1 .. SPDX-License-Identifier: GPL-2.0
9 The V4L2 control API seems simple enough, but quickly becomes very hard to
10 implement correctly in drivers. But much of the code needed to handle controls
11 is actually not driver specific and can be moved to the V4L core framework.
13 After all, the only part that a driver developer is interested in is:
15 1) How do I add a control?
16 2) How do I set the control's value? (i.e. s_ctrl)
20 3) How do I get the control's value? (i.e. g_volatile_ctrl)
21 4) How do I validate the user's proposed control value? (i.e. try_ctrl)
23 All the rest is something that can be done centrally.
25 The control framework was created in order to implement all the rules of the
26 V4L2 specification with respect to controls in a central place. And to make
27 life as easy as possible for the driver developer.
29 Note that the control framework relies on the presence of a struct
30 :c:type:`v4l2_device` for V4L2 drivers and struct :c:type:`v4l2_subdev` for
34 Objects in the framework
35 ------------------------
37 There are two main objects:
39 The :c:type:`v4l2_ctrl` object describes the control properties and keeps
40 track of the control's value (both the current value and the proposed new
43 :c:type:`v4l2_ctrl_handler` is the object that keeps track of controls. It
44 maintains a list of v4l2_ctrl objects that it owns and another list of
45 references to controls, possibly to controls owned by other handlers.
48 Basic usage for V4L2 and sub-device drivers
49 -------------------------------------------
51 1) Prepare the driver:
53 1.1) Add the handler to your driver's top-level struct:
61 struct v4l2_device v4l2_dev;
63 struct v4l2_ctrl_handler ctrl_handler;
67 For sub-device drivers:
73 struct v4l2_subdev sd;
75 struct v4l2_ctrl_handler ctrl_handler;
79 1.2) Initialize the handler:
83 v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
85 The second argument is a hint telling the function how many controls this
86 handler is expected to handle. It will allocate a hashtable based on this
87 information. It is a hint only.
89 1.3) Hook the control handler into the driver:
95 foo->v4l2_dev.ctrl_handler = &foo->ctrl_handler;
97 Finally, remove all control functions from your v4l2_ioctl_ops (if any):
98 vidioc_queryctrl, vidioc_query_ext_ctrl, vidioc_querymenu, vidioc_g_ctrl,
99 vidioc_s_ctrl, vidioc_g_ext_ctrls, vidioc_try_ext_ctrls and vidioc_s_ext_ctrls.
100 Those are now no longer needed.
102 For sub-device drivers:
106 foo->sd.ctrl_handler = &foo->ctrl_handler;
108 1.4) Clean up the handler at the end:
112 v4l2_ctrl_handler_free(&foo->ctrl_handler);
117 You add non-menu controls by calling :c:func:`v4l2_ctrl_new_std`:
121 struct v4l2_ctrl *v4l2_ctrl_new_std(struct v4l2_ctrl_handler *hdl,
122 const struct v4l2_ctrl_ops *ops,
123 u32 id, s32 min, s32 max, u32 step, s32 def);
125 Menu and integer menu controls are added by calling
126 :c:func:`v4l2_ctrl_new_std_menu`:
130 struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
131 const struct v4l2_ctrl_ops *ops,
132 u32 id, s32 max, s32 skip_mask, s32 def);
134 Menu controls with a driver specific menu are added by calling
135 :c:func:`v4l2_ctrl_new_std_menu_items`:
139 struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
140 struct v4l2_ctrl_handler *hdl,
141 const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
142 s32 skip_mask, s32 def, const char * const *qmenu);
144 Integer menu controls with a driver specific menu can be added by calling
145 :c:func:`v4l2_ctrl_new_int_menu`:
149 struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
150 const struct v4l2_ctrl_ops *ops,
151 u32 id, s32 max, s32 def, const s64 *qmenu_int);
153 These functions are typically called right after the
154 :c:func:`v4l2_ctrl_handler_init`:
158 static const s64 exp_bias_qmenu[] = {
161 static const char * const test_pattern[] = {
168 v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
169 v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
170 V4L2_CID_BRIGHTNESS, 0, 255, 1, 128);
171 v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
172 V4L2_CID_CONTRAST, 0, 255, 1, 128);
173 v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops,
174 V4L2_CID_POWER_LINE_FREQUENCY,
175 V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0,
176 V4L2_CID_POWER_LINE_FREQUENCY_DISABLED);
177 v4l2_ctrl_new_int_menu(&foo->ctrl_handler, &foo_ctrl_ops,
178 V4L2_CID_EXPOSURE_BIAS,
179 ARRAY_SIZE(exp_bias_qmenu) - 1,
180 ARRAY_SIZE(exp_bias_qmenu) / 2 - 1,
182 v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
183 V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
186 if (foo->ctrl_handler.error) {
187 int err = foo->ctrl_handler.error;
189 v4l2_ctrl_handler_free(&foo->ctrl_handler);
193 The :c:func:`v4l2_ctrl_new_std` function returns the v4l2_ctrl pointer to
194 the new control, but if you do not need to access the pointer outside the
195 control ops, then there is no need to store it.
197 The :c:func:`v4l2_ctrl_new_std` function will fill in most fields based on
198 the control ID except for the min, max, step and default values. These are
199 passed in the last four arguments. These values are driver specific while
200 control attributes like type, name, flags are all global. The control's
201 current value will be set to the default value.
203 The :c:func:`v4l2_ctrl_new_std_menu` function is very similar but it is
204 used for menu controls. There is no min argument since that is always 0 for
205 menu controls, and instead of a step there is a skip_mask argument: if bit
206 X is 1, then menu item X is skipped.
208 The :c:func:`v4l2_ctrl_new_int_menu` function creates a new standard
209 integer menu control with driver-specific items in the menu. It differs
210 from v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and
211 takes as the last argument an array of signed 64-bit integers that form an
212 exact menu item list.
214 The :c:func:`v4l2_ctrl_new_std_menu_items` function is very similar to
215 v4l2_ctrl_new_std_menu but takes an extra parameter qmenu, which is the
216 driver specific menu for an otherwise standard menu control. A good example
217 for this control is the test pattern control for capture/display/sensors
218 devices that have the capability to generate test patterns. These test
219 patterns are hardware specific, so the contents of the menu will vary from
222 Note that if something fails, the function will return NULL or an error and
223 set ctrl_handler->error to the error code. If ctrl_handler->error was already
224 set, then it will just return and do nothing. This is also true for
225 v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
227 This makes it easy to init the handler and just add all controls and only check
228 the error code at the end. Saves a lot of repetitive error checking.
230 It is recommended to add controls in ascending control ID order: it will be
231 a bit faster that way.
233 3) Optionally force initial control setup:
237 v4l2_ctrl_handler_setup(&foo->ctrl_handler);
239 This will call s_ctrl for all controls unconditionally. Effectively this
240 initializes the hardware to the default control values. It is recommended
241 that you do this as this ensures that both the internal data structures and
242 the hardware are in sync.
244 4) Finally: implement the :c:type:`v4l2_ctrl_ops`
248 static const struct v4l2_ctrl_ops foo_ctrl_ops = {
249 .s_ctrl = foo_s_ctrl,
252 Usually all you need is s_ctrl:
256 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
258 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
261 case V4L2_CID_BRIGHTNESS:
262 write_reg(0x123, ctrl->val);
264 case V4L2_CID_CONTRAST:
265 write_reg(0x456, ctrl->val);
271 The control ops are called with the v4l2_ctrl pointer as argument.
272 The new control value has already been validated, so all you need to do is
273 to actually update the hardware registers.
275 You're done! And this is sufficient for most of the drivers we have. No need
276 to do any validation of control values, or implement QUERYCTRL, QUERY_EXT_CTRL
277 and QUERYMENU. And G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
282 The remainder sections deal with more advanced controls topics and scenarios.
283 In practice the basic usage as described above is sufficient for most drivers.
289 When a sub-device is registered with a V4L2 driver by calling
290 v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
291 and v4l2_device are set, then the controls of the subdev will become
292 automatically available in the V4L2 driver as well. If the subdev driver
293 contains controls that already exist in the V4L2 driver, then those will be
294 skipped (so a V4L2 driver can always override a subdev control).
296 What happens here is that v4l2_device_register_subdev() calls
297 v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
301 Accessing Control Values
302 ------------------------
304 The following union is used inside the control framework to access control
309 union v4l2_ctrl_ptr {
316 The v4l2_ctrl struct contains these fields that can be used to access both
317 current and new values:
327 union v4l2_ctrl_ptr p_new;
328 union v4l2_ctrl_ptr p_cur;
330 If the control has a simple s32 type type, then:
334 &ctrl->val == ctrl->p_new.p_s32
335 &ctrl->cur.val == ctrl->p_cur.p_s32
337 For all other types use ctrl->p_cur.p<something>. Basically the val
338 and cur.val fields can be considered an alias since these are used so often.
340 Within the control ops you can freely use these. The val and cur.val speak for
341 themselves. The p_char pointers point to character buffers of length
342 ctrl->maximum + 1, and are always 0-terminated.
344 Unless the control is marked volatile the p_cur field points to the the
345 current cached control value. When you create a new control this value is made
346 identical to the default value. After calling v4l2_ctrl_handler_setup() this
347 value is passed to the hardware. It is generally a good idea to call this
350 Whenever a new value is set that new value is automatically cached. This means
351 that most drivers do not need to implement the g_volatile_ctrl() op. The
352 exception is for controls that return a volatile register such as a signal
353 strength read-out that changes continuously. In that case you will need to
354 implement g_volatile_ctrl like this:
358 static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl)
361 case V4L2_CID_BRIGHTNESS:
362 ctrl->val = read_reg(0x123);
367 Note that you use the 'new value' union as well in g_volatile_ctrl. In general
368 controls that need to implement g_volatile_ctrl are read-only controls. If they
369 are not, a V4L2_EVENT_CTRL_CH_VALUE will not be generated when the control
372 To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
376 ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
378 ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
380 For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
381 you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
382 contains the current value, which you can use (but not change!) as well.
384 If s_ctrl returns 0 (OK), then the control framework will copy the new final
385 values to the 'cur' union.
387 While in g_volatile/s/try_ctrl you can access the value of all controls owned
388 by the same handler since the handler's lock is held. If you need to access
389 the value of controls owned by other handlers, then you have to be very careful
390 not to introduce deadlocks.
392 Outside of the control ops you have to go through to helper functions to get
393 or set a single control value safely in your driver:
397 s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl);
398 int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val);
400 These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
401 do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
402 will result in a deadlock since these helpers lock the handler as well.
404 You can also take the handler lock yourself:
408 mutex_lock(&state->ctrl_handler.lock);
409 pr_info("String value is '%s'\n", ctrl1->p_cur.p_char);
410 pr_info("Integer value is '%s'\n", ctrl2->cur.val);
411 mutex_unlock(&state->ctrl_handler.lock);
417 The v4l2_ctrl struct contains this union:
426 For menu controls menu_skip_mask is used. What it does is that it allows you
427 to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
428 implementation where you can return -EINVAL if a certain menu item is not
429 present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
432 A good example is the MPEG Audio Layer II Bitrate menu control where the
433 menu is a list of standardized possible bitrates. But in practice hardware
434 implementations will only support a subset of those. By setting the skip
435 mask you can tell the framework which menu items should be skipped. Setting
436 it to 0 means that all menu items are supported.
438 You set this mask either through the v4l2_ctrl_config struct for a custom
439 control, or by calling v4l2_ctrl_new_std_menu().
445 Driver specific controls can be created using v4l2_ctrl_new_custom():
449 static const struct v4l2_ctrl_config ctrl_filter = {
450 .ops = &ctrl_custom_ops,
451 .id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER,
452 .name = "Spatial Filter",
453 .type = V4L2_CTRL_TYPE_INTEGER,
454 .flags = V4L2_CTRL_FLAG_SLIDER,
459 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL);
461 The last argument is the priv pointer which can be set to driver-specific
464 The v4l2_ctrl_config struct also has a field to set the is_private flag.
466 If the name field is not set, then the framework will assume this is a standard
467 control and will fill in the name, type and flags fields accordingly.
470 Active and Grabbed Controls
471 ---------------------------
473 If you get more complex relationships between controls, then you may have to
474 activate and deactivate controls. For example, if the Chroma AGC control is
475 on, then the Chroma Gain control is inactive. That is, you may set it, but
476 the value will not be used by the hardware as long as the automatic gain
477 control is on. Typically user interfaces can disable such input fields.
479 You can set the 'active' status using v4l2_ctrl_activate(). By default all
480 controls are active. Note that the framework does not check for this flag.
481 It is meant purely for GUIs. The function is typically called from within
484 The other flag is the 'grabbed' flag. A grabbed control means that you cannot
485 change it because it is in use by some resource. Typical examples are MPEG
486 bitrate controls that cannot be changed while capturing is in progress.
488 If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
489 will return -EBUSY if an attempt is made to set this control. The
490 v4l2_ctrl_grab() function is typically called from the driver when it
491 starts or stops streaming.
497 By default all controls are independent from the others. But in more
498 complex scenarios you can get dependencies from one control to another.
499 In that case you need to 'cluster' them:
504 struct v4l2_ctrl_handler ctrl_handler;
505 #define AUDIO_CL_VOLUME (0)
506 #define AUDIO_CL_MUTE (1)
507 struct v4l2_ctrl *audio_cluster[2];
511 state->audio_cluster[AUDIO_CL_VOLUME] =
512 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
513 state->audio_cluster[AUDIO_CL_MUTE] =
514 v4l2_ctrl_new_std(&state->ctrl_handler, ...);
515 v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
517 From now on whenever one or more of the controls belonging to the same
518 cluster is set (or 'gotten', or 'tried'), only the control ops of the first
519 control ('volume' in this example) is called. You effectively create a new
520 composite control. Similar to how a 'struct' works in C.
522 So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
523 all two controls belonging to the audio_cluster:
527 static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
529 struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
532 case V4L2_CID_AUDIO_VOLUME: {
533 struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
535 write_reg(0x123, mute->val ? 0 : ctrl->val);
538 case V4L2_CID_CONTRAST:
539 write_reg(0x456, ctrl->val);
545 In the example above the following are equivalent for the VOLUME case:
549 ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
550 ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
552 In practice using cluster arrays like this becomes very tiresome. So instead
553 the following equivalent method is used:
559 struct v4l2_ctrl *volume;
560 struct v4l2_ctrl *mute;
563 The anonymous struct is used to clearly 'cluster' these two control pointers,
564 but it serves no other purpose. The effect is the same as creating an
565 array with two control pointers. So you can just do:
569 state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
570 state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
571 v4l2_ctrl_cluster(2, &state->volume);
573 And in foo_s_ctrl you can use these pointers directly: state->mute->val.
575 Note that controls in a cluster may be NULL. For example, if for some
576 reason mute was never added (because the hardware doesn't support that
577 particular feature), then mute will be NULL. So in that case we have a
578 cluster of 2 controls, of which only 1 is actually instantiated. The
579 only restriction is that the first control of the cluster must always be
580 present, since that is the 'master' control of the cluster. The master
581 control is the one that identifies the cluster and that provides the
582 pointer to the v4l2_ctrl_ops struct that is used for that cluster.
584 Obviously, all controls in the cluster array must be initialized to either
585 a valid control or to NULL.
587 In rare cases you might want to know which controls of a cluster actually
588 were set explicitly by the user. For this you can check the 'is_new' flag of
589 each control. For example, in the case of a volume/mute cluster the 'is_new'
590 flag of the mute control would be set if the user called VIDIOC_S_CTRL for
591 mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
592 controls, then the 'is_new' flag would be 1 for both controls.
594 The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
597 Handling autogain/gain-type Controls with Auto Clusters
598 -------------------------------------------------------
600 A common type of control cluster is one that handles 'auto-foo/foo'-type
601 controls. Typical examples are autogain/gain, autoexposure/exposure,
602 autowhitebalance/red balance/blue balance. In all cases you have one control
603 that determines whether another control is handled automatically by the hardware,
604 or whether it is under manual control from the user.
606 If the cluster is in automatic mode, then the manual controls should be
607 marked inactive and volatile. When the volatile controls are read the
608 g_volatile_ctrl operation should return the value that the hardware's automatic
609 mode set up automatically.
611 If the cluster is put in manual mode, then the manual controls should become
612 active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
613 called while in manual mode). In addition just before switching to manual mode
614 the current values as determined by the auto mode are copied as the new manual
617 Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
618 changing that control affects the control flags of the manual controls.
620 In order to simplify this a special variation of v4l2_ctrl_cluster was
625 void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
626 u8 manual_val, bool set_volatile);
628 The first two arguments are identical to v4l2_ctrl_cluster. The third argument
629 tells the framework which value switches the cluster into manual mode. The
630 last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
631 If it is false, then the manual controls are never volatile. You would typically
632 use that if the hardware does not give you the option to read back to values as
633 determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
634 you to obtain the current gain value).
636 The first control of the cluster is assumed to be the 'auto' control.
638 Using this function will ensure that you don't need to handle all the complex
639 flag and volatile handling.
642 VIDIOC_LOG_STATUS Support
643 -------------------------
645 This ioctl allow you to dump the current status of a driver to the kernel log.
646 The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
647 value of the controls owned by the given handler to the log. You can supply a
648 prefix as well. If the prefix didn't end with a space, then ': ' will be added
652 Different Handlers for Different Video Nodes
653 --------------------------------------------
655 Usually the V4L2 driver has just one control handler that is global for
656 all video nodes. But you can also specify different control handlers for
657 different video nodes. You can do that by manually setting the ctrl_handler
658 field of struct video_device.
660 That is no problem if there are no subdevs involved but if there are, then
661 you need to block the automatic merging of subdev controls to the global
662 control handler. You do that by simply setting the ctrl_handler field in
663 struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
664 merge subdev controls.
666 After each subdev was added, you will then have to call v4l2_ctrl_add_handler
667 manually to add the subdev's control handler (sd->ctrl_handler) to the desired
668 control handler. This control handler may be specific to the video_device or
669 for a subset of video_device's. For example: the radio device nodes only have
670 audio controls, while the video and vbi device nodes share the same control
671 handler for the audio and video controls.
673 If you want to have one handler (e.g. for a radio device node) have a subset
674 of another handler (e.g. for a video device node), then you should first add
675 the controls to the first handler, add the other controls to the second
676 handler and finally add the first handler to the second. For example:
680 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
681 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
682 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
683 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
684 v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL);
686 The last argument to v4l2_ctrl_add_handler() is a filter function that allows
687 you to filter which controls will be added. Set it to NULL if you want to add
690 Or you can add specific controls to a handler:
694 volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
695 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
696 v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
698 What you should not do is make two identical controls for two handlers.
703 v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
704 v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
706 This would be bad since muting the radio would not change the video mute
707 control. The rule is to have one control for each hardware 'knob' that you
714 Normally you have created the controls yourself and you can store the struct
715 v4l2_ctrl pointer into your own struct.
717 But sometimes you need to find a control from another handler that you do
718 not own. For example, if you have to find a volume control from a subdev.
720 You can do that by calling v4l2_ctrl_find:
724 struct v4l2_ctrl *volume;
726 volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
728 Since v4l2_ctrl_find will lock the handler you have to be careful where you
729 use it. For example, this is not a good idea:
733 struct v4l2_ctrl_handler ctrl_handler;
735 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
736 v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
738 ...and in video_ops.s_ctrl:
742 case V4L2_CID_BRIGHTNESS:
743 contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
746 When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
747 attempting to find another control from the same handler will deadlock.
749 It is recommended not to use this function from inside the control ops.
755 When one control handler is added to another using v4l2_ctrl_add_handler, then
756 by default all controls from one are merged to the other. But a subdev might
757 have low-level controls that make sense for some advanced embedded system, but
758 not when it is used in consumer-level hardware. In that case you want to keep
759 those low-level controls local to the subdev. You can do this by simply
760 setting the 'is_private' flag of the control to 1:
764 static const struct v4l2_ctrl_config ctrl_private = {
765 .ops = &ctrl_custom_ops,
767 .name = "Some Private Control",
768 .type = V4L2_CTRL_TYPE_INTEGER,
774 ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
776 These controls will now be skipped when v4l2_ctrl_add_handler is called.
779 V4L2_CTRL_TYPE_CTRL_CLASS Controls
780 ----------------------------------
782 Controls of this type can be used by GUIs to get the name of the control class.
783 A fully featured GUI can make a dialog with multiple tabs with each tab
784 containing the controls belonging to a particular control class. The name of
785 each tab can be found by querying a special control with ID <control class | 1>.
787 Drivers do not have to care about this. The framework will automatically add
788 a control of this type whenever the first control belonging to a new control
792 Adding Notify Callbacks
793 -----------------------
795 Sometimes the platform or bridge driver needs to be notified when a control
796 from a sub-device driver changes. You can set a notify callback by calling
801 void v4l2_ctrl_notify(struct v4l2_ctrl *ctrl,
802 void (*notify)(struct v4l2_ctrl *ctrl, void *priv), void *priv);
804 Whenever the give control changes value the notify callback will be called
805 with a pointer to the control and the priv pointer that was passed with
806 v4l2_ctrl_notify. Note that the control's handler lock is held when the
807 notify function is called.
809 There can be only one notify function per control handler. Any attempt
810 to set another notify function will cause a WARN_ON.
812 v4l2_ctrl functions and data structures
813 ---------------------------------------
815 .. kernel-doc:: include/media/v4l2-ctrls.h