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2a1fcdf0 HV |
1 | Overview of the V4L2 driver framework |
2 | ===================================== | |
3 | ||
4 | This text documents the various structures provided by the V4L2 framework and | |
5 | their relationships. | |
6 | ||
7 | ||
8 | Introduction | |
9 | ------------ | |
10 | ||
11 | The V4L2 drivers tend to be very complex due to the complexity of the | |
12 | hardware: most devices have multiple ICs, export multiple device nodes in | |
13 | /dev, and create also non-V4L2 devices such as DVB, ALSA, FB, I2C and input | |
14 | (IR) devices. | |
15 | ||
16 | Especially the fact that V4L2 drivers have to setup supporting ICs to | |
17 | do audio/video muxing/encoding/decoding makes it more complex than most. | |
18 | Usually these ICs are connected to the main bridge driver through one or | |
19 | more I2C busses, but other busses can also be used. Such devices are | |
20 | called 'sub-devices'. | |
21 | ||
22 | For a long time the framework was limited to the video_device struct for | |
23 | creating V4L device nodes and video_buf for handling the video buffers | |
24 | (note that this document does not discuss the video_buf framework). | |
25 | ||
26 | This meant that all drivers had to do the setup of device instances and | |
27 | connecting to sub-devices themselves. Some of this is quite complicated | |
28 | to do right and many drivers never did do it correctly. | |
29 | ||
30 | There is also a lot of common code that could never be refactored due to | |
31 | the lack of a framework. | |
32 | ||
33 | So this framework sets up the basic building blocks that all drivers | |
34 | need and this same framework should make it much easier to refactor | |
35 | common code into utility functions shared by all drivers. | |
36 | ||
37 | ||
38 | Structure of a driver | |
39 | --------------------- | |
40 | ||
41 | All drivers have the following structure: | |
42 | ||
43 | 1) A struct for each device instance containing the device state. | |
44 | ||
45 | 2) A way of initializing and commanding sub-devices (if any). | |
46 | ||
47 | 3) Creating V4L2 device nodes (/dev/videoX, /dev/vbiX, /dev/radioX and | |
48 | /dev/vtxX) and keeping track of device-node specific data. | |
49 | ||
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 | |
87 | to v4l2_dev. Registration will also set v4l2_dev->name to a value derived from | |
88 | dev (driver name followed by the bus_id, to be precise). You may change the | |
89 | name after registration if you want. | |
90 | ||
a47ddf14 HV |
91 | The first 'dev' argument is normally the struct device pointer of a pci_dev, |
92 | usb_device or platform_device. | |
93 | ||
2a1fcdf0 HV |
94 | You unregister with: |
95 | ||
96 | v4l2_device_unregister(struct v4l2_device *v4l2_dev); | |
97 | ||
98 | Unregistering will also automatically unregister all subdevs from the device. | |
99 | ||
100 | Sometimes you need to iterate over all devices registered by a specific | |
101 | driver. This is usually the case if multiple device drivers use the same | |
102 | hardware. E.g. the ivtvfb driver is a framebuffer driver that uses the ivtv | |
103 | hardware. The same is true for alsa drivers for example. | |
104 | ||
105 | You can iterate over all registered devices as follows: | |
106 | ||
107 | static int callback(struct device *dev, void *p) | |
108 | { | |
109 | struct v4l2_device *v4l2_dev = dev_get_drvdata(dev); | |
110 | ||
111 | /* test if this device was inited */ | |
112 | if (v4l2_dev == NULL) | |
113 | return 0; | |
114 | ... | |
115 | return 0; | |
116 | } | |
117 | ||
118 | int iterate(void *p) | |
119 | { | |
120 | struct device_driver *drv; | |
121 | int err; | |
122 | ||
123 | /* Find driver 'ivtv' on the PCI bus. | |
124 | pci_bus_type is a global. For USB busses use usb_bus_type. */ | |
125 | drv = driver_find("ivtv", &pci_bus_type); | |
126 | /* iterate over all ivtv device instances */ | |
127 | err = driver_for_each_device(drv, NULL, p, callback); | |
128 | put_driver(drv); | |
129 | return err; | |
130 | } | |
131 | ||
132 | Sometimes you need to keep a running counter of the device instance. This is | |
133 | commonly used to map a device instance to an index of a module option array. | |
134 | ||
135 | The recommended approach is as follows: | |
136 | ||
137 | static atomic_t drv_instance = ATOMIC_INIT(0); | |
138 | ||
89aec3e1 | 139 | static int __devinit drv_probe(struct pci_dev *pdev, |
2a1fcdf0 HV |
140 | const struct pci_device_id *pci_id) |
141 | { | |
142 | ... | |
143 | state->instance = atomic_inc_return(&drv_instance) - 1; | |
144 | } | |
145 | ||
146 | ||
147 | struct v4l2_subdev | |
148 | ------------------ | |
149 | ||
150 | Many drivers need to communicate with sub-devices. These devices can do all | |
151 | sort of tasks, but most commonly they handle audio and/or video muxing, | |
152 | encoding or decoding. For webcams common sub-devices are sensors and camera | |
153 | controllers. | |
154 | ||
155 | Usually these are I2C devices, but not necessarily. In order to provide the | |
156 | driver with a consistent interface to these sub-devices the v4l2_subdev struct | |
157 | (v4l2-subdev.h) was created. | |
158 | ||
159 | Each sub-device driver must have a v4l2_subdev struct. This struct can be | |
160 | stand-alone for simple sub-devices or it might be embedded in a larger struct | |
161 | if more state information needs to be stored. Usually there is a low-level | |
162 | device struct (e.g. i2c_client) that contains the device data as setup | |
163 | by the kernel. It is recommended to store that pointer in the private | |
164 | data of v4l2_subdev using v4l2_set_subdevdata(). That makes it easy to go | |
165 | from a v4l2_subdev to the actual low-level bus-specific device data. | |
166 | ||
167 | You also need a way to go from the low-level struct to v4l2_subdev. For the | |
168 | common i2c_client struct the i2c_set_clientdata() call is used to store a | |
169 | v4l2_subdev pointer, for other busses you may have to use other methods. | |
170 | ||
171 | From the bridge driver perspective you load the sub-device module and somehow | |
172 | obtain the v4l2_subdev pointer. For i2c devices this is easy: you call | |
173 | i2c_get_clientdata(). For other busses something similar needs to be done. | |
174 | Helper functions exists for sub-devices on an I2C bus that do most of this | |
175 | tricky work for you. | |
176 | ||
177 | Each v4l2_subdev contains function pointers that sub-device drivers can | |
178 | implement (or leave NULL if it is not applicable). Since sub-devices can do | |
179 | so many different things and you do not want to end up with a huge ops struct | |
180 | of which only a handful of ops are commonly implemented, the function pointers | |
181 | are sorted according to category and each category has its own ops struct. | |
182 | ||
183 | The top-level ops struct contains pointers to the category ops structs, which | |
184 | may be NULL if the subdev driver does not support anything from that category. | |
185 | ||
186 | It looks like this: | |
187 | ||
188 | struct v4l2_subdev_core_ops { | |
aecde8b5 | 189 | int (*g_chip_ident)(struct v4l2_subdev *sd, struct v4l2_dbg_chip_ident *chip); |
2a1fcdf0 HV |
190 | int (*log_status)(struct v4l2_subdev *sd); |
191 | int (*init)(struct v4l2_subdev *sd, u32 val); | |
192 | ... | |
193 | }; | |
194 | ||
195 | struct v4l2_subdev_tuner_ops { | |
196 | ... | |
197 | }; | |
198 | ||
199 | struct v4l2_subdev_audio_ops { | |
200 | ... | |
201 | }; | |
202 | ||
203 | struct v4l2_subdev_video_ops { | |
204 | ... | |
205 | }; | |
206 | ||
207 | struct v4l2_subdev_ops { | |
208 | const struct v4l2_subdev_core_ops *core; | |
209 | const struct v4l2_subdev_tuner_ops *tuner; | |
210 | const struct v4l2_subdev_audio_ops *audio; | |
211 | const struct v4l2_subdev_video_ops *video; | |
212 | }; | |
213 | ||
214 | The core ops are common to all subdevs, the other categories are implemented | |
215 | depending on the sub-device. E.g. a video device is unlikely to support the | |
216 | audio ops and vice versa. | |
217 | ||
218 | This setup limits the number of function pointers while still making it easy | |
219 | to add new ops and categories. | |
220 | ||
221 | A sub-device driver initializes the v4l2_subdev struct using: | |
222 | ||
89aec3e1 | 223 | v4l2_subdev_init(sd, &ops); |
2a1fcdf0 HV |
224 | |
225 | Afterwards you need to initialize subdev->name with a unique name and set the | |
226 | module owner. This is done for you if you use the i2c helper functions. | |
227 | ||
228 | A device (bridge) driver needs to register the v4l2_subdev with the | |
229 | v4l2_device: | |
230 | ||
89aec3e1 | 231 | int err = v4l2_device_register_subdev(v4l2_dev, sd); |
2a1fcdf0 HV |
232 | |
233 | This can fail if the subdev module disappeared before it could be registered. | |
234 | After this function was called successfully the subdev->dev field points to | |
235 | the v4l2_device. | |
236 | ||
237 | You can unregister a sub-device using: | |
238 | ||
89aec3e1 | 239 | v4l2_device_unregister_subdev(sd); |
2a1fcdf0 | 240 | |
89aec3e1 | 241 | Afterwards the subdev module can be unloaded and sd->dev == NULL. |
2a1fcdf0 HV |
242 | |
243 | You can call an ops function either directly: | |
244 | ||
89aec3e1 | 245 | err = sd->ops->core->g_chip_ident(sd, &chip); |
2a1fcdf0 HV |
246 | |
247 | but it is better and easier to use this macro: | |
248 | ||
89aec3e1 | 249 | err = v4l2_subdev_call(sd, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
250 | |
251 | The macro will to the right NULL pointer checks and returns -ENODEV if subdev | |
252 | is NULL, -ENOIOCTLCMD if either subdev->core or subdev->core->g_chip_ident is | |
253 | NULL, or the actual result of the subdev->ops->core->g_chip_ident ops. | |
254 | ||
255 | It is also possible to call all or a subset of the sub-devices: | |
256 | ||
89aec3e1 | 257 | v4l2_device_call_all(v4l2_dev, 0, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
258 | |
259 | Any subdev that does not support this ops is skipped and error results are | |
260 | ignored. If you want to check for errors use this: | |
261 | ||
89aec3e1 | 262 | err = v4l2_device_call_until_err(v4l2_dev, 0, core, g_chip_ident, &chip); |
2a1fcdf0 HV |
263 | |
264 | Any error except -ENOIOCTLCMD will exit the loop with that error. If no | |
265 | errors (except -ENOIOCTLCMD) occured, then 0 is returned. | |
266 | ||
267 | The second argument to both calls is a group ID. If 0, then all subdevs are | |
268 | called. If non-zero, then only those whose group ID match that value will | |
269 | be called. Before a bridge driver registers a subdev it can set subdev->grp_id | |
270 | to whatever value it wants (it's 0 by default). This value is owned by the | |
271 | bridge driver and the sub-device driver will never modify or use it. | |
272 | ||
273 | The group ID gives the bridge driver more control how callbacks are called. | |
274 | For example, there may be multiple audio chips on a board, each capable of | |
275 | changing the volume. But usually only one will actually be used when the | |
276 | user want to change the volume. You can set the group ID for that subdev to | |
277 | e.g. AUDIO_CONTROLLER and specify that as the group ID value when calling | |
278 | v4l2_device_call_all(). That ensures that it will only go to the subdev | |
279 | that needs it. | |
280 | ||
281 | The advantage of using v4l2_subdev is that it is a generic struct and does | |
282 | not contain any knowledge about the underlying hardware. So a driver might | |
283 | contain several subdevs that use an I2C bus, but also a subdev that is | |
284 | controlled through GPIO pins. This distinction is only relevant when setting | |
285 | up the device, but once the subdev is registered it is completely transparent. | |
286 | ||
287 | ||
288 | I2C sub-device drivers | |
289 | ---------------------- | |
290 | ||
291 | Since these drivers are so common, special helper functions are available to | |
292 | ease the use of these drivers (v4l2-common.h). | |
293 | ||
294 | The recommended method of adding v4l2_subdev support to an I2C driver is to | |
295 | embed the v4l2_subdev struct into the state struct that is created for each | |
296 | I2C device instance. Very simple devices have no state struct and in that case | |
297 | you can just create a v4l2_subdev directly. | |
298 | ||
299 | A typical state struct would look like this (where 'chipname' is replaced by | |
300 | the name of the chip): | |
301 | ||
302 | struct chipname_state { | |
303 | struct v4l2_subdev sd; | |
304 | ... /* additional state fields */ | |
305 | }; | |
306 | ||
307 | Initialize the v4l2_subdev struct as follows: | |
308 | ||
309 | v4l2_i2c_subdev_init(&state->sd, client, subdev_ops); | |
310 | ||
311 | This function will fill in all the fields of v4l2_subdev and ensure that the | |
312 | v4l2_subdev and i2c_client both point to one another. | |
313 | ||
314 | You should also add a helper inline function to go from a v4l2_subdev pointer | |
315 | to a chipname_state struct: | |
316 | ||
317 | static inline struct chipname_state *to_state(struct v4l2_subdev *sd) | |
318 | { | |
319 | return container_of(sd, struct chipname_state, sd); | |
320 | } | |
321 | ||
322 | Use this to go from the v4l2_subdev struct to the i2c_client struct: | |
323 | ||
324 | struct i2c_client *client = v4l2_get_subdevdata(sd); | |
325 | ||
326 | And this to go from an i2c_client to a v4l2_subdev struct: | |
327 | ||
328 | struct v4l2_subdev *sd = i2c_get_clientdata(client); | |
329 | ||
330 | Finally you need to make a command function to make driver->command() | |
331 | call the right subdev_ops functions: | |
332 | ||
333 | static int subdev_command(struct i2c_client *client, unsigned cmd, void *arg) | |
334 | { | |
335 | return v4l2_subdev_command(i2c_get_clientdata(client), cmd, arg); | |
336 | } | |
337 | ||
338 | If driver->command is never used then you can leave this out. Eventually the | |
339 | driver->command usage should be removed from v4l. | |
340 | ||
341 | Make sure to call v4l2_device_unregister_subdev(sd) when the remove() callback | |
342 | is called. This will unregister the sub-device from the bridge driver. It is | |
343 | safe to call this even if the sub-device was never registered. | |
344 | ||
f5360bdc HV |
345 | You need to do this because when the bridge driver destroys the i2c adapter |
346 | the remove() callbacks are called of the i2c devices on that adapter. | |
347 | After that the corresponding v4l2_subdev structures are invalid, so they | |
348 | have to be unregistered first. Calling v4l2_device_unregister_subdev(sd) | |
349 | from the remove() callback ensures that this is always done correctly. | |
350 | ||
2a1fcdf0 HV |
351 | |
352 | The bridge driver also has some helper functions it can use: | |
353 | ||
354 | struct v4l2_subdev *sd = v4l2_i2c_new_subdev(adapter, "module_foo", "chipid", 0x36); | |
355 | ||
356 | This loads the given module (can be NULL if no module needs to be loaded) and | |
357 | calls i2c_new_device() with the given i2c_adapter and chip/address arguments. | |
358 | If all goes well, then it registers the subdev with the v4l2_device. It gets | |
359 | the v4l2_device by calling i2c_get_adapdata(adapter), so you should make sure | |
360 | that adapdata is set to v4l2_device when you setup the i2c_adapter in your | |
361 | driver. | |
362 | ||
363 | You can also use v4l2_i2c_new_probed_subdev() which is very similar to | |
364 | v4l2_i2c_new_subdev(), except that it has an array of possible I2C addresses | |
365 | that it should probe. Internally it calls i2c_new_probed_device(). | |
366 | ||
367 | Both functions return NULL if something went wrong. | |
368 | ||
369 | ||
370 | struct video_device | |
371 | ------------------- | |
372 | ||
a47ddf14 HV |
373 | The actual device nodes in the /dev directory are created using the |
374 | video_device struct (v4l2-dev.h). This struct can either be allocated | |
375 | dynamically or embedded in a larger struct. | |
376 | ||
377 | To allocate it dynamically use: | |
378 | ||
379 | struct video_device *vdev = video_device_alloc(); | |
380 | ||
381 | if (vdev == NULL) | |
382 | return -ENOMEM; | |
383 | ||
384 | vdev->release = video_device_release; | |
385 | ||
386 | If you embed it in a larger struct, then you must set the release() | |
387 | callback to your own function: | |
388 | ||
389 | struct video_device *vdev = &my_vdev->vdev; | |
390 | ||
391 | vdev->release = my_vdev_release; | |
392 | ||
393 | The release callback must be set and it is called when the last user | |
394 | of the video device exits. | |
395 | ||
396 | The default video_device_release() callback just calls kfree to free the | |
397 | allocated memory. | |
398 | ||
399 | You should also set these fields: | |
400 | ||
dfa9a5ae | 401 | - v4l2_dev: set to the v4l2_device parent device. |
a47ddf14 | 402 | - name: set to something descriptive and unique. |
c7dd09da | 403 | - fops: set to the v4l2_file_operations struct. |
a47ddf14 HV |
404 | - ioctl_ops: if you use the v4l2_ioctl_ops to simplify ioctl maintenance |
405 | (highly recommended to use this and it might become compulsory in the | |
406 | future!), then set this to your v4l2_ioctl_ops struct. | |
407 | ||
c7dd09da HV |
408 | If you use v4l2_ioctl_ops, then you should set either .unlocked_ioctl or |
409 | .ioctl to video_ioctl2 in your v4l2_file_operations struct. | |
410 | ||
411 | The v4l2_file_operations struct is a subset of file_operations. The main | |
412 | difference is that the inode argument is omitted since it is never used. | |
a47ddf14 HV |
413 | |
414 | ||
415 | video_device registration | |
416 | ------------------------- | |
417 | ||
418 | Next you register the video device: this will create the character device | |
419 | for you. | |
420 | ||
421 | err = video_register_device(vdev, VFL_TYPE_GRABBER, -1); | |
422 | if (err) { | |
50a2a8b3 | 423 | video_device_release(vdev); /* or kfree(my_vdev); */ |
a47ddf14 HV |
424 | return err; |
425 | } | |
426 | ||
427 | Which device is registered depends on the type argument. The following | |
428 | types exist: | |
429 | ||
430 | VFL_TYPE_GRABBER: videoX for video input/output devices | |
431 | VFL_TYPE_VBI: vbiX for vertical blank data (i.e. closed captions, teletext) | |
432 | VFL_TYPE_RADIO: radioX for radio tuners | |
433 | VFL_TYPE_VTX: vtxX for teletext devices (deprecated, don't use) | |
434 | ||
435 | The last argument gives you a certain amount of control over the device | |
436 | kernel number used (i.e. the X in videoX). Normally you will pass -1 to | |
437 | let the v4l2 framework pick the first free number. But if a driver creates | |
438 | many devices, then it can be useful to have different video devices in | |
439 | separate ranges. For example, video capture devices start at 0, video | |
440 | output devices start at 16. | |
441 | ||
442 | So you can use the last argument to specify a minimum kernel number and | |
443 | the v4l2 framework will try to pick the first free number that is equal | |
444 | or higher to what you passed. If that fails, then it will just pick the | |
445 | first free number. | |
446 | ||
447 | Whenever a device node is created some attributes are also created for you. | |
448 | If you look in /sys/class/video4linux you see the devices. Go into e.g. | |
449 | video0 and you will see 'name' and 'index' attributes. The 'name' attribute | |
450 | is the 'name' field of the video_device struct. The 'index' attribute is | |
451 | a device node index that can be assigned by the driver, or that is calculated | |
452 | for you. | |
453 | ||
454 | If you call video_register_device(), then the index is just increased by | |
455 | 1 for each device node you register. The first video device node you register | |
456 | always starts off with 0. | |
457 | ||
458 | Alternatively you can call video_register_device_index() which is identical | |
459 | to video_register_device(), but with an extra index argument. Here you can | |
460 | pass a specific index value (between 0 and 31) that should be used. | |
461 | ||
462 | Users can setup udev rules that utilize the index attribute to make fancy | |
463 | device names (e.g. 'mpegX' for MPEG video capture device nodes). | |
464 | ||
465 | After the device was successfully registered, then you can use these fields: | |
466 | ||
467 | - vfl_type: the device type passed to video_register_device. | |
468 | - minor: the assigned device minor number. | |
469 | - num: the device kernel number (i.e. the X in videoX). | |
470 | - index: the device index number (calculated or set explicitly using | |
471 | video_register_device_index). | |
472 | ||
473 | If the registration failed, then you need to call video_device_release() | |
474 | to free the allocated video_device struct, or free your own struct if the | |
475 | video_device was embedded in it. The vdev->release() callback will never | |
476 | be called if the registration failed, nor should you ever attempt to | |
477 | unregister the device if the registration failed. | |
478 | ||
479 | ||
480 | video_device cleanup | |
481 | -------------------- | |
482 | ||
483 | When the video device nodes have to be removed, either during the unload | |
484 | of the driver or because the USB device was disconnected, then you should | |
485 | unregister them: | |
486 | ||
487 | video_unregister_device(vdev); | |
488 | ||
489 | This will remove the device nodes from sysfs (causing udev to remove them | |
490 | from /dev). | |
491 | ||
492 | After video_unregister_device() returns no new opens can be done. | |
493 | ||
494 | However, in the case of USB devices some application might still have one | |
495 | of these device nodes open. You should block all new accesses to read, | |
496 | write, poll, etc. except possibly for certain ioctl operations like | |
497 | queueing buffers. | |
498 | ||
499 | When the last user of the video device node exits, then the vdev->release() | |
500 | callback is called and you can do the final cleanup there. | |
501 | ||
502 | ||
503 | video_device helper functions | |
504 | ----------------------------- | |
505 | ||
506 | There are a few useful helper functions: | |
507 | ||
508 | You can set/get driver private data in the video_device struct using: | |
509 | ||
89aec3e1 HV |
510 | void *video_get_drvdata(struct video_device *vdev); |
511 | void video_set_drvdata(struct video_device *vdev, void *data); | |
a47ddf14 HV |
512 | |
513 | Note that you can safely call video_set_drvdata() before calling | |
514 | video_register_device(). | |
515 | ||
516 | And this function: | |
517 | ||
518 | struct video_device *video_devdata(struct file *file); | |
519 | ||
520 | returns the video_device belonging to the file struct. | |
521 | ||
522 | The final helper function combines video_get_drvdata with | |
523 | video_devdata: | |
524 | ||
525 | void *video_drvdata(struct file *file); | |
526 | ||
527 | You can go from a video_device struct to the v4l2_device struct using: | |
528 | ||
dfa9a5ae | 529 | struct v4l2_device *v4l2_dev = vdev->v4l2_dev; |
44061c05 MCC |
530 | |
531 | video buffer helper functions | |
532 | ----------------------------- | |
533 | ||
534 | The v4l2 core API provides a standard method for dealing with video | |
535 | buffers. Those methods allow a driver to implement read(), mmap() and | |
536 | overlay() on a consistent way. | |
537 | ||
538 | There are currently methods for using video buffers on devices that | |
539 | supports DMA with scatter/gather method (videobuf-dma-sg), DMA with | |
540 | linear access (videobuf-dma-contig), and vmalloced buffers, mostly | |
541 | used on USB drivers (videobuf-vmalloc). | |
542 | ||
543 | Any driver using videobuf should provide operations (callbacks) for | |
544 | four handlers: | |
545 | ||
546 | ops->buf_setup - calculates the size of the video buffers and avoid they | |
547 | to waste more than some maximum limit of RAM; | |
548 | ops->buf_prepare - fills the video buffer structs and calls | |
549 | videobuf_iolock() to alloc and prepare mmaped memory; | |
550 | ops->buf_queue - advices the driver that another buffer were | |
551 | requested (by read() or by QBUF); | |
552 | ops->buf_release - frees any buffer that were allocated. | |
553 | ||
554 | In order to use it, the driver need to have a code (generally called at | |
555 | interrupt context) that will properly handle the buffer request lists, | |
556 | announcing that a new buffer were filled. | |
557 | ||
558 | The irq handling code should handle the videobuf task lists, in order | |
559 | to advice videobuf that a new frame were filled, in order to honor to a | |
560 | request. The code is generally like this one: | |
561 | if (list_empty(&dma_q->active)) | |
562 | return; | |
563 | ||
564 | buf = list_entry(dma_q->active.next, struct vbuffer, vb.queue); | |
565 | ||
566 | if (!waitqueue_active(&buf->vb.done)) | |
567 | return; | |
568 | ||
569 | /* Some logic to handle the buf may be needed here */ | |
570 | ||
571 | list_del(&buf->vb.queue); | |
572 | do_gettimeofday(&buf->vb.ts); | |
573 | wake_up(&buf->vb.done); | |
574 | ||
575 | Those are the videobuffer functions used on drivers, implemented on | |
576 | videobuf-core: | |
577 | ||
578 | - videobuf_queue_core_init() | |
579 | Initializes the videobuf infrastructure. This function should be | |
580 | called before any other videobuf function. | |
581 | ||
582 | - videobuf_iolock() | |
583 | Prepares the videobuf memory for the proper method (read, mmap, overlay). | |
584 | ||
585 | - videobuf_queue_is_busy() | |
586 | Checks if a videobuf is streaming. | |
587 | ||
588 | - videobuf_queue_cancel() | |
589 | Stops video handling. | |
590 | ||
591 | - videobuf_mmap_free() | |
592 | frees mmap buffers. | |
593 | ||
594 | - videobuf_stop() | |
595 | Stops video handling, ends mmap and frees mmap and other buffers. | |
596 | ||
597 | - V4L2 api functions. Those functions correspond to VIDIOC_foo ioctls: | |
598 | videobuf_reqbufs(), videobuf_querybuf(), videobuf_qbuf(), | |
599 | videobuf_dqbuf(), videobuf_streamon(), videobuf_streamoff(). | |
600 | ||
601 | - V4L1 api function (corresponds to VIDIOCMBUF ioctl): | |
602 | videobuf_cgmbuf() | |
603 | This function is used to provide backward compatibility with V4L1 | |
604 | API. | |
605 | ||
606 | - Some help functions for read()/poll() operations: | |
607 | videobuf_read_stream() | |
608 | For continuous stream read() | |
609 | videobuf_read_one() | |
610 | For snapshot read() | |
611 | videobuf_poll_stream() | |
612 | polling help function | |
613 | ||
614 | The better way to understand it is to take a look at vivi driver. One | |
615 | of the main reasons for vivi is to be a videobuf usage example. the | |
616 | vivi_thread_tick() does the task that the IRQ callback would do on PCI | |
617 | drivers (or the irq callback on USB). |