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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 | ||
515f3287 SN |
133 | Or alternatively for integer menu controls, by calling v4l2_ctrl_new_int_menu: |
134 | ||
135 | struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl, | |
136 | const struct v4l2_ctrl_ops *ops, | |
137 | u32 id, s32 max, s32 def, const s64 *qmenu_int); | |
138 | ||
a42b57f5 HV |
139 | These functions are typically called right after the v4l2_ctrl_handler_init: |
140 | ||
515f3287 SN |
141 | static const s64 exp_bias_qmenu[] = { |
142 | -2, -1, 0, 1, 2 | |
143 | }; | |
144 | ||
a42b57f5 HV |
145 | v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls); |
146 | v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops, | |
147 | V4L2_CID_BRIGHTNESS, 0, 255, 1, 128); | |
148 | v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops, | |
149 | V4L2_CID_CONTRAST, 0, 255, 1, 128); | |
150 | v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops, | |
151 | V4L2_CID_POWER_LINE_FREQUENCY, | |
152 | V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0, | |
153 | V4L2_CID_POWER_LINE_FREQUENCY_DISABLED); | |
515f3287 SN |
154 | v4l2_ctrl_new_int_menu(&foo->ctrl_handler, &foo_ctrl_ops, |
155 | V4L2_CID_EXPOSURE_BIAS, | |
156 | ARRAY_SIZE(exp_bias_qmenu) - 1, | |
157 | ARRAY_SIZE(exp_bias_qmenu) / 2 - 1, | |
158 | exp_bias_qmenu); | |
a42b57f5 HV |
159 | ... |
160 | if (foo->ctrl_handler.error) { | |
161 | int err = foo->ctrl_handler.error; | |
162 | ||
163 | v4l2_ctrl_handler_free(&foo->ctrl_handler); | |
164 | return err; | |
165 | } | |
166 | ||
167 | The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new | |
168 | control, but if you do not need to access the pointer outside the control ops, | |
169 | then there is no need to store it. | |
170 | ||
171 | The v4l2_ctrl_new_std function will fill in most fields based on the control | |
172 | ID except for the min, max, step and default values. These are passed in the | |
173 | last four arguments. These values are driver specific while control attributes | |
174 | like type, name, flags are all global. The control's current value will be set | |
175 | to the default value. | |
176 | ||
177 | The v4l2_ctrl_new_std_menu function is very similar but it is used for menu | |
178 | controls. There is no min argument since that is always 0 for menu controls, | |
179 | and instead of a step there is a skip_mask argument: if bit X is 1, then menu | |
180 | item X is skipped. | |
181 | ||
515f3287 SN |
182 | The v4l2_ctrl_new_int_menu function creates a new standard integer menu |
183 | control with driver-specific items in the menu. It differs from | |
184 | v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and takes | |
185 | as the last argument an array of signed 64-bit integers that form an exact | |
186 | menu item list. | |
187 | ||
a42b57f5 HV |
188 | Note that if something fails, the function will return NULL or an error and |
189 | set ctrl_handler->error to the error code. If ctrl_handler->error was already | |
190 | set, then it will just return and do nothing. This is also true for | |
191 | v4l2_ctrl_handler_init if it cannot allocate the internal data structure. | |
192 | ||
193 | This makes it easy to init the handler and just add all controls and only check | |
194 | the error code at the end. Saves a lot of repetitive error checking. | |
195 | ||
196 | It is recommended to add controls in ascending control ID order: it will be | |
197 | a bit faster that way. | |
198 | ||
199 | 3) Optionally force initial control setup: | |
200 | ||
201 | v4l2_ctrl_handler_setup(&foo->ctrl_handler); | |
202 | ||
203 | This will call s_ctrl for all controls unconditionally. Effectively this | |
204 | initializes the hardware to the default control values. It is recommended | |
205 | that you do this as this ensures that both the internal data structures and | |
206 | the hardware are in sync. | |
207 | ||
208 | 4) Finally: implement the v4l2_ctrl_ops | |
209 | ||
210 | static const struct v4l2_ctrl_ops foo_ctrl_ops = { | |
211 | .s_ctrl = foo_s_ctrl, | |
212 | }; | |
213 | ||
214 | Usually all you need is s_ctrl: | |
215 | ||
216 | static int foo_s_ctrl(struct v4l2_ctrl *ctrl) | |
217 | { | |
218 | struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler); | |
219 | ||
220 | switch (ctrl->id) { | |
221 | case V4L2_CID_BRIGHTNESS: | |
222 | write_reg(0x123, ctrl->val); | |
223 | break; | |
224 | case V4L2_CID_CONTRAST: | |
225 | write_reg(0x456, ctrl->val); | |
226 | break; | |
227 | } | |
228 | return 0; | |
229 | } | |
230 | ||
231 | The control ops are called with the v4l2_ctrl pointer as argument. | |
232 | The new control value has already been validated, so all you need to do is | |
233 | to actually update the hardware registers. | |
234 | ||
235 | You're done! And this is sufficient for most of the drivers we have. No need | |
236 | to do any validation of control values, or implement QUERYCTRL/QUERYMENU. And | |
237 | G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported. | |
238 | ||
239 | ||
240 | ============================================================================== | |
241 | ||
242 | The remainder of this document deals with more advanced topics and scenarios. | |
243 | In practice the basic usage as described above is sufficient for most drivers. | |
244 | ||
245 | =============================================================================== | |
246 | ||
247 | ||
248 | Inheriting Controls | |
249 | =================== | |
250 | ||
251 | When a sub-device is registered with a V4L2 driver by calling | |
252 | v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev | |
253 | and v4l2_device are set, then the controls of the subdev will become | |
254 | automatically available in the V4L2 driver as well. If the subdev driver | |
255 | contains controls that already exist in the V4L2 driver, then those will be | |
256 | skipped (so a V4L2 driver can always override a subdev control). | |
257 | ||
258 | What happens here is that v4l2_device_register_subdev() calls | |
259 | v4l2_ctrl_add_handler() adding the controls of the subdev to the controls | |
260 | of v4l2_device. | |
261 | ||
262 | ||
263 | Accessing Control Values | |
264 | ======================== | |
265 | ||
266 | The v4l2_ctrl struct contains these two unions: | |
267 | ||
268 | /* The current control value. */ | |
269 | union { | |
270 | s32 val; | |
271 | s64 val64; | |
272 | char *string; | |
273 | } cur; | |
274 | ||
275 | /* The new control value. */ | |
276 | union { | |
277 | s32 val; | |
278 | s64 val64; | |
279 | char *string; | |
280 | }; | |
281 | ||
282 | Within the control ops you can freely use these. The val and val64 speak for | |
283 | themselves. The string pointers point to character buffers of length | |
284 | ctrl->maximum + 1, and are always 0-terminated. | |
285 | ||
286 | In most cases 'cur' contains the current cached control value. When you create | |
287 | a new control this value is made identical to the default value. After calling | |
288 | v4l2_ctrl_handler_setup() this value is passed to the hardware. It is generally | |
289 | a good idea to call this function. | |
290 | ||
291 | Whenever a new value is set that new value is automatically cached. This means | |
292 | that most drivers do not need to implement the g_volatile_ctrl() op. The | |
293 | exception is for controls that return a volatile register such as a signal | |
294 | strength read-out that changes continuously. In that case you will need to | |
295 | implement g_volatile_ctrl like this: | |
296 | ||
297 | static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl) | |
298 | { | |
299 | switch (ctrl->id) { | |
300 | case V4L2_CID_BRIGHTNESS: | |
78866efe | 301 | ctrl->val = read_reg(0x123); |
a42b57f5 HV |
302 | break; |
303 | } | |
304 | } | |
305 | ||
78866efe HV |
306 | Note that you use the 'new value' union as well in g_volatile_ctrl. In general |
307 | controls that need to implement g_volatile_ctrl are read-only controls. | |
2a863793 | 308 | |
88365105 | 309 | To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE: |
a42b57f5 HV |
310 | |
311 | ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...); | |
312 | if (ctrl) | |
88365105 | 313 | ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE; |
a42b57f5 HV |
314 | |
315 | For try/s_ctrl the new values (i.e. as passed by the user) are filled in and | |
316 | you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union | |
317 | contains the current value, which you can use (but not change!) as well. | |
318 | ||
319 | If s_ctrl returns 0 (OK), then the control framework will copy the new final | |
320 | values to the 'cur' union. | |
321 | ||
322 | While in g_volatile/s/try_ctrl you can access the value of all controls owned | |
323 | by the same handler since the handler's lock is held. If you need to access | |
324 | the value of controls owned by other handlers, then you have to be very careful | |
325 | not to introduce deadlocks. | |
326 | ||
327 | Outside of the control ops you have to go through to helper functions to get | |
328 | or set a single control value safely in your driver: | |
329 | ||
330 | s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl); | |
331 | int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val); | |
332 | ||
333 | These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls | |
334 | do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that | |
335 | will result in a deadlock since these helpers lock the handler as well. | |
336 | ||
337 | You can also take the handler lock yourself: | |
338 | ||
339 | mutex_lock(&state->ctrl_handler.lock); | |
340 | printk(KERN_INFO "String value is '%s'\n", ctrl1->cur.string); | |
341 | printk(KERN_INFO "Integer value is '%s'\n", ctrl2->cur.val); | |
342 | mutex_unlock(&state->ctrl_handler.lock); | |
343 | ||
344 | ||
345 | Menu Controls | |
346 | ============= | |
347 | ||
348 | The v4l2_ctrl struct contains this union: | |
349 | ||
350 | union { | |
351 | u32 step; | |
352 | u32 menu_skip_mask; | |
353 | }; | |
354 | ||
355 | For menu controls menu_skip_mask is used. What it does is that it allows you | |
356 | to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU | |
357 | implementation where you can return -EINVAL if a certain menu item is not | |
358 | present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for | |
359 | menu controls. | |
360 | ||
361 | A good example is the MPEG Audio Layer II Bitrate menu control where the | |
362 | menu is a list of standardized possible bitrates. But in practice hardware | |
363 | implementations will only support a subset of those. By setting the skip | |
364 | mask you can tell the framework which menu items should be skipped. Setting | |
365 | it to 0 means that all menu items are supported. | |
366 | ||
367 | You set this mask either through the v4l2_ctrl_config struct for a custom | |
368 | control, or by calling v4l2_ctrl_new_std_menu(). | |
369 | ||
370 | ||
371 | Custom Controls | |
372 | =============== | |
373 | ||
374 | Driver specific controls can be created using v4l2_ctrl_new_custom(): | |
375 | ||
376 | static const struct v4l2_ctrl_config ctrl_filter = { | |
377 | .ops = &ctrl_custom_ops, | |
378 | .id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER, | |
379 | .name = "Spatial Filter", | |
380 | .type = V4L2_CTRL_TYPE_INTEGER, | |
381 | .flags = V4L2_CTRL_FLAG_SLIDER, | |
382 | .max = 15, | |
383 | .step = 1, | |
384 | }; | |
385 | ||
386 | ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL); | |
387 | ||
388 | The last argument is the priv pointer which can be set to driver-specific | |
389 | private data. | |
390 | ||
88365105 | 391 | The v4l2_ctrl_config struct also has a field to set the is_private flag. |
a42b57f5 HV |
392 | |
393 | If the name field is not set, then the framework will assume this is a standard | |
394 | control and will fill in the name, type and flags fields accordingly. | |
395 | ||
396 | ||
397 | Active and Grabbed Controls | |
398 | =========================== | |
399 | ||
400 | If you get more complex relationships between controls, then you may have to | |
401 | activate and deactivate controls. For example, if the Chroma AGC control is | |
402 | on, then the Chroma Gain control is inactive. That is, you may set it, but | |
403 | the value will not be used by the hardware as long as the automatic gain | |
404 | control is on. Typically user interfaces can disable such input fields. | |
405 | ||
406 | You can set the 'active' status using v4l2_ctrl_activate(). By default all | |
407 | controls are active. Note that the framework does not check for this flag. | |
408 | It is meant purely for GUIs. The function is typically called from within | |
409 | s_ctrl. | |
410 | ||
411 | The other flag is the 'grabbed' flag. A grabbed control means that you cannot | |
412 | change it because it is in use by some resource. Typical examples are MPEG | |
413 | bitrate controls that cannot be changed while capturing is in progress. | |
414 | ||
415 | If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework | |
416 | will return -EBUSY if an attempt is made to set this control. The | |
417 | v4l2_ctrl_grab() function is typically called from the driver when it | |
418 | starts or stops streaming. | |
419 | ||
420 | ||
421 | Control Clusters | |
422 | ================ | |
423 | ||
424 | By default all controls are independent from the others. But in more | |
425 | complex scenarios you can get dependencies from one control to another. | |
426 | In that case you need to 'cluster' them: | |
427 | ||
428 | struct foo { | |
429 | struct v4l2_ctrl_handler ctrl_handler; | |
430 | #define AUDIO_CL_VOLUME (0) | |
431 | #define AUDIO_CL_MUTE (1) | |
432 | struct v4l2_ctrl *audio_cluster[2]; | |
433 | ... | |
434 | }; | |
435 | ||
436 | state->audio_cluster[AUDIO_CL_VOLUME] = | |
437 | v4l2_ctrl_new_std(&state->ctrl_handler, ...); | |
438 | state->audio_cluster[AUDIO_CL_MUTE] = | |
439 | v4l2_ctrl_new_std(&state->ctrl_handler, ...); | |
440 | v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster); | |
441 | ||
442 | From now on whenever one or more of the controls belonging to the same | |
443 | cluster is set (or 'gotten', or 'tried'), only the control ops of the first | |
444 | control ('volume' in this example) is called. You effectively create a new | |
445 | composite control. Similar to how a 'struct' works in C. | |
446 | ||
447 | So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set | |
448 | all two controls belonging to the audio_cluster: | |
449 | ||
450 | static int foo_s_ctrl(struct v4l2_ctrl *ctrl) | |
451 | { | |
452 | struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler); | |
453 | ||
454 | switch (ctrl->id) { | |
455 | case V4L2_CID_AUDIO_VOLUME: { | |
456 | struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE]; | |
457 | ||
458 | write_reg(0x123, mute->val ? 0 : ctrl->val); | |
459 | break; | |
460 | } | |
461 | case V4L2_CID_CONTRAST: | |
462 | write_reg(0x456, ctrl->val); | |
463 | break; | |
464 | } | |
465 | return 0; | |
466 | } | |
467 | ||
468 | In the example above the following are equivalent for the VOLUME case: | |
469 | ||
470 | ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME] | |
471 | ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE] | |
472 | ||
c76cd635 HV |
473 | In practice using cluster arrays like this becomes very tiresome. So instead |
474 | the following equivalent method is used: | |
475 | ||
476 | struct { | |
477 | /* audio cluster */ | |
478 | struct v4l2_ctrl *volume; | |
479 | struct v4l2_ctrl *mute; | |
480 | }; | |
481 | ||
482 | The anonymous struct is used to clearly 'cluster' these two control pointers, | |
483 | but it serves no other purpose. The effect is the same as creating an | |
484 | array with two control pointers. So you can just do: | |
485 | ||
486 | state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...); | |
487 | state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...); | |
488 | v4l2_ctrl_cluster(2, &state->volume); | |
489 | ||
490 | And in foo_s_ctrl you can use these pointers directly: state->mute->val. | |
491 | ||
a42b57f5 HV |
492 | Note that controls in a cluster may be NULL. For example, if for some |
493 | reason mute was never added (because the hardware doesn't support that | |
494 | particular feature), then mute will be NULL. So in that case we have a | |
495 | cluster of 2 controls, of which only 1 is actually instantiated. The | |
496 | only restriction is that the first control of the cluster must always be | |
497 | present, since that is the 'master' control of the cluster. The master | |
498 | control is the one that identifies the cluster and that provides the | |
499 | pointer to the v4l2_ctrl_ops struct that is used for that cluster. | |
500 | ||
501 | Obviously, all controls in the cluster array must be initialized to either | |
502 | a valid control or to NULL. | |
503 | ||
2a863793 HV |
504 | In rare cases you might want to know which controls of a cluster actually |
505 | were set explicitly by the user. For this you can check the 'is_new' flag of | |
506 | each control. For example, in the case of a volume/mute cluster the 'is_new' | |
507 | flag of the mute control would be set if the user called VIDIOC_S_CTRL for | |
508 | mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume | |
509 | controls, then the 'is_new' flag would be 1 for both controls. | |
510 | ||
511 | The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup(). | |
512 | ||
a42b57f5 | 513 | |
c76cd635 HV |
514 | Handling autogain/gain-type Controls with Auto Clusters |
515 | ======================================================= | |
516 | ||
517 | A common type of control cluster is one that handles 'auto-foo/foo'-type | |
518 | controls. Typical examples are autogain/gain, autoexposure/exposure, | |
882a935c | 519 | autowhitebalance/red balance/blue balance. In all cases you have one control |
c76cd635 HV |
520 | that determines whether another control is handled automatically by the hardware, |
521 | or whether it is under manual control from the user. | |
522 | ||
523 | If the cluster is in automatic mode, then the manual controls should be | |
882a935c HV |
524 | marked inactive and volatile. When the volatile controls are read the |
525 | g_volatile_ctrl operation should return the value that the hardware's automatic | |
526 | mode set up automatically. | |
c76cd635 HV |
527 | |
528 | If the cluster is put in manual mode, then the manual controls should become | |
882a935c HV |
529 | active again and the volatile flag is cleared (so g_volatile_ctrl is no longer |
530 | called while in manual mode). In addition just before switching to manual mode | |
531 | the current values as determined by the auto mode are copied as the new manual | |
532 | values. | |
c76cd635 HV |
533 | |
534 | Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since | |
535 | changing that control affects the control flags of the manual controls. | |
536 | ||
537 | In order to simplify this a special variation of v4l2_ctrl_cluster was | |
538 | introduced: | |
539 | ||
540 | void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls, | |
541 | u8 manual_val, bool set_volatile); | |
542 | ||
543 | The first two arguments are identical to v4l2_ctrl_cluster. The third argument | |
544 | tells the framework which value switches the cluster into manual mode. The | |
88365105 | 545 | last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls. |
882a935c HV |
546 | If it is false, then the manual controls are never volatile. You would typically |
547 | use that if the hardware does not give you the option to read back to values as | |
548 | determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow | |
549 | you to obtain the current gain value). | |
c76cd635 HV |
550 | |
551 | The first control of the cluster is assumed to be the 'auto' control. | |
552 | ||
553 | Using this function will ensure that you don't need to handle all the complex | |
554 | flag and volatile handling. | |
555 | ||
556 | ||
a42b57f5 HV |
557 | VIDIOC_LOG_STATUS Support |
558 | ========================= | |
559 | ||
560 | This ioctl allow you to dump the current status of a driver to the kernel log. | |
561 | The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the | |
562 | value of the controls owned by the given handler to the log. You can supply a | |
563 | prefix as well. If the prefix didn't end with a space, then ': ' will be added | |
564 | for you. | |
565 | ||
566 | ||
567 | Different Handlers for Different Video Nodes | |
568 | ============================================ | |
569 | ||
570 | Usually the V4L2 driver has just one control handler that is global for | |
571 | all video nodes. But you can also specify different control handlers for | |
572 | different video nodes. You can do that by manually setting the ctrl_handler | |
573 | field of struct video_device. | |
574 | ||
575 | That is no problem if there are no subdevs involved but if there are, then | |
576 | you need to block the automatic merging of subdev controls to the global | |
577 | control handler. You do that by simply setting the ctrl_handler field in | |
578 | struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer | |
579 | merge subdev controls. | |
580 | ||
581 | After each subdev was added, you will then have to call v4l2_ctrl_add_handler | |
582 | manually to add the subdev's control handler (sd->ctrl_handler) to the desired | |
583 | control handler. This control handler may be specific to the video_device or | |
584 | for a subset of video_device's. For example: the radio device nodes only have | |
585 | audio controls, while the video and vbi device nodes share the same control | |
586 | handler for the audio and video controls. | |
587 | ||
588 | If you want to have one handler (e.g. for a radio device node) have a subset | |
589 | of another handler (e.g. for a video device node), then you should first add | |
590 | the controls to the first handler, add the other controls to the second | |
591 | handler and finally add the first handler to the second. For example: | |
592 | ||
593 | v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...); | |
594 | v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...); | |
595 | v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...); | |
596 | v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...); | |
34a6b7d0 HV |
597 | v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL); |
598 | ||
599 | The last argument to v4l2_ctrl_add_handler() is a filter function that allows | |
600 | you to filter which controls will be added. Set it to NULL if you want to add | |
601 | all controls. | |
a42b57f5 HV |
602 | |
603 | Or you can add specific controls to a handler: | |
604 | ||
605 | volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...); | |
606 | v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...); | |
607 | v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...); | |
608 | v4l2_ctrl_add_ctrl(&radio_ctrl_handler, volume); | |
609 | ||
610 | What you should not do is make two identical controls for two handlers. | |
611 | For example: | |
612 | ||
613 | v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...); | |
614 | v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...); | |
615 | ||
616 | This would be bad since muting the radio would not change the video mute | |
617 | control. The rule is to have one control for each hardware 'knob' that you | |
618 | can twiddle. | |
619 | ||
620 | ||
621 | Finding Controls | |
622 | ================ | |
623 | ||
624 | Normally you have created the controls yourself and you can store the struct | |
625 | v4l2_ctrl pointer into your own struct. | |
626 | ||
627 | But sometimes you need to find a control from another handler that you do | |
628 | not own. For example, if you have to find a volume control from a subdev. | |
629 | ||
630 | You can do that by calling v4l2_ctrl_find: | |
631 | ||
632 | struct v4l2_ctrl *volume; | |
633 | ||
634 | volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME); | |
635 | ||
636 | Since v4l2_ctrl_find will lock the handler you have to be careful where you | |
637 | use it. For example, this is not a good idea: | |
638 | ||
639 | struct v4l2_ctrl_handler ctrl_handler; | |
640 | ||
641 | v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...); | |
642 | v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...); | |
643 | ||
644 | ...and in video_ops.s_ctrl: | |
645 | ||
646 | case V4L2_CID_BRIGHTNESS: | |
647 | contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST); | |
648 | ... | |
649 | ||
650 | When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so | |
651 | attempting to find another control from the same handler will deadlock. | |
652 | ||
653 | It is recommended not to use this function from inside the control ops. | |
654 | ||
655 | ||
656 | Inheriting Controls | |
657 | =================== | |
658 | ||
659 | When one control handler is added to another using v4l2_ctrl_add_handler, then | |
660 | by default all controls from one are merged to the other. But a subdev might | |
661 | have low-level controls that make sense for some advanced embedded system, but | |
662 | not when it is used in consumer-level hardware. In that case you want to keep | |
663 | those low-level controls local to the subdev. You can do this by simply | |
664 | setting the 'is_private' flag of the control to 1: | |
665 | ||
666 | static const struct v4l2_ctrl_config ctrl_private = { | |
667 | .ops = &ctrl_custom_ops, | |
668 | .id = V4L2_CID_..., | |
669 | .name = "Some Private Control", | |
670 | .type = V4L2_CTRL_TYPE_INTEGER, | |
671 | .max = 15, | |
672 | .step = 1, | |
673 | .is_private = 1, | |
674 | }; | |
675 | ||
676 | ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL); | |
677 | ||
678 | These controls will now be skipped when v4l2_ctrl_add_handler is called. | |
679 | ||
680 | ||
681 | V4L2_CTRL_TYPE_CTRL_CLASS Controls | |
682 | ================================== | |
683 | ||
684 | Controls of this type can be used by GUIs to get the name of the control class. | |
685 | A fully featured GUI can make a dialog with multiple tabs with each tab | |
686 | containing the controls belonging to a particular control class. The name of | |
687 | each tab can be found by querying a special control with ID <control class | 1>. | |
688 | ||
689 | Drivers do not have to care about this. The framework will automatically add | |
690 | a control of this type whenever the first control belonging to a new control | |
691 | class is added. | |
692 | ||
693 | ||
a42b57f5 HV |
694 | Proposals for Extensions |
695 | ======================== | |
696 | ||
697 | Some ideas for future extensions to the spec: | |
698 | ||
699 | 1) Add a V4L2_CTRL_FLAG_HEX to have values shown as hexadecimal instead of | |
700 | decimal. Useful for e.g. video_mute_yuv. | |
701 | ||
702 | 2) It is possible to mark in the controls array which controls have been | |
703 | successfully written and which failed by for example adding a bit to the | |
704 | control ID. Not sure if it is worth the effort, though. |