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1 | Introduction |
2 | ============ | |
3 | ||
4 | The V4L2 control API seems simple enough, but quickly becomes very hard to | |
5 | implement correctly in drivers. But much of the code needed to handle controls | |
6 | is actually not driver specific and can be moved to the V4L core framework. | |
7 | ||
8 | After all, the only part that a driver developer is interested in is: | |
9 | ||
10 | 1) How do I add a control? | |
11 | 2) How do I set the control's value? (i.e. s_ctrl) | |
12 | ||
13 | And occasionally: | |
14 | ||
15 | 3) How do I get the control's value? (i.e. g_volatile_ctrl) | |
16 | 4) How do I validate the user's proposed control value? (i.e. try_ctrl) | |
17 | ||
18 | All the rest is something that can be done centrally. | |
19 | ||
20 | The control framework was created in order to implement all the rules of the | |
21 | V4L2 specification with respect to controls in a central place. And to make | |
22 | life as easy as possible for the driver developer. | |
23 | ||
24 | Note that the control framework relies on the presence of a struct v4l2_device | |
25 | for V4L2 drivers and struct v4l2_subdev for sub-device drivers. | |
26 | ||
27 | ||
28 | Objects in the framework | |
29 | ======================== | |
30 | ||
31 | There are two main objects: | |
32 | ||
33 | The v4l2_ctrl object describes the control properties and keeps track of the | |
34 | control's value (both the current value and the proposed new value). | |
35 | ||
36 | v4l2_ctrl_handler is the object that keeps track of controls. It maintains a | |
37 | list of v4l2_ctrl objects that it owns and another list of references to | |
38 | controls, possibly to controls owned by other handlers. | |
39 | ||
40 | ||
41 | Basic usage for V4L2 and sub-device drivers | |
42 | =========================================== | |
43 | ||
44 | 1) Prepare the driver: | |
45 | ||
46 | 1.1) Add the handler to your driver's top-level struct: | |
47 | ||
48 | struct foo_dev { | |
49 | ... | |
50 | struct v4l2_ctrl_handler ctrl_handler; | |
51 | ... | |
52 | }; | |
53 | ||
54 | struct foo_dev *foo; | |
55 | ||
56 | 1.2) Initialize the handler: | |
57 | ||
58 | v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls); | |
59 | ||
60 | The second argument is a hint telling the function how many controls this | |
61 | handler is expected to handle. It will allocate a hashtable based on this | |
62 | information. It is a hint only. | |
63 | ||
64 | 1.3) Hook the control handler into the driver: | |
65 | ||
66 | 1.3.1) For V4L2 drivers do this: | |
67 | ||
68 | struct foo_dev { | |
69 | ... | |
70 | struct v4l2_device v4l2_dev; | |
71 | ... | |
72 | struct v4l2_ctrl_handler ctrl_handler; | |
73 | ... | |
74 | }; | |
75 | ||
76 | foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler; | |
77 | ||
78 | Where foo->v4l2_dev is of type struct v4l2_device. | |
79 | ||
80 | Finally, remove all control functions from your v4l2_ioctl_ops: | |
81 | vidioc_queryctrl, vidioc_querymenu, vidioc_g_ctrl, vidioc_s_ctrl, | |
82 | vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls. | |
83 | Those are now no longer needed. | |
84 | ||
85 | 1.3.2) For sub-device drivers do this: | |
86 | ||
87 | struct foo_dev { | |
88 | ... | |
89 | struct v4l2_subdev sd; | |
90 | ... | |
91 | struct v4l2_ctrl_handler ctrl_handler; | |
92 | ... | |
93 | }; | |
94 | ||
95 | foo->sd.ctrl_handler = &foo->ctrl_handler; | |
96 | ||
97 | Where foo->sd is of type struct v4l2_subdev. | |
98 | ||
99 | And set all core control ops in your struct v4l2_subdev_core_ops to these | |
100 | helpers: | |
101 | ||
102 | .queryctrl = v4l2_subdev_queryctrl, | |
103 | .querymenu = v4l2_subdev_querymenu, | |
104 | .g_ctrl = v4l2_subdev_g_ctrl, | |
105 | .s_ctrl = v4l2_subdev_s_ctrl, | |
106 | .g_ext_ctrls = v4l2_subdev_g_ext_ctrls, | |
107 | .try_ext_ctrls = v4l2_subdev_try_ext_ctrls, | |
108 | .s_ext_ctrls = v4l2_subdev_s_ext_ctrls, | |
109 | ||
110 | Note: this is a temporary solution only. Once all V4L2 drivers that depend | |
111 | on subdev drivers are converted to the control framework these helpers will | |
112 | no longer be needed. | |
113 | ||
114 | 1.4) Clean up the handler at the end: | |
115 | ||
116 | v4l2_ctrl_handler_free(&foo->ctrl_handler); | |
117 | ||
118 | ||
119 | 2) Add controls: | |
120 | ||
121 | You add non-menu controls by calling v4l2_ctrl_new_std: | |
122 | ||
123 | struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl, | |
124 | const struct v4l2_ctrl_ops *ops, | |
125 | u32 id, s32 min, s32 max, u32 step, s32 def); | |
126 | ||
127 | Menu controls are added by calling v4l2_ctrl_new_std_menu: | |
128 | ||
129 | struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl, | |
130 | const struct v4l2_ctrl_ops *ops, | |
131 | u32 id, s32 max, s32 skip_mask, s32 def); | |
132 | ||
133 | These functions are typically called right after the v4l2_ctrl_handler_init: | |
134 | ||
135 | v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls); | |
136 | v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops, | |
137 | V4L2_CID_BRIGHTNESS, 0, 255, 1, 128); | |
138 | v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops, | |
139 | V4L2_CID_CONTRAST, 0, 255, 1, 128); | |
140 | v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops, | |
141 | V4L2_CID_POWER_LINE_FREQUENCY, | |
142 | V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0, | |
143 | V4L2_CID_POWER_LINE_FREQUENCY_DISABLED); | |
144 | ... | |
145 | if (foo->ctrl_handler.error) { | |
146 | int err = foo->ctrl_handler.error; | |
147 | ||
148 | v4l2_ctrl_handler_free(&foo->ctrl_handler); | |
149 | return err; | |
150 | } | |
151 | ||
152 | The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new | |
153 | control, but if you do not need to access the pointer outside the control ops, | |
154 | then there is no need to store it. | |
155 | ||
156 | The v4l2_ctrl_new_std function will fill in most fields based on the control | |
157 | ID except for the min, max, step and default values. These are passed in the | |
158 | last four arguments. These values are driver specific while control attributes | |
159 | like type, name, flags are all global. The control's current value will be set | |
160 | to the default value. | |
161 | ||
162 | The v4l2_ctrl_new_std_menu function is very similar but it is used for menu | |
163 | controls. There is no min argument since that is always 0 for menu controls, | |
164 | and instead of a step there is a skip_mask argument: if bit X is 1, then menu | |
165 | item X is skipped. | |
166 | ||
167 | Note that if something fails, the function will return NULL or an error and | |
168 | set ctrl_handler->error to the error code. If ctrl_handler->error was already | |
169 | set, then it will just return and do nothing. This is also true for | |
170 | v4l2_ctrl_handler_init if it cannot allocate the internal data structure. | |
171 | ||
172 | This makes it easy to init the handler and just add all controls and only check | |
173 | the error code at the end. Saves a lot of repetitive error checking. | |
174 | ||
175 | It is recommended to add controls in ascending control ID order: it will be | |
176 | a bit faster that way. | |
177 | ||
178 | 3) Optionally force initial control setup: | |
179 | ||
180 | v4l2_ctrl_handler_setup(&foo->ctrl_handler); | |
181 | ||
182 | This will call s_ctrl for all controls unconditionally. Effectively this | |
183 | initializes the hardware to the default control values. It is recommended | |
184 | that you do this as this ensures that both the internal data structures and | |
185 | the hardware are in sync. | |
186 | ||
187 | 4) Finally: implement the v4l2_ctrl_ops | |
188 | ||
189 | static const struct v4l2_ctrl_ops foo_ctrl_ops = { | |
190 | .s_ctrl = foo_s_ctrl, | |
191 | }; | |
192 | ||
193 | Usually all you need is s_ctrl: | |
194 | ||
195 | static int foo_s_ctrl(struct v4l2_ctrl *ctrl) | |
196 | { | |
197 | struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler); | |
198 | ||
199 | switch (ctrl->id) { | |
200 | case V4L2_CID_BRIGHTNESS: | |
201 | write_reg(0x123, ctrl->val); | |
202 | break; | |
203 | case V4L2_CID_CONTRAST: | |
204 | write_reg(0x456, ctrl->val); | |
205 | break; | |
206 | } | |
207 | return 0; | |
208 | } | |
209 | ||
210 | The control ops are called with the v4l2_ctrl pointer as argument. | |
211 | The new control value has already been validated, so all you need to do is | |
212 | to actually update the hardware registers. | |
213 | ||
214 | You're done! And this is sufficient for most of the drivers we have. No need | |
215 | to do any validation of control values, or implement QUERYCTRL/QUERYMENU. And | |
216 | G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported. | |
217 | ||
218 | ||
219 | ============================================================================== | |
220 | ||
221 | The remainder of this document deals with more advanced topics and scenarios. | |
222 | In practice the basic usage as described above is sufficient for most drivers. | |
223 | ||
224 | =============================================================================== | |
225 | ||
226 | ||
227 | Inheriting Controls | |
228 | =================== | |
229 | ||
230 | When a sub-device is registered with a V4L2 driver by calling | |
231 | v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev | |
232 | and v4l2_device are set, then the controls of the subdev will become | |
233 | automatically available in the V4L2 driver as well. If the subdev driver | |
234 | contains controls that already exist in the V4L2 driver, then those will be | |
235 | skipped (so a V4L2 driver can always override a subdev control). | |
236 | ||
237 | What happens here is that v4l2_device_register_subdev() calls | |
238 | v4l2_ctrl_add_handler() adding the controls of the subdev to the controls | |
239 | of v4l2_device. | |
240 | ||
241 | ||
242 | Accessing Control Values | |
243 | ======================== | |
244 | ||
245 | The v4l2_ctrl struct contains these two unions: | |
246 | ||
247 | /* The current control value. */ | |
248 | union { | |
249 | s32 val; | |
250 | s64 val64; | |
251 | char *string; | |
252 | } cur; | |
253 | ||
254 | /* The new control value. */ | |
255 | union { | |
256 | s32 val; | |
257 | s64 val64; | |
258 | char *string; | |
259 | }; | |
260 | ||
261 | Within the control ops you can freely use these. The val and val64 speak for | |
262 | themselves. The string pointers point to character buffers of length | |
263 | ctrl->maximum + 1, and are always 0-terminated. | |
264 | ||
265 | In most cases 'cur' contains the current cached control value. When you create | |
266 | a new control this value is made identical to the default value. After calling | |
267 | v4l2_ctrl_handler_setup() this value is passed to the hardware. It is generally | |
268 | a good idea to call this function. | |
269 | ||
270 | Whenever a new value is set that new value is automatically cached. This means | |
271 | that most drivers do not need to implement the g_volatile_ctrl() op. The | |
272 | exception is for controls that return a volatile register such as a signal | |
273 | strength read-out that changes continuously. In that case you will need to | |
274 | implement g_volatile_ctrl like this: | |
275 | ||
276 | static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl) | |
277 | { | |
278 | switch (ctrl->id) { | |
279 | case V4L2_CID_BRIGHTNESS: | |
280 | ctrl->cur.val = read_reg(0x123); | |
281 | break; | |
282 | } | |
283 | } | |
284 | ||
285 | The 'new value' union is not used in g_volatile_ctrl. In general controls | |
286 | that need to implement g_volatile_ctrl are read-only controls. | |
287 | ||
288 | To mark a control as volatile you have to set the is_volatile flag: | |
289 | ||
290 | ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...); | |
291 | if (ctrl) | |
292 | ctrl->is_volatile = 1; | |
293 | ||
294 | For try/s_ctrl the new values (i.e. as passed by the user) are filled in and | |
295 | you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union | |
296 | contains the current value, which you can use (but not change!) as well. | |
297 | ||
298 | If s_ctrl returns 0 (OK), then the control framework will copy the new final | |
299 | values to the 'cur' union. | |
300 | ||
301 | While in g_volatile/s/try_ctrl you can access the value of all controls owned | |
302 | by the same handler since the handler's lock is held. If you need to access | |
303 | the value of controls owned by other handlers, then you have to be very careful | |
304 | not to introduce deadlocks. | |
305 | ||
306 | Outside of the control ops you have to go through to helper functions to get | |
307 | or set a single control value safely in your driver: | |
308 | ||
309 | s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl); | |
310 | int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val); | |
311 | ||
312 | These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls | |
313 | do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that | |
314 | will result in a deadlock since these helpers lock the handler as well. | |
315 | ||
316 | You can also take the handler lock yourself: | |
317 | ||
318 | mutex_lock(&state->ctrl_handler.lock); | |
319 | printk(KERN_INFO "String value is '%s'\n", ctrl1->cur.string); | |
320 | printk(KERN_INFO "Integer value is '%s'\n", ctrl2->cur.val); | |
321 | mutex_unlock(&state->ctrl_handler.lock); | |
322 | ||
323 | ||
324 | Menu Controls | |
325 | ============= | |
326 | ||
327 | The v4l2_ctrl struct contains this union: | |
328 | ||
329 | union { | |
330 | u32 step; | |
331 | u32 menu_skip_mask; | |
332 | }; | |
333 | ||
334 | For menu controls menu_skip_mask is used. What it does is that it allows you | |
335 | to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU | |
336 | implementation where you can return -EINVAL if a certain menu item is not | |
337 | present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for | |
338 | menu controls. | |
339 | ||
340 | A good example is the MPEG Audio Layer II Bitrate menu control where the | |
341 | menu is a list of standardized possible bitrates. But in practice hardware | |
342 | implementations will only support a subset of those. By setting the skip | |
343 | mask you can tell the framework which menu items should be skipped. Setting | |
344 | it to 0 means that all menu items are supported. | |
345 | ||
346 | You set this mask either through the v4l2_ctrl_config struct for a custom | |
347 | control, or by calling v4l2_ctrl_new_std_menu(). | |
348 | ||
349 | ||
350 | Custom Controls | |
351 | =============== | |
352 | ||
353 | Driver specific controls can be created using v4l2_ctrl_new_custom(): | |
354 | ||
355 | static const struct v4l2_ctrl_config ctrl_filter = { | |
356 | .ops = &ctrl_custom_ops, | |
357 | .id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER, | |
358 | .name = "Spatial Filter", | |
359 | .type = V4L2_CTRL_TYPE_INTEGER, | |
360 | .flags = V4L2_CTRL_FLAG_SLIDER, | |
361 | .max = 15, | |
362 | .step = 1, | |
363 | }; | |
364 | ||
365 | ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL); | |
366 | ||
367 | The last argument is the priv pointer which can be set to driver-specific | |
368 | private data. | |
369 | ||
370 | The v4l2_ctrl_config struct also has fields to set the is_private and is_volatile | |
371 | flags. | |
372 | ||
373 | If the name field is not set, then the framework will assume this is a standard | |
374 | control and will fill in the name, type and flags fields accordingly. | |
375 | ||
376 | ||
377 | Active and Grabbed Controls | |
378 | =========================== | |
379 | ||
380 | If you get more complex relationships between controls, then you may have to | |
381 | activate and deactivate controls. For example, if the Chroma AGC control is | |
382 | on, then the Chroma Gain control is inactive. That is, you may set it, but | |
383 | the value will not be used by the hardware as long as the automatic gain | |
384 | control is on. Typically user interfaces can disable such input fields. | |
385 | ||
386 | You can set the 'active' status using v4l2_ctrl_activate(). By default all | |
387 | controls are active. Note that the framework does not check for this flag. | |
388 | It is meant purely for GUIs. The function is typically called from within | |
389 | s_ctrl. | |
390 | ||
391 | The other flag is the 'grabbed' flag. A grabbed control means that you cannot | |
392 | change it because it is in use by some resource. Typical examples are MPEG | |
393 | bitrate controls that cannot be changed while capturing is in progress. | |
394 | ||
395 | If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework | |
396 | will return -EBUSY if an attempt is made to set this control. The | |
397 | v4l2_ctrl_grab() function is typically called from the driver when it | |
398 | starts or stops streaming. | |
399 | ||
400 | ||
401 | Control Clusters | |
402 | ================ | |
403 | ||
404 | By default all controls are independent from the others. But in more | |
405 | complex scenarios you can get dependencies from one control to another. | |
406 | In that case you need to 'cluster' them: | |
407 | ||
408 | struct foo { | |
409 | struct v4l2_ctrl_handler ctrl_handler; | |
410 | #define AUDIO_CL_VOLUME (0) | |
411 | #define AUDIO_CL_MUTE (1) | |
412 | struct v4l2_ctrl *audio_cluster[2]; | |
413 | ... | |
414 | }; | |
415 | ||
416 | state->audio_cluster[AUDIO_CL_VOLUME] = | |
417 | v4l2_ctrl_new_std(&state->ctrl_handler, ...); | |
418 | state->audio_cluster[AUDIO_CL_MUTE] = | |
419 | v4l2_ctrl_new_std(&state->ctrl_handler, ...); | |
420 | v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster); | |
421 | ||
422 | From now on whenever one or more of the controls belonging to the same | |
423 | cluster is set (or 'gotten', or 'tried'), only the control ops of the first | |
424 | control ('volume' in this example) is called. You effectively create a new | |
425 | composite control. Similar to how a 'struct' works in C. | |
426 | ||
427 | So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set | |
428 | all two controls belonging to the audio_cluster: | |
429 | ||
430 | static int foo_s_ctrl(struct v4l2_ctrl *ctrl) | |
431 | { | |
432 | struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler); | |
433 | ||
434 | switch (ctrl->id) { | |
435 | case V4L2_CID_AUDIO_VOLUME: { | |
436 | struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE]; | |
437 | ||
438 | write_reg(0x123, mute->val ? 0 : ctrl->val); | |
439 | break; | |
440 | } | |
441 | case V4L2_CID_CONTRAST: | |
442 | write_reg(0x456, ctrl->val); | |
443 | break; | |
444 | } | |
445 | return 0; | |
446 | } | |
447 | ||
448 | In the example above the following are equivalent for the VOLUME case: | |
449 | ||
450 | ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME] | |
451 | ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE] | |
452 | ||
453 | Note that controls in a cluster may be NULL. For example, if for some | |
454 | reason mute was never added (because the hardware doesn't support that | |
455 | particular feature), then mute will be NULL. So in that case we have a | |
456 | cluster of 2 controls, of which only 1 is actually instantiated. The | |
457 | only restriction is that the first control of the cluster must always be | |
458 | present, since that is the 'master' control of the cluster. The master | |
459 | control is the one that identifies the cluster and that provides the | |
460 | pointer to the v4l2_ctrl_ops struct that is used for that cluster. | |
461 | ||
462 | Obviously, all controls in the cluster array must be initialized to either | |
463 | a valid control or to NULL. | |
464 | ||
465 | ||
466 | VIDIOC_LOG_STATUS Support | |
467 | ========================= | |
468 | ||
469 | This ioctl allow you to dump the current status of a driver to the kernel log. | |
470 | The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the | |
471 | value of the controls owned by the given handler to the log. You can supply a | |
472 | prefix as well. If the prefix didn't end with a space, then ': ' will be added | |
473 | for you. | |
474 | ||
475 | ||
476 | Different Handlers for Different Video Nodes | |
477 | ============================================ | |
478 | ||
479 | Usually the V4L2 driver has just one control handler that is global for | |
480 | all video nodes. But you can also specify different control handlers for | |
481 | different video nodes. You can do that by manually setting the ctrl_handler | |
482 | field of struct video_device. | |
483 | ||
484 | That is no problem if there are no subdevs involved but if there are, then | |
485 | you need to block the automatic merging of subdev controls to the global | |
486 | control handler. You do that by simply setting the ctrl_handler field in | |
487 | struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer | |
488 | merge subdev controls. | |
489 | ||
490 | After each subdev was added, you will then have to call v4l2_ctrl_add_handler | |
491 | manually to add the subdev's control handler (sd->ctrl_handler) to the desired | |
492 | control handler. This control handler may be specific to the video_device or | |
493 | for a subset of video_device's. For example: the radio device nodes only have | |
494 | audio controls, while the video and vbi device nodes share the same control | |
495 | handler for the audio and video controls. | |
496 | ||
497 | If you want to have one handler (e.g. for a radio device node) have a subset | |
498 | of another handler (e.g. for a video device node), then you should first add | |
499 | the controls to the first handler, add the other controls to the second | |
500 | handler and finally add the first handler to the second. For example: | |
501 | ||
502 | v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...); | |
503 | v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...); | |
504 | v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...); | |
505 | v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...); | |
506 | v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler); | |
507 | ||
508 | Or you can add specific controls to a handler: | |
509 | ||
510 | volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...); | |
511 | v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...); | |
512 | v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...); | |
513 | v4l2_ctrl_add_ctrl(&radio_ctrl_handler, volume); | |
514 | ||
515 | What you should not do is make two identical controls for two handlers. | |
516 | For example: | |
517 | ||
518 | v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...); | |
519 | v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...); | |
520 | ||
521 | This would be bad since muting the radio would not change the video mute | |
522 | control. The rule is to have one control for each hardware 'knob' that you | |
523 | can twiddle. | |
524 | ||
525 | ||
526 | Finding Controls | |
527 | ================ | |
528 | ||
529 | Normally you have created the controls yourself and you can store the struct | |
530 | v4l2_ctrl pointer into your own struct. | |
531 | ||
532 | But sometimes you need to find a control from another handler that you do | |
533 | not own. For example, if you have to find a volume control from a subdev. | |
534 | ||
535 | You can do that by calling v4l2_ctrl_find: | |
536 | ||
537 | struct v4l2_ctrl *volume; | |
538 | ||
539 | volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME); | |
540 | ||
541 | Since v4l2_ctrl_find will lock the handler you have to be careful where you | |
542 | use it. For example, this is not a good idea: | |
543 | ||
544 | struct v4l2_ctrl_handler ctrl_handler; | |
545 | ||
546 | v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...); | |
547 | v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...); | |
548 | ||
549 | ...and in video_ops.s_ctrl: | |
550 | ||
551 | case V4L2_CID_BRIGHTNESS: | |
552 | contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST); | |
553 | ... | |
554 | ||
555 | When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so | |
556 | attempting to find another control from the same handler will deadlock. | |
557 | ||
558 | It is recommended not to use this function from inside the control ops. | |
559 | ||
560 | ||
561 | Inheriting Controls | |
562 | =================== | |
563 | ||
564 | When one control handler is added to another using v4l2_ctrl_add_handler, then | |
565 | by default all controls from one are merged to the other. But a subdev might | |
566 | have low-level controls that make sense for some advanced embedded system, but | |
567 | not when it is used in consumer-level hardware. In that case you want to keep | |
568 | those low-level controls local to the subdev. You can do this by simply | |
569 | setting the 'is_private' flag of the control to 1: | |
570 | ||
571 | static const struct v4l2_ctrl_config ctrl_private = { | |
572 | .ops = &ctrl_custom_ops, | |
573 | .id = V4L2_CID_..., | |
574 | .name = "Some Private Control", | |
575 | .type = V4L2_CTRL_TYPE_INTEGER, | |
576 | .max = 15, | |
577 | .step = 1, | |
578 | .is_private = 1, | |
579 | }; | |
580 | ||
581 | ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL); | |
582 | ||
583 | These controls will now be skipped when v4l2_ctrl_add_handler is called. | |
584 | ||
585 | ||
586 | V4L2_CTRL_TYPE_CTRL_CLASS Controls | |
587 | ================================== | |
588 | ||
589 | Controls of this type can be used by GUIs to get the name of the control class. | |
590 | A fully featured GUI can make a dialog with multiple tabs with each tab | |
591 | containing the controls belonging to a particular control class. The name of | |
592 | each tab can be found by querying a special control with ID <control class | 1>. | |
593 | ||
594 | Drivers do not have to care about this. The framework will automatically add | |
595 | a control of this type whenever the first control belonging to a new control | |
596 | class is added. | |
597 | ||
598 | ||
599 | Differences from the Spec | |
600 | ========================= | |
601 | ||
602 | There are a few places where the framework acts slightly differently from the | |
603 | V4L2 Specification. Those differences are described in this section. We will | |
604 | have to see whether we need to adjust the spec or not. | |
605 | ||
606 | 1) It is no longer required to have all controls contained in a | |
607 | v4l2_ext_control array be from the same control class. The framework will be | |
608 | able to handle any type of control in the array. You need to set ctrl_class | |
609 | to 0 in order to enable this. If ctrl_class is non-zero, then it will still | |
610 | check that all controls belong to that control class. | |
611 | ||
612 | If you set ctrl_class to 0 and count to 0, then it will only return an error | |
613 | if there are no controls at all. | |
614 | ||
615 | 2) Clarified the way error_idx works. For get and set it will be equal to | |
616 | count if nothing was done yet. If it is less than count then only the controls | |
617 | up to error_idx-1 were successfully applied. | |
618 | ||
619 | 3) When attempting to read a button control the framework will return -EACCES | |
620 | instead of -EINVAL as stated in the spec. It seems to make more sense since | |
621 | button controls are write-only controls. | |
622 | ||
623 | 4) Attempting to write to a read-only control will return -EACCES instead of | |
624 | -EINVAL as the spec says. | |
625 | ||
626 | 5) The spec does not mention what should happen when you try to set/get a | |
627 | control class controls. ivtv currently returns -EINVAL (indicating that the | |
628 | control ID does not exist) while the framework will return -EACCES, which | |
629 | makes more sense. | |
630 | ||
631 | ||
632 | Proposals for Extensions | |
633 | ======================== | |
634 | ||
635 | Some ideas for future extensions to the spec: | |
636 | ||
637 | 1) Add a V4L2_CTRL_FLAG_HEX to have values shown as hexadecimal instead of | |
638 | decimal. Useful for e.g. video_mute_yuv. | |
639 | ||
640 | 2) It is possible to mark in the controls array which controls have been | |
641 | successfully written and which failed by for example adding a bit to the | |
642 | control ID. Not sure if it is worth the effort, though. | |
643 | ||
644 | 3) Trying to set volatile inactive controls should result in -EACCESS. | |
645 | ||
646 | 4) Add a new flag to mark volatile controls. Any application that wants | |
647 | to store the state of the controls can then skip volatile inactive controls. | |
648 | Currently it is not possible to detect such controls. |