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.
8 After all, the only part that a driver developer is interested in is:
10 1) How do I add a control?
11 2) How do I set the control's value? (i.e. s_ctrl)
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)
18 All the rest is something that can be done centrally.
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.
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.
28 Objects in the framework
29 ========================
31 There are two main objects:
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).
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.
41 Basic usage for V4L2 and sub-device drivers
42 ===========================================
44 1) Prepare the driver:
46 1.1) Add the handler to your driver's top-level struct:
50 struct v4l2_ctrl_handler ctrl_handler;
56 1.2) Initialize the handler:
58 v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
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.
64 1.3) Hook the control handler into the driver:
66 1.3.1) For V4L2 drivers do this:
70 struct v4l2_device v4l2_dev;
72 struct v4l2_ctrl_handler ctrl_handler;
76 foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
78 Where foo->v4l2_dev is of type struct v4l2_device.
80 Finally, remove all control functions from your v4l2_ioctl_ops (if any):
81 vidioc_queryctrl, vidioc_query_ext_ctrl, vidioc_querymenu, vidioc_g_ctrl,
82 vidioc_s_ctrl, vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls.
83 Those are now no longer needed.
85 1.3.2) For sub-device drivers do this:
89 struct v4l2_subdev sd;
91 struct v4l2_ctrl_handler ctrl_handler;
95 foo->sd.ctrl_handler = &foo->ctrl_handler;
97 Where foo->sd is of type struct v4l2_subdev.
99 1.4) Clean up the handler at the end:
101 v4l2_ctrl_handler_free(&foo->ctrl_handler);
106 You add non-menu controls by calling v4l2_ctrl_new_std:
108 struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl,
109 const struct v4l2_ctrl_ops *ops,
110 u32 id, s32 min, s32 max, u32 step, s32 def);
112 Menu and integer menu controls are added by calling v4l2_ctrl_new_std_menu:
114 struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
115 const struct v4l2_ctrl_ops *ops,
116 u32 id, s32 max, s32 skip_mask, s32 def);
118 Menu controls with a driver specific menu are added by calling
119 v4l2_ctrl_new_std_menu_items:
121 struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
122 struct v4l2_ctrl_handler *hdl,
123 const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
124 s32 skip_mask, s32 def, const char * const *qmenu);
126 Integer menu controls with a driver specific menu can be added by calling
127 v4l2_ctrl_new_int_menu:
129 struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
130 const struct v4l2_ctrl_ops *ops,
131 u32 id, s32 max, s32 def, const s64 *qmenu_int);
133 These functions are typically called right after the v4l2_ctrl_handler_init:
135 static const s64 exp_bias_qmenu[] = {
138 static const char * const test_pattern[] = {
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);
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,
159 v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
160 V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
163 if (foo->ctrl_handler.error) {
164 int err = foo->ctrl_handler.error;
166 v4l2_ctrl_handler_free(&foo->ctrl_handler);
170 The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new
171 control, but if you do not need to access the pointer outside the control ops,
172 then there is no need to store it.
174 The v4l2_ctrl_new_std function will fill in most fields based on the control
175 ID except for the min, max, step and default values. These are passed in the
176 last four arguments. These values are driver specific while control attributes
177 like type, name, flags are all global. The control's current value will be set
178 to the default value.
180 The v4l2_ctrl_new_std_menu function is very similar but it is used for menu
181 controls. There is no min argument since that is always 0 for menu controls,
182 and instead of a step there is a skip_mask argument: if bit X is 1, then menu
185 The v4l2_ctrl_new_int_menu function creates a new standard integer menu
186 control with driver-specific items in the menu. It differs from
187 v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and takes
188 as the last argument an array of signed 64-bit integers that form an exact
191 The v4l2_ctrl_new_std_menu_items function is very similar to
192 v4l2_ctrl_new_std_menu but takes an extra parameter qmenu, which is the driver
193 specific menu for an otherwise standard menu control. A good example for this
194 control is the test pattern control for capture/display/sensors devices that
195 have the capability to generate test patterns. These test patterns are hardware
196 specific, so the contents of the menu will vary from device to device.
198 Note that if something fails, the function will return NULL or an error and
199 set ctrl_handler->error to the error code. If ctrl_handler->error was already
200 set, then it will just return and do nothing. This is also true for
201 v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
203 This makes it easy to init the handler and just add all controls and only check
204 the error code at the end. Saves a lot of repetitive error checking.
206 It is recommended to add controls in ascending control ID order: it will be
207 a bit faster that way.
209 3) Optionally force initial control setup:
211 v4l2_ctrl_handler_setup(&foo->ctrl_handler);
213 This will call s_ctrl for all controls unconditionally. Effectively this
214 initializes the hardware to the default control values. It is recommended
215 that you do this as this ensures that both the internal data structures and
216 the hardware are in sync.
218 4) Finally: implement the v4l2_ctrl_ops
220 static const struct v4l2_ctrl_ops foo_ctrl_ops = {
221 .s_ctrl = foo_s_ctrl,
224 Usually all you need is s_ctrl:
226 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
228 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
231 case V4L2_CID_BRIGHTNESS:
232 write_reg(0x123, ctrl->val);
234 case V4L2_CID_CONTRAST:
235 write_reg(0x456, ctrl->val);
241 The control ops are called with the v4l2_ctrl pointer as argument.
242 The new control value has already been validated, so all you need to do is
243 to actually update the hardware registers.
245 You're done! And this is sufficient for most of the drivers we have. No need
246 to do any validation of control values, or implement QUERYCTRL, QUERY_EXT_CTRL
247 and QUERYMENU. And G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
250 ==============================================================================
252 The remainder of this document deals with more advanced topics and scenarios.
253 In practice the basic usage as described above is sufficient for most drivers.
255 ===============================================================================
261 When a sub-device is registered with a V4L2 driver by calling
262 v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
263 and v4l2_device are set, then the controls of the subdev will become
264 automatically available in the V4L2 driver as well. If the subdev driver
265 contains controls that already exist in the V4L2 driver, then those will be
266 skipped (so a V4L2 driver can always override a subdev control).
268 What happens here is that v4l2_device_register_subdev() calls
269 v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
273 Accessing Control Values
274 ========================
276 The following union is used inside the control framework to access control
279 union v4l2_ctrl_ptr {
286 The v4l2_ctrl struct contains these fields that can be used to access both
287 current and new values:
295 union v4l2_ctrl_ptr p_new;
296 union v4l2_ctrl_ptr p_cur;
298 If the control has a simple s32 type type, then:
300 &ctrl->val == ctrl->p_new.p_s32
301 &ctrl->cur.val == ctrl->p_cur.p_s32
303 For all other types use ctrl->p_cur.p<something>. Basically the val
304 and cur.val fields can be considered an alias since these are used so often.
306 Within the control ops you can freely use these. The val and cur.val speak for
307 themselves. The p_char pointers point to character buffers of length
308 ctrl->maximum + 1, and are always 0-terminated.
310 Unless the control is marked volatile the p_cur field points to the the
311 current cached control value. When you create a new control this value is made
312 identical to the default value. After calling v4l2_ctrl_handler_setup() this
313 value is passed to the hardware. It is generally a good idea to call this
316 Whenever a new value is set that new value is automatically cached. This means
317 that most drivers do not need to implement the g_volatile_ctrl() op. The
318 exception is for controls that return a volatile register such as a signal
319 strength read-out that changes continuously. In that case you will need to
320 implement g_volatile_ctrl like this:
322 static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl)
325 case V4L2_CID_BRIGHTNESS:
326 ctrl->val = read_reg(0x123);
331 Note that you use the 'new value' union as well in g_volatile_ctrl. In general
332 controls that need to implement g_volatile_ctrl are read-only controls. If they
333 are not, a V4L2_EVENT_CTRL_CH_VALUE will not be generated when the control
336 To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
338 ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
340 ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
342 For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
343 you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
344 contains the current value, which you can use (but not change!) as well.
346 If s_ctrl returns 0 (OK), then the control framework will copy the new final
347 values to the 'cur' union.
349 While in g_volatile/s/try_ctrl you can access the value of all controls owned
350 by the same handler since the handler's lock is held. If you need to access
351 the value of controls owned by other handlers, then you have to be very careful
352 not to introduce deadlocks.
354 Outside of the control ops you have to go through to helper functions to get
355 or set a single control value safely in your driver:
357 s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl);
358 int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val);
360 These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
361 do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
362 will result in a deadlock since these helpers lock the handler as well.
364 You can also take the handler lock yourself:
366 mutex_lock(&state->ctrl_handler.lock);
367 pr_info("String value is '%s'\n", ctrl1->p_cur.p_char);
368 pr_info("Integer value is '%s'\n", ctrl2->cur.val);
369 mutex_unlock(&state->ctrl_handler.lock);
375 The v4l2_ctrl struct contains this union:
382 For menu controls menu_skip_mask is used. What it does is that it allows you
383 to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
384 implementation where you can return -EINVAL if a certain menu item is not
385 present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
388 A good example is the MPEG Audio Layer II Bitrate menu control where the
389 menu is a list of standardized possible bitrates. But in practice hardware
390 implementations will only support a subset of those. By setting the skip
391 mask you can tell the framework which menu items should be skipped. Setting
392 it to 0 means that all menu items are supported.
394 You set this mask either through the v4l2_ctrl_config struct for a custom
395 control, or by calling v4l2_ctrl_new_std_menu().
401 Driver specific controls can be created using v4l2_ctrl_new_custom():
403 static const struct v4l2_ctrl_config ctrl_filter = {
404 .ops = &ctrl_custom_ops,
405 .id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER,
406 .name = "Spatial Filter",
407 .type = V4L2_CTRL_TYPE_INTEGER,
408 .flags = V4L2_CTRL_FLAG_SLIDER,
413 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL);
415 The last argument is the priv pointer which can be set to driver-specific
418 The v4l2_ctrl_config struct also has a field to set the is_private flag.
420 If the name field is not set, then the framework will assume this is a standard
421 control and will fill in the name, type and flags fields accordingly.
424 Active and Grabbed Controls
425 ===========================
427 If you get more complex relationships between controls, then you may have to
428 activate and deactivate controls. For example, if the Chroma AGC control is
429 on, then the Chroma Gain control is inactive. That is, you may set it, but
430 the value will not be used by the hardware as long as the automatic gain
431 control is on. Typically user interfaces can disable such input fields.
433 You can set the 'active' status using v4l2_ctrl_activate(). By default all
434 controls are active. Note that the framework does not check for this flag.
435 It is meant purely for GUIs. The function is typically called from within
438 The other flag is the 'grabbed' flag. A grabbed control means that you cannot
439 change it because it is in use by some resource. Typical examples are MPEG
440 bitrate controls that cannot be changed while capturing is in progress.
442 If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
443 will return -EBUSY if an attempt is made to set this control. The
444 v4l2_ctrl_grab() function is typically called from the driver when it
445 starts or stops streaming.
451 By default all controls are independent from the others. But in more
452 complex scenarios you can get dependencies from one control to another.
453 In that case you need to 'cluster' them:
456 struct v4l2_ctrl_handler ctrl_handler;
457 #define AUDIO_CL_VOLUME (0)
458 #define AUDIO_CL_MUTE (1)
459 struct v4l2_ctrl *audio_cluster[2];
463 state->audio_cluster[AUDIO_CL_VOLUME] =
464 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
465 state->audio_cluster[AUDIO_CL_MUTE] =
466 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
467 v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
469 From now on whenever one or more of the controls belonging to the same
470 cluster is set (or 'gotten', or 'tried'), only the control ops of the first
471 control ('volume' in this example) is called. You effectively create a new
472 composite control. Similar to how a 'struct' works in C.
474 So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
475 all two controls belonging to the audio_cluster:
477 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
479 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
482 case V4L2_CID_AUDIO_VOLUME: {
483 struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
485 write_reg(0x123, mute->val ? 0 : ctrl->val);
488 case V4L2_CID_CONTRAST:
489 write_reg(0x456, ctrl->val);
495 In the example above the following are equivalent for the VOLUME case:
497 ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
498 ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
500 In practice using cluster arrays like this becomes very tiresome. So instead
501 the following equivalent method is used:
505 struct v4l2_ctrl *volume;
506 struct v4l2_ctrl *mute;
509 The anonymous struct is used to clearly 'cluster' these two control pointers,
510 but it serves no other purpose. The effect is the same as creating an
511 array with two control pointers. So you can just do:
513 state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
514 state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
515 v4l2_ctrl_cluster(2, &state->volume);
517 And in foo_s_ctrl you can use these pointers directly: state->mute->val.
519 Note that controls in a cluster may be NULL. For example, if for some
520 reason mute was never added (because the hardware doesn't support that
521 particular feature), then mute will be NULL. So in that case we have a
522 cluster of 2 controls, of which only 1 is actually instantiated. The
523 only restriction is that the first control of the cluster must always be
524 present, since that is the 'master' control of the cluster. The master
525 control is the one that identifies the cluster and that provides the
526 pointer to the v4l2_ctrl_ops struct that is used for that cluster.
528 Obviously, all controls in the cluster array must be initialized to either
529 a valid control or to NULL.
531 In rare cases you might want to know which controls of a cluster actually
532 were set explicitly by the user. For this you can check the 'is_new' flag of
533 each control. For example, in the case of a volume/mute cluster the 'is_new'
534 flag of the mute control would be set if the user called VIDIOC_S_CTRL for
535 mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
536 controls, then the 'is_new' flag would be 1 for both controls.
538 The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
541 Handling autogain/gain-type Controls with Auto Clusters
542 =======================================================
544 A common type of control cluster is one that handles 'auto-foo/foo'-type
545 controls. Typical examples are autogain/gain, autoexposure/exposure,
546 autowhitebalance/red balance/blue balance. In all cases you have one control
547 that determines whether another control is handled automatically by the hardware,
548 or whether it is under manual control from the user.
550 If the cluster is in automatic mode, then the manual controls should be
551 marked inactive and volatile. When the volatile controls are read the
552 g_volatile_ctrl operation should return the value that the hardware's automatic
553 mode set up automatically.
555 If the cluster is put in manual mode, then the manual controls should become
556 active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
557 called while in manual mode). In addition just before switching to manual mode
558 the current values as determined by the auto mode are copied as the new manual
561 Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
562 changing that control affects the control flags of the manual controls.
564 In order to simplify this a special variation of v4l2_ctrl_cluster was
567 void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
568 u8 manual_val, bool set_volatile);
570 The first two arguments are identical to v4l2_ctrl_cluster. The third argument
571 tells the framework which value switches the cluster into manual mode. The
572 last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
573 If it is false, then the manual controls are never volatile. You would typically
574 use that if the hardware does not give you the option to read back to values as
575 determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
576 you to obtain the current gain value).
578 The first control of the cluster is assumed to be the 'auto' control.
580 Using this function will ensure that you don't need to handle all the complex
581 flag and volatile handling.
584 VIDIOC_LOG_STATUS Support
585 =========================
587 This ioctl allow you to dump the current status of a driver to the kernel log.
588 The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
589 value of the controls owned by the given handler to the log. You can supply a
590 prefix as well. If the prefix didn't end with a space, then ': ' will be added
594 Different Handlers for Different Video Nodes
595 ============================================
597 Usually the V4L2 driver has just one control handler that is global for
598 all video nodes. But you can also specify different control handlers for
599 different video nodes. You can do that by manually setting the ctrl_handler
600 field of struct video_device.
602 That is no problem if there are no subdevs involved but if there are, then
603 you need to block the automatic merging of subdev controls to the global
604 control handler. You do that by simply setting the ctrl_handler field in
605 struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
606 merge subdev controls.
608 After each subdev was added, you will then have to call v4l2_ctrl_add_handler
609 manually to add the subdev's control handler (sd->ctrl_handler) to the desired
610 control handler. This control handler may be specific to the video_device or
611 for a subset of video_device's. For example: the radio device nodes only have
612 audio controls, while the video and vbi device nodes share the same control
613 handler for the audio and video controls.
615 If you want to have one handler (e.g. for a radio device node) have a subset
616 of another handler (e.g. for a video device node), then you should first add
617 the controls to the first handler, add the other controls to the second
618 handler and finally add the first handler to the second. For example:
620 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
621 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
622 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
623 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
624 v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL);
626 The last argument to v4l2_ctrl_add_handler() is a filter function that allows
627 you to filter which controls will be added. Set it to NULL if you want to add
630 Or you can add specific controls to a handler:
632 volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
633 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
634 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
636 What you should not do is make two identical controls for two handlers.
639 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
640 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
642 This would be bad since muting the radio would not change the video mute
643 control. The rule is to have one control for each hardware 'knob' that you
650 Normally you have created the controls yourself and you can store the struct
651 v4l2_ctrl pointer into your own struct.
653 But sometimes you need to find a control from another handler that you do
654 not own. For example, if you have to find a volume control from a subdev.
656 You can do that by calling v4l2_ctrl_find:
658 struct v4l2_ctrl *volume;
660 volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
662 Since v4l2_ctrl_find will lock the handler you have to be careful where you
663 use it. For example, this is not a good idea:
665 struct v4l2_ctrl_handler ctrl_handler;
667 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
668 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
670 ...and in video_ops.s_ctrl:
672 case V4L2_CID_BRIGHTNESS:
673 contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
676 When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
677 attempting to find another control from the same handler will deadlock.
679 It is recommended not to use this function from inside the control ops.
685 When one control handler is added to another using v4l2_ctrl_add_handler, then
686 by default all controls from one are merged to the other. But a subdev might
687 have low-level controls that make sense for some advanced embedded system, but
688 not when it is used in consumer-level hardware. In that case you want to keep
689 those low-level controls local to the subdev. You can do this by simply
690 setting the 'is_private' flag of the control to 1:
692 static const struct v4l2_ctrl_config ctrl_private = {
693 .ops = &ctrl_custom_ops,
695 .name = "Some Private Control",
696 .type = V4L2_CTRL_TYPE_INTEGER,
702 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
704 These controls will now be skipped when v4l2_ctrl_add_handler is called.
707 V4L2_CTRL_TYPE_CTRL_CLASS Controls
708 ==================================
710 Controls of this type can be used by GUIs to get the name of the control class.
711 A fully featured GUI can make a dialog with multiple tabs with each tab
712 containing the controls belonging to a particular control class. The name of
713 each tab can be found by querying a special control with ID <control class | 1>.
715 Drivers do not have to care about this. The framework will automatically add
716 a control of this type whenever the first control belonging to a new control
720 Adding Notify Callbacks
721 =======================
723 Sometimes the platform or bridge driver needs to be notified when a control
724 from a sub-device driver changes. You can set a notify callback by calling
727 void v4l2_ctrl_notify(struct v4l2_ctrl *ctrl,
728 void (*notify)(struct v4l2_ctrl *ctrl, void *priv), void *priv);
730 Whenever the give control changes value the notify callback will be called
731 with a pointer to the control and the priv pointer that was passed with
732 v4l2_ctrl_notify. Note that the control's handler lock is held when the
733 notify function is called.
735 There can be only one notify function per control handler. Any attempt
736 to set another notify function will cause a WARN_ON.