633 | to use the videobuf layer. | |
6cd84b78 SA |
634 | |
635 | struct v4l2_fh | |
636 | -------------- | |
637 | ||
638 | struct v4l2_fh provides a way to easily keep file handle specific data | |
639 | that is used by the V4L2 framework. Using v4l2_fh is optional for | |
640 | drivers. | |
641 | ||
642 | The users of v4l2_fh (in the V4L2 framework, not the driver) know | |
643 | whether a driver uses v4l2_fh as its file->private_data pointer by | |
644 | testing the V4L2_FL_USES_V4L2_FH bit in video_device->flags. | |
645 | ||
646 | Useful functions: | |
647 | ||
648 | - v4l2_fh_init() | |
649 | ||
650 | Initialise the file handle. This *MUST* be performed in the driver's | |
651 | v4l2_file_operations->open() handler. | |
652 | ||
653 | - v4l2_fh_add() | |
654 | ||
655 | Add a v4l2_fh to video_device file handle list. May be called after | |
656 | initialising the file handle. | |
657 | ||
658 | - v4l2_fh_del() | |
659 | ||
660 | Unassociate the file handle from video_device(). The file handle | |
661 | exit function may now be called. | |
662 | ||
663 | - v4l2_fh_exit() | |
664 | ||
665 | Uninitialise the file handle. After uninitialisation the v4l2_fh | |
666 | memory can be freed. | |
667 | ||
668 | struct v4l2_fh is allocated as a part of the driver's own file handle | |
669 | structure and is set to file->private_data in the driver's open | |
670 | function by the driver. Drivers can extract their own file handle | |
671 | structure by using the container_of macro. Example: | |
672 | ||
673 | struct my_fh { | |
674 | int blah; | |
675 | struct v4l2_fh fh; | |
676 | }; | |
677 | ||
678 | ... | |
679 | ||
680 | int my_open(struct file *file) | |
681 | { | |
682 | struct my_fh *my_fh; | |
683 | struct video_device *vfd; | |
684 | int ret; | |
685 | ||
686 | ... | |
687 | ||
688 | ret = v4l2_fh_init(&my_fh->fh, vfd); | |
689 | if (ret) | |
690 | return ret; | |
691 | ||
692 | v4l2_fh_add(&my_fh->fh); | |
693 | ||
694 | file->private_data = &my_fh->fh; | |
695 | ||
696 | ... | |
697 | } | |
698 | ||
699 | int my_release(struct file *file) | |
700 | { | |
701 | struct v4l2_fh *fh = file->private_data; | |
702 | struct my_fh *my_fh = container_of(fh, struct my_fh, fh); | |
703 | ||
704 | ... | |
705 | } | |
dd966083 SA |
706 | |
707 | V4L2 events | |
708 | ----------- | |
709 | ||
710 | The V4L2 events provide a generic way to pass events to user space. | |
711 | The driver must use v4l2_fh to be able to support V4L2 events. | |
712 | ||
713 | Useful functions: | |
714 | ||
715 | - v4l2_event_alloc() | |
716 | ||
717 | To use events, the driver must allocate events for the file handle. By | |
718 | calling the function more than once, the driver may assure that at least n | |
719 | events in total have been allocated. The function may not be called in | |
720 | atomic context. | |
721 | ||
722 | - v4l2_event_queue() | |
723 | ||
724 | Queue events to video device. The driver's only responsibility is to fill | |
725 | in the type and the data fields. The other fields will be filled in by | |
726 | V4L2. | |
727 | ||
728 | - v4l2_event_subscribe() | |
729 | ||
730 | The video_device->ioctl_ops->vidioc_subscribe_event must check the driver | |
731 | is able to produce events with specified event id. Then it calls | |
732 | v4l2_event_subscribe() to subscribe the event. | |
733 | ||
734 | - v4l2_event_unsubscribe() | |
735 | ||
736 | vidioc_unsubscribe_event in struct v4l2_ioctl_ops. A driver may use | |
737 | v4l2_event_unsubscribe() directly unless it wants to be involved in | |
738 | unsubscription process. | |
739 | ||
740 | The special type V4L2_EVENT_ALL may be used to unsubscribe all events. The | |
741 | drivers may want to handle this in a special way. | |
742 | ||
743 | - v4l2_event_pending() | |
744 | ||
745 | Returns the number of pending events. Useful when implementing poll. | |
746 | ||
747 | Drivers do not initialise events directly. The events are initialised | |
748 | through v4l2_fh_init() if video_device->ioctl_ops->vidioc_subscribe_event is | |
749 | non-NULL. This *MUST* be performed in the driver's | |
750 | v4l2_file_operations->open() handler. | |
751 | ||
752 | Events are delivered to user space through the poll system call. The driver | |
753 | can use v4l2_fh->events->wait wait_queue_head_t as the argument for | |
754 | poll_wait(). | |
755 | ||
756 | There are standard and private events. New standard events must use the | |
757 | smallest available event type. The drivers must allocate their events from | |
758 | their own class starting from class base. Class base is | |
759 | V4L2_EVENT_PRIVATE_START + n * 1000 where n is the lowest available number. | |
760 | The first event type in the class is reserved for future use, so the first | |
761 | available event type is 'class base + 1'. | |
762 | ||
763 | An example on how the V4L2 events may be used can be found in the OMAP | |
764 | 3 ISP driver available at <URL:http://gitorious.org/omap3camera> as of | |
765 | writing this. |