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Hans Verkuila42b57f2010-08-01 14:35:53 -03001Introduction
2============
3
4The V4L2 control API seems simple enough, but quickly becomes very hard to
5implement correctly in drivers. But much of the code needed to handle controls
6is actually not driver specific and can be moved to the V4L core framework.
7
8After all, the only part that a driver developer is interested in is:
9
101) How do I add a control?
112) How do I set the control's value? (i.e. s_ctrl)
12
13And occasionally:
14
153) How do I get the control's value? (i.e. g_volatile_ctrl)
164) How do I validate the user's proposed control value? (i.e. try_ctrl)
17
18All the rest is something that can be done centrally.
19
20The control framework was created in order to implement all the rules of the
21V4L2 specification with respect to controls in a central place. And to make
22life as easy as possible for the driver developer.
23
24Note that the control framework relies on the presence of a struct v4l2_device
25for V4L2 drivers and struct v4l2_subdev for sub-device drivers.
26
27
28Objects in the framework
29========================
30
31There are two main objects:
32
33The v4l2_ctrl object describes the control properties and keeps track of the
34control's value (both the current value and the proposed new value).
35
36v4l2_ctrl_handler is the object that keeps track of controls. It maintains a
37list of v4l2_ctrl objects that it owns and another list of references to
38controls, possibly to controls owned by other handlers.
39
40
41Basic usage for V4L2 and sub-device drivers
42===========================================
43
441) Prepare the driver:
45
461.1) 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
561.2) 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
641.3) Hook the control handler into the driver:
65
661.3.1) 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
851.3.2) 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
1141.4) Clean up the handler at the end:
115
116 v4l2_ctrl_handler_free(&foo->ctrl_handler);
117
118
1192) Add controls:
120
121You 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
127Menu 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
133These 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
152The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new
153control, but if you do not need to access the pointer outside the control ops,
154then there is no need to store it.
155
156The v4l2_ctrl_new_std function will fill in most fields based on the control
157ID except for the min, max, step and default values. These are passed in the
158last four arguments. These values are driver specific while control attributes
159like type, name, flags are all global. The control's current value will be set
160to the default value.
161
162The v4l2_ctrl_new_std_menu function is very similar but it is used for menu
163controls. There is no min argument since that is always 0 for menu controls,
164and instead of a step there is a skip_mask argument: if bit X is 1, then menu
165item X is skipped.
166
167Note that if something fails, the function will return NULL or an error and
168set ctrl_handler->error to the error code. If ctrl_handler->error was already
169set, then it will just return and do nothing. This is also true for
170v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
171
172This makes it easy to init the handler and just add all controls and only check
173the error code at the end. Saves a lot of repetitive error checking.
174
175It is recommended to add controls in ascending control ID order: it will be
176a bit faster that way.
177
1783) Optionally force initial control setup:
179
180 v4l2_ctrl_handler_setup(&foo->ctrl_handler);
181
182This will call s_ctrl for all controls unconditionally. Effectively this
183initializes the hardware to the default control values. It is recommended
184that you do this as this ensures that both the internal data structures and
185the hardware are in sync.
186
1874) 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
193Usually 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
210The control ops are called with the v4l2_ctrl pointer as argument.
211The new control value has already been validated, so all you need to do is
212to actually update the hardware registers.
213
214You're done! And this is sufficient for most of the drivers we have. No need
215to do any validation of control values, or implement QUERYCTRL/QUERYMENU. And
216G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
217
218
219==============================================================================
220
221The remainder of this document deals with more advanced topics and scenarios.
222In practice the basic usage as described above is sufficient for most drivers.
223
224===============================================================================
225
226
227Inheriting Controls
228===================
229
230When a sub-device is registered with a V4L2 driver by calling
231v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
232and v4l2_device are set, then the controls of the subdev will become
233automatically available in the V4L2 driver as well. If the subdev driver
234contains controls that already exist in the V4L2 driver, then those will be
235skipped (so a V4L2 driver can always override a subdev control).
236
237What happens here is that v4l2_device_register_subdev() calls
238v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
239of v4l2_device.
240
241
242Accessing Control Values
243========================
244
245The 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
261Within the control ops you can freely use these. The val and val64 speak for
262themselves. The string pointers point to character buffers of length
263ctrl->maximum + 1, and are always 0-terminated.
264
265In most cases 'cur' contains the current cached control value. When you create
266a new control this value is made identical to the default value. After calling
267v4l2_ctrl_handler_setup() this value is passed to the hardware. It is generally
268a good idea to call this function.
269
270Whenever a new value is set that new value is automatically cached. This means
271that most drivers do not need to implement the g_volatile_ctrl() op. The
272exception is for controls that return a volatile register such as a signal
273strength read-out that changes continuously. In that case you will need to
274implement 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:
Hans Verkuil78866ef2011-05-27 08:53:37 -0300280 ctrl->val = read_reg(0x123);
Hans Verkuila42b57f2010-08-01 14:35:53 -0300281 break;
282 }
283 }
284
Hans Verkuil78866ef2011-05-27 08:53:37 -0300285Note that you use the 'new value' union as well in g_volatile_ctrl. In general
286controls that need to implement g_volatile_ctrl are read-only controls.
Hans Verkuil2a863792011-01-11 14:45:03 -0300287
Hans Verkuil88365102011-08-26 07:35:14 -0300288To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
Hans Verkuila42b57f2010-08-01 14:35:53 -0300289
290 ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
291 if (ctrl)
Hans Verkuil88365102011-08-26 07:35:14 -0300292 ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
Hans Verkuila42b57f2010-08-01 14:35:53 -0300293
294For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
295you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
296contains the current value, which you can use (but not change!) as well.
297
298If s_ctrl returns 0 (OK), then the control framework will copy the new final
299values to the 'cur' union.
300
301While in g_volatile/s/try_ctrl you can access the value of all controls owned
302by the same handler since the handler's lock is held. If you need to access
303the value of controls owned by other handlers, then you have to be very careful
304not to introduce deadlocks.
305
306Outside of the control ops you have to go through to helper functions to get
307or 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
312These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
313do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
314will result in a deadlock since these helpers lock the handler as well.
315
316You 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
324Menu Controls
325=============
326
327The v4l2_ctrl struct contains this union:
328
329 union {
330 u32 step;
331 u32 menu_skip_mask;
332 };
333
334For menu controls menu_skip_mask is used. What it does is that it allows you
335to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
336implementation where you can return -EINVAL if a certain menu item is not
337present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
338menu controls.
339
340A good example is the MPEG Audio Layer II Bitrate menu control where the
341menu is a list of standardized possible bitrates. But in practice hardware
342implementations will only support a subset of those. By setting the skip
343mask you can tell the framework which menu items should be skipped. Setting
344it to 0 means that all menu items are supported.
345
346You set this mask either through the v4l2_ctrl_config struct for a custom
347control, or by calling v4l2_ctrl_new_std_menu().
348
349
350Custom Controls
351===============
352
353Driver 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
367The last argument is the priv pointer which can be set to driver-specific
368private data.
369
Hans Verkuil88365102011-08-26 07:35:14 -0300370The v4l2_ctrl_config struct also has a field to set the is_private flag.
Hans Verkuila42b57f2010-08-01 14:35:53 -0300371
372If the name field is not set, then the framework will assume this is a standard
373control and will fill in the name, type and flags fields accordingly.
374
375
376Active and Grabbed Controls
377===========================
378
379If you get more complex relationships between controls, then you may have to
380activate and deactivate controls. For example, if the Chroma AGC control is
381on, then the Chroma Gain control is inactive. That is, you may set it, but
382the value will not be used by the hardware as long as the automatic gain
383control is on. Typically user interfaces can disable such input fields.
384
385You can set the 'active' status using v4l2_ctrl_activate(). By default all
386controls are active. Note that the framework does not check for this flag.
387It is meant purely for GUIs. The function is typically called from within
388s_ctrl.
389
390The other flag is the 'grabbed' flag. A grabbed control means that you cannot
391change it because it is in use by some resource. Typical examples are MPEG
392bitrate controls that cannot be changed while capturing is in progress.
393
394If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
395will return -EBUSY if an attempt is made to set this control. The
396v4l2_ctrl_grab() function is typically called from the driver when it
397starts or stops streaming.
398
399
400Control Clusters
401================
402
403By default all controls are independent from the others. But in more
404complex scenarios you can get dependencies from one control to another.
405In that case you need to 'cluster' them:
406
407 struct foo {
408 struct v4l2_ctrl_handler ctrl_handler;
409#define AUDIO_CL_VOLUME (0)
410#define AUDIO_CL_MUTE (1)
411 struct v4l2_ctrl *audio_cluster[2];
412 ...
413 };
414
415 state->audio_cluster[AUDIO_CL_VOLUME] =
416 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
417 state->audio_cluster[AUDIO_CL_MUTE] =
418 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
419 v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
420
421From now on whenever one or more of the controls belonging to the same
422cluster is set (or 'gotten', or 'tried'), only the control ops of the first
423control ('volume' in this example) is called. You effectively create a new
424composite control. Similar to how a 'struct' works in C.
425
426So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
427all two controls belonging to the audio_cluster:
428
429 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
430 {
431 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
432
433 switch (ctrl->id) {
434 case V4L2_CID_AUDIO_VOLUME: {
435 struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
436
437 write_reg(0x123, mute->val ? 0 : ctrl->val);
438 break;
439 }
440 case V4L2_CID_CONTRAST:
441 write_reg(0x456, ctrl->val);
442 break;
443 }
444 return 0;
445 }
446
447In the example above the following are equivalent for the VOLUME case:
448
449 ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
450 ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
451
Hans Verkuilc76cd632011-06-07 05:46:53 -0300452In practice using cluster arrays like this becomes very tiresome. So instead
453the following equivalent method is used:
454
455 struct {
456 /* audio cluster */
457 struct v4l2_ctrl *volume;
458 struct v4l2_ctrl *mute;
459 };
460
461The anonymous struct is used to clearly 'cluster' these two control pointers,
462but it serves no other purpose. The effect is the same as creating an
463array with two control pointers. So you can just do:
464
465 state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
466 state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
467 v4l2_ctrl_cluster(2, &state->volume);
468
469And in foo_s_ctrl you can use these pointers directly: state->mute->val.
470
Hans Verkuila42b57f2010-08-01 14:35:53 -0300471Note that controls in a cluster may be NULL. For example, if for some
472reason mute was never added (because the hardware doesn't support that
473particular feature), then mute will be NULL. So in that case we have a
474cluster of 2 controls, of which only 1 is actually instantiated. The
475only restriction is that the first control of the cluster must always be
476present, since that is the 'master' control of the cluster. The master
477control is the one that identifies the cluster and that provides the
478pointer to the v4l2_ctrl_ops struct that is used for that cluster.
479
480Obviously, all controls in the cluster array must be initialized to either
481a valid control or to NULL.
482
Hans Verkuil2a863792011-01-11 14:45:03 -0300483In rare cases you might want to know which controls of a cluster actually
484were set explicitly by the user. For this you can check the 'is_new' flag of
485each control. For example, in the case of a volume/mute cluster the 'is_new'
486flag of the mute control would be set if the user called VIDIOC_S_CTRL for
487mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
488controls, then the 'is_new' flag would be 1 for both controls.
489
490The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
491
Hans Verkuila42b57f2010-08-01 14:35:53 -0300492
Hans Verkuilc76cd632011-06-07 05:46:53 -0300493Handling autogain/gain-type Controls with Auto Clusters
494=======================================================
495
496A common type of control cluster is one that handles 'auto-foo/foo'-type
497controls. Typical examples are autogain/gain, autoexposure/exposure,
Hans Verkuil882a9352011-08-26 08:35:59 -0300498autowhitebalance/red balance/blue balance. In all cases you have one control
Hans Verkuilc76cd632011-06-07 05:46:53 -0300499that determines whether another control is handled automatically by the hardware,
500or whether it is under manual control from the user.
501
502If the cluster is in automatic mode, then the manual controls should be
Hans Verkuil882a9352011-08-26 08:35:59 -0300503marked inactive and volatile. When the volatile controls are read the
504g_volatile_ctrl operation should return the value that the hardware's automatic
505mode set up automatically.
Hans Verkuilc76cd632011-06-07 05:46:53 -0300506
507If the cluster is put in manual mode, then the manual controls should become
Hans Verkuil882a9352011-08-26 08:35:59 -0300508active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
509called while in manual mode). In addition just before switching to manual mode
510the current values as determined by the auto mode are copied as the new manual
511values.
Hans Verkuilc76cd632011-06-07 05:46:53 -0300512
513Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
514changing that control affects the control flags of the manual controls.
515
516In order to simplify this a special variation of v4l2_ctrl_cluster was
517introduced:
518
519void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
520 u8 manual_val, bool set_volatile);
521
522The first two arguments are identical to v4l2_ctrl_cluster. The third argument
523tells the framework which value switches the cluster into manual mode. The
Hans Verkuil88365102011-08-26 07:35:14 -0300524last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
Hans Verkuil882a9352011-08-26 08:35:59 -0300525If it is false, then the manual controls are never volatile. You would typically
526use that if the hardware does not give you the option to read back to values as
527determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
528you to obtain the current gain value).
Hans Verkuilc76cd632011-06-07 05:46:53 -0300529
530The first control of the cluster is assumed to be the 'auto' control.
531
532Using this function will ensure that you don't need to handle all the complex
533flag and volatile handling.
534
535
Hans Verkuila42b57f2010-08-01 14:35:53 -0300536VIDIOC_LOG_STATUS Support
537=========================
538
539This ioctl allow you to dump the current status of a driver to the kernel log.
540The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
541value of the controls owned by the given handler to the log. You can supply a
542prefix as well. If the prefix didn't end with a space, then ': ' will be added
543for you.
544
545
546Different Handlers for Different Video Nodes
547============================================
548
549Usually the V4L2 driver has just one control handler that is global for
550all video nodes. But you can also specify different control handlers for
551different video nodes. You can do that by manually setting the ctrl_handler
552field of struct video_device.
553
554That is no problem if there are no subdevs involved but if there are, then
555you need to block the automatic merging of subdev controls to the global
556control handler. You do that by simply setting the ctrl_handler field in
557struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
558merge subdev controls.
559
560After each subdev was added, you will then have to call v4l2_ctrl_add_handler
561manually to add the subdev's control handler (sd->ctrl_handler) to the desired
562control handler. This control handler may be specific to the video_device or
563for a subset of video_device's. For example: the radio device nodes only have
564audio controls, while the video and vbi device nodes share the same control
565handler for the audio and video controls.
566
567If you want to have one handler (e.g. for a radio device node) have a subset
568of another handler (e.g. for a video device node), then you should first add
569the controls to the first handler, add the other controls to the second
570handler and finally add the first handler to the second. For example:
571
572 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
573 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
574 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
575 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
576 v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler);
577
578Or you can add specific controls to a handler:
579
580 volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
581 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
582 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
583 v4l2_ctrl_add_ctrl(&radio_ctrl_handler, volume);
584
585What you should not do is make two identical controls for two handlers.
586For example:
587
588 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
589 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
590
591This would be bad since muting the radio would not change the video mute
592control. The rule is to have one control for each hardware 'knob' that you
593can twiddle.
594
595
596Finding Controls
597================
598
599Normally you have created the controls yourself and you can store the struct
600v4l2_ctrl pointer into your own struct.
601
602But sometimes you need to find a control from another handler that you do
603not own. For example, if you have to find a volume control from a subdev.
604
605You can do that by calling v4l2_ctrl_find:
606
607 struct v4l2_ctrl *volume;
608
609 volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
610
611Since v4l2_ctrl_find will lock the handler you have to be careful where you
612use it. For example, this is not a good idea:
613
614 struct v4l2_ctrl_handler ctrl_handler;
615
616 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
617 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
618
619...and in video_ops.s_ctrl:
620
621 case V4L2_CID_BRIGHTNESS:
622 contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
623 ...
624
625When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
626attempting to find another control from the same handler will deadlock.
627
628It is recommended not to use this function from inside the control ops.
629
630
631Inheriting Controls
632===================
633
634When one control handler is added to another using v4l2_ctrl_add_handler, then
635by default all controls from one are merged to the other. But a subdev might
636have low-level controls that make sense for some advanced embedded system, but
637not when it is used in consumer-level hardware. In that case you want to keep
638those low-level controls local to the subdev. You can do this by simply
639setting the 'is_private' flag of the control to 1:
640
641 static const struct v4l2_ctrl_config ctrl_private = {
642 .ops = &ctrl_custom_ops,
643 .id = V4L2_CID_...,
644 .name = "Some Private Control",
645 .type = V4L2_CTRL_TYPE_INTEGER,
646 .max = 15,
647 .step = 1,
648 .is_private = 1,
649 };
650
651 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
652
653These controls will now be skipped when v4l2_ctrl_add_handler is called.
654
655
656V4L2_CTRL_TYPE_CTRL_CLASS Controls
657==================================
658
659Controls of this type can be used by GUIs to get the name of the control class.
660A fully featured GUI can make a dialog with multiple tabs with each tab
661containing the controls belonging to a particular control class. The name of
662each tab can be found by querying a special control with ID <control class | 1>.
663
664Drivers do not have to care about this. The framework will automatically add
665a control of this type whenever the first control belonging to a new control
666class is added.
667
668
Hans Verkuila42b57f2010-08-01 14:35:53 -0300669Proposals for Extensions
670========================
671
672Some ideas for future extensions to the spec:
673
6741) Add a V4L2_CTRL_FLAG_HEX to have values shown as hexadecimal instead of
675decimal. Useful for e.g. video_mute_yuv.
676
6772) It is possible to mark in the controls array which controls have been
678successfully written and which failed by for example adding a bit to the
679control ID. Not sure if it is worth the effort, though.