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Documentation / video4linux / v4l2-controls.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

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 and integer menu controls are added by calling v4l2_ctrl_new_std_menu:
128	
129		struct v4l2_ctrl *v4l2_ctrl_new_std_menu(struct v4l2_ctrl_handler *hdl,
130				const struct v4l2_ctrl_ops *ops,
131				u32 id, s32 max, s32 skip_mask, s32 def);
132	
133	Menu controls with a driver specific menu are added by calling
134	v4l2_ctrl_new_std_menu_items:
135	
136	       struct v4l2_ctrl *v4l2_ctrl_new_std_menu_items(
137	                       struct v4l2_ctrl_handler *hdl,
138	                       const struct v4l2_ctrl_ops *ops, u32 id, s32 max,
139	                       s32 skip_mask, s32 def, const char * const *qmenu);
140	
141	Integer menu controls with a driver specific menu can be added by calling
142	v4l2_ctrl_new_int_menu:
143	
144		struct v4l2_ctrl *v4l2_ctrl_new_int_menu(struct v4l2_ctrl_handler *hdl,
145				const struct v4l2_ctrl_ops *ops,
146				u32 id, s32 max, s32 def, const s64 *qmenu_int);
147	
148	These functions are typically called right after the v4l2_ctrl_handler_init:
149	
150		static const s64 exp_bias_qmenu[] = {
151		       -2, -1, 0, 1, 2
152		};
153		static const char * const test_pattern[] = {
154			"Disabled",
155			"Vertical Bars",
156			"Solid Black",
157			"Solid White",
158		};
159	
160		v4l2_ctrl_handler_init(&foo->ctrl_handler, nr_of_controls);
161		v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
162				V4L2_CID_BRIGHTNESS, 0, 255, 1, 128);
163		v4l2_ctrl_new_std(&foo->ctrl_handler, &foo_ctrl_ops,
164				V4L2_CID_CONTRAST, 0, 255, 1, 128);
165		v4l2_ctrl_new_std_menu(&foo->ctrl_handler, &foo_ctrl_ops,
166				V4L2_CID_POWER_LINE_FREQUENCY,
167				V4L2_CID_POWER_LINE_FREQUENCY_60HZ, 0,
168				V4L2_CID_POWER_LINE_FREQUENCY_DISABLED);
169		v4l2_ctrl_new_int_menu(&foo->ctrl_handler, &foo_ctrl_ops,
170				V4L2_CID_EXPOSURE_BIAS,
171				ARRAY_SIZE(exp_bias_qmenu) - 1,
172				ARRAY_SIZE(exp_bias_qmenu) / 2 - 1,
173				exp_bias_qmenu);
174		v4l2_ctrl_new_std_menu_items(&foo->ctrl_handler, &foo_ctrl_ops,
175				V4L2_CID_TEST_PATTERN, ARRAY_SIZE(test_pattern) - 1, 0,
176				0, test_pattern);
177		...
178		if (foo->ctrl_handler.error) {
179			int err = foo->ctrl_handler.error;
180	
181			v4l2_ctrl_handler_free(&foo->ctrl_handler);
182			return err;
183		}
184	
185	The v4l2_ctrl_new_std function returns the v4l2_ctrl pointer to the new
186	control, but if you do not need to access the pointer outside the control ops,
187	then there is no need to store it.
188	
189	The v4l2_ctrl_new_std function will fill in most fields based on the control
190	ID except for the min, max, step and default values. These are passed in the
191	last four arguments. These values are driver specific while control attributes
192	like type, name, flags are all global. The control's current value will be set
193	to the default value.
194	
195	The v4l2_ctrl_new_std_menu function is very similar but it is used for menu
196	controls. There is no min argument since that is always 0 for menu controls,
197	and instead of a step there is a skip_mask argument: if bit X is 1, then menu
198	item X is skipped.
199	
200	The v4l2_ctrl_new_int_menu function creates a new standard integer menu
201	control with driver-specific items in the menu. It differs from
202	v4l2_ctrl_new_std_menu in that it doesn't have the mask argument and takes
203	as the last argument an array of signed 64-bit integers that form an exact
204	menu item list.
205	
206	The v4l2_ctrl_new_std_menu_items function is very similar to
207	v4l2_ctrl_new_std_menu but takes an extra parameter qmenu, which is the driver
208	specific menu for an otherwise standard menu control. A good example for this
209	control is the test pattern control for capture/display/sensors devices that
210	have the capability to generate test patterns. These test patterns are hardware
211	specific, so the contents of the menu will vary from device to device.
212	
213	Note that if something fails, the function will return NULL or an error and
214	set ctrl_handler->error to the error code. If ctrl_handler->error was already
215	set, then it will just return and do nothing. This is also true for
216	v4l2_ctrl_handler_init if it cannot allocate the internal data structure.
217	
218	This makes it easy to init the handler and just add all controls and only check
219	the error code at the end. Saves a lot of repetitive error checking.
220	
221	It is recommended to add controls in ascending control ID order: it will be
222	a bit faster that way.
223	
224	3) Optionally force initial control setup:
225	
226		v4l2_ctrl_handler_setup(&foo->ctrl_handler);
227	
228	This will call s_ctrl for all controls unconditionally. Effectively this
229	initializes the hardware to the default control values. It is recommended
230	that you do this as this ensures that both the internal data structures and
231	the hardware are in sync.
232	
233	4) Finally: implement the v4l2_ctrl_ops
234	
235		static const struct v4l2_ctrl_ops foo_ctrl_ops = {
236			.s_ctrl = foo_s_ctrl,
237		};
238	
239	Usually all you need is s_ctrl:
240	
241		static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
242		{
243			struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
244	
245			switch (ctrl->id) {
246			case V4L2_CID_BRIGHTNESS:
247				write_reg(0x123, ctrl->val);
248				break;
249			case V4L2_CID_CONTRAST:
250				write_reg(0x456, ctrl->val);
251				break;
252			}
253			return 0;
254		}
255	
256	The control ops are called with the v4l2_ctrl pointer as argument.
257	The new control value has already been validated, so all you need to do is
258	to actually update the hardware registers.
259	
260	You're done! And this is sufficient for most of the drivers we have. No need
261	to do any validation of control values, or implement QUERYCTRL/QUERYMENU. And
262	G/S_CTRL as well as G/TRY/S_EXT_CTRLS are automatically supported.
263	
264	
265	==============================================================================
266	
267	The remainder of this document deals with more advanced topics and scenarios.
268	In practice the basic usage as described above is sufficient for most drivers.
269	
270	===============================================================================
271	
272	
273	Inheriting Controls
274	===================
275	
276	When a sub-device is registered with a V4L2 driver by calling
277	v4l2_device_register_subdev() and the ctrl_handler fields of both v4l2_subdev
278	and v4l2_device are set, then the controls of the subdev will become
279	automatically available in the V4L2 driver as well. If the subdev driver
280	contains controls that already exist in the V4L2 driver, then those will be
281	skipped (so a V4L2 driver can always override a subdev control).
282	
283	What happens here is that v4l2_device_register_subdev() calls
284	v4l2_ctrl_add_handler() adding the controls of the subdev to the controls
285	of v4l2_device.
286	
287	
288	Accessing Control Values
289	========================
290	
291	The v4l2_ctrl struct contains these two unions:
292	
293		/* The current control value. */
294		union {
295			s32 val;
296			s64 val64;
297			char *string;
298		} cur;
299	
300		/* The new control value. */
301		union {
302			s32 val;
303			s64 val64;
304			char *string;
305		};
306	
307	Within the control ops you can freely use these. The val and val64 speak for
308	themselves. The string pointers point to character buffers of length
309	ctrl->maximum + 1, and are always 0-terminated.
310	
311	In most cases 'cur' contains the current cached control value. When you create
312	a new control this value is made identical to the default value. After calling
313	v4l2_ctrl_handler_setup() this value is passed to the hardware. It is generally
314	a good idea to call this function.
315	
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:
321	
322		static int foo_g_volatile_ctrl(struct v4l2_ctrl *ctrl)
323		{
324			switch (ctrl->id) {
325			case V4L2_CID_BRIGHTNESS:
326				ctrl->val = read_reg(0x123);
327				break;
328			}
329		}
330	
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.
333	
334	To mark a control as volatile you have to set V4L2_CTRL_FLAG_VOLATILE:
335	
336		ctrl = v4l2_ctrl_new_std(&sd->ctrl_handler, ...);
337		if (ctrl)
338			ctrl->flags |= V4L2_CTRL_FLAG_VOLATILE;
339	
340	For try/s_ctrl the new values (i.e. as passed by the user) are filled in and
341	you can modify them in try_ctrl or set them in s_ctrl. The 'cur' union
342	contains the current value, which you can use (but not change!) as well.
343	
344	If s_ctrl returns 0 (OK), then the control framework will copy the new final
345	values to the 'cur' union.
346	
347	While in g_volatile/s/try_ctrl you can access the value of all controls owned
348	by the same handler since the handler's lock is held. If you need to access
349	the value of controls owned by other handlers, then you have to be very careful
350	not to introduce deadlocks.
351	
352	Outside of the control ops you have to go through to helper functions to get
353	or set a single control value safely in your driver:
354	
355		s32 v4l2_ctrl_g_ctrl(struct v4l2_ctrl *ctrl);
356		int v4l2_ctrl_s_ctrl(struct v4l2_ctrl *ctrl, s32 val);
357	
358	These functions go through the control framework just as VIDIOC_G/S_CTRL ioctls
359	do. Don't use these inside the control ops g_volatile/s/try_ctrl, though, that
360	will result in a deadlock since these helpers lock the handler as well.
361	
362	You can also take the handler lock yourself:
363	
364		mutex_lock(&state->ctrl_handler.lock);
365		printk(KERN_INFO "String value is '%s'\n", ctrl1->cur.string);
366		printk(KERN_INFO "Integer value is '%s'\n", ctrl2->cur.val);
367		mutex_unlock(&state->ctrl_handler.lock);
368	
369	
370	Menu Controls
371	=============
372	
373	The v4l2_ctrl struct contains this union:
374	
375		union {
376			u32 step;
377			u32 menu_skip_mask;
378		};
379	
380	For menu controls menu_skip_mask is used. What it does is that it allows you
381	to easily exclude certain menu items. This is used in the VIDIOC_QUERYMENU
382	implementation where you can return -EINVAL if a certain menu item is not
383	present. Note that VIDIOC_QUERYCTRL always returns a step value of 1 for
384	menu controls.
385	
386	A good example is the MPEG Audio Layer II Bitrate menu control where the
387	menu is a list of standardized possible bitrates. But in practice hardware
388	implementations will only support a subset of those. By setting the skip
389	mask you can tell the framework which menu items should be skipped. Setting
390	it to 0 means that all menu items are supported.
391	
392	You set this mask either through the v4l2_ctrl_config struct for a custom
393	control, or by calling v4l2_ctrl_new_std_menu().
394	
395	
396	Custom Controls
397	===============
398	
399	Driver specific controls can be created using v4l2_ctrl_new_custom():
400	
401		static const struct v4l2_ctrl_config ctrl_filter = {
402			.ops = &ctrl_custom_ops,
403			.id = V4L2_CID_MPEG_CX2341X_VIDEO_SPATIAL_FILTER,
404			.name = "Spatial Filter",
405			.type = V4L2_CTRL_TYPE_INTEGER,
406			.flags = V4L2_CTRL_FLAG_SLIDER,
407			.max = 15,
408			.step = 1,
409		};
410	
411		ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_filter, NULL);
412	
413	The last argument is the priv pointer which can be set to driver-specific
414	private data.
415	
416	The v4l2_ctrl_config struct also has a field to set the is_private flag.
417	
418	If the name field is not set, then the framework will assume this is a standard
419	control and will fill in the name, type and flags fields accordingly.
420	
421	
422	Active and Grabbed Controls
423	===========================
424	
425	If you get more complex relationships between controls, then you may have to
426	activate and deactivate controls. For example, if the Chroma AGC control is
427	on, then the Chroma Gain control is inactive. That is, you may set it, but
428	the value will not be used by the hardware as long as the automatic gain
429	control is on. Typically user interfaces can disable such input fields.
430	
431	You can set the 'active' status using v4l2_ctrl_activate(). By default all
432	controls are active. Note that the framework does not check for this flag.
433	It is meant purely for GUIs. The function is typically called from within
434	s_ctrl.
435	
436	The other flag is the 'grabbed' flag. A grabbed control means that you cannot
437	change it because it is in use by some resource. Typical examples are MPEG
438	bitrate controls that cannot be changed while capturing is in progress.
439	
440	If a control is set to 'grabbed' using v4l2_ctrl_grab(), then the framework
441	will return -EBUSY if an attempt is made to set this control. The
442	v4l2_ctrl_grab() function is typically called from the driver when it
443	starts or stops streaming.
444	
445	
446	Control Clusters
447	================
448	
449	By default all controls are independent from the others. But in more
450	complex scenarios you can get dependencies from one control to another.
451	In that case you need to 'cluster' them:
452	
453		struct foo {
454			struct v4l2_ctrl_handler ctrl_handler;
455	#define AUDIO_CL_VOLUME (0)
456	#define AUDIO_CL_MUTE   (1)
457			struct v4l2_ctrl *audio_cluster[2];
458			...
459		};
460	
461		state->audio_cluster[AUDIO_CL_VOLUME] =
462			v4l2_ctrl_new_std(&state->ctrl_handler, ...);
463		state->audio_cluster[AUDIO_CL_MUTE] =
464			v4l2_ctrl_new_std(&state->ctrl_handler, ...);
465		v4l2_ctrl_cluster(ARRAY_SIZE(state->audio_cluster), state->audio_cluster);
466	
467	From now on whenever one or more of the controls belonging to the same
468	cluster is set (or 'gotten', or 'tried'), only the control ops of the first
469	control ('volume' in this example) is called. You effectively create a new
470	composite control. Similar to how a 'struct' works in C.
471	
472	So when s_ctrl is called with V4L2_CID_AUDIO_VOLUME as argument, you should set
473	all two controls belonging to the audio_cluster:
474	
475		static int foo_s_ctrl(struct v4l2_ctrl *ctrl)
476		{
477			struct foo *state = container_of(ctrl->handler, struct foo, ctrl_handler);
478	
479			switch (ctrl->id) {
480			case V4L2_CID_AUDIO_VOLUME: {
481				struct v4l2_ctrl *mute = ctrl->cluster[AUDIO_CL_MUTE];
482	
483				write_reg(0x123, mute->val ? 0 : ctrl->val);
484				break;
485			}
486			case V4L2_CID_CONTRAST:
487				write_reg(0x456, ctrl->val);
488				break;
489			}
490			return 0;
491		}
492	
493	In the example above the following are equivalent for the VOLUME case:
494	
495		ctrl == ctrl->cluster[AUDIO_CL_VOLUME] == state->audio_cluster[AUDIO_CL_VOLUME]
496		ctrl->cluster[AUDIO_CL_MUTE] == state->audio_cluster[AUDIO_CL_MUTE]
497	
498	In practice using cluster arrays like this becomes very tiresome. So instead
499	the following equivalent method is used:
500	
501		struct {
502			/* audio cluster */
503			struct v4l2_ctrl *volume;
504			struct v4l2_ctrl *mute;
505		};
506	
507	The anonymous struct is used to clearly 'cluster' these two control pointers,
508	but it serves no other purpose. The effect is the same as creating an
509	array with two control pointers. So you can just do:
510	
511		state->volume = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
512		state->mute = v4l2_ctrl_new_std(&state->ctrl_handler, ...);
513		v4l2_ctrl_cluster(2, &state->volume);
514	
515	And in foo_s_ctrl you can use these pointers directly: state->mute->val.
516	
517	Note that controls in a cluster may be NULL. For example, if for some
518	reason mute was never added (because the hardware doesn't support that
519	particular feature), then mute will be NULL. So in that case we have a
520	cluster of 2 controls, of which only 1 is actually instantiated. The
521	only restriction is that the first control of the cluster must always be
522	present, since that is the 'master' control of the cluster. The master
523	control is the one that identifies the cluster and that provides the
524	pointer to the v4l2_ctrl_ops struct that is used for that cluster.
525	
526	Obviously, all controls in the cluster array must be initialized to either
527	a valid control or to NULL.
528	
529	In rare cases you might want to know which controls of a cluster actually
530	were set explicitly by the user. For this you can check the 'is_new' flag of
531	each control. For example, in the case of a volume/mute cluster the 'is_new'
532	flag of the mute control would be set if the user called VIDIOC_S_CTRL for
533	mute only. If the user would call VIDIOC_S_EXT_CTRLS for both mute and volume
534	controls, then the 'is_new' flag would be 1 for both controls.
535	
536	The 'is_new' flag is always 1 when called from v4l2_ctrl_handler_setup().
537	
538	
539	Handling autogain/gain-type Controls with Auto Clusters
540	=======================================================
541	
542	A common type of control cluster is one that handles 'auto-foo/foo'-type
543	controls. Typical examples are autogain/gain, autoexposure/exposure,
544	autowhitebalance/red balance/blue balance. In all cases you have one control
545	that determines whether another control is handled automatically by the hardware,
546	or whether it is under manual control from the user.
547	
548	If the cluster is in automatic mode, then the manual controls should be
549	marked inactive and volatile. When the volatile controls are read the
550	g_volatile_ctrl operation should return the value that the hardware's automatic
551	mode set up automatically.
552	
553	If the cluster is put in manual mode, then the manual controls should become
554	active again and the volatile flag is cleared (so g_volatile_ctrl is no longer
555	called while in manual mode). In addition just before switching to manual mode
556	the current values as determined by the auto mode are copied as the new manual
557	values.
558	
559	Finally the V4L2_CTRL_FLAG_UPDATE should be set for the auto control since
560	changing that control affects the control flags of the manual controls.
561	
562	In order to simplify this a special variation of v4l2_ctrl_cluster was
563	introduced:
564	
565	void v4l2_ctrl_auto_cluster(unsigned ncontrols, struct v4l2_ctrl **controls,
566				u8 manual_val, bool set_volatile);
567	
568	The first two arguments are identical to v4l2_ctrl_cluster. The third argument
569	tells the framework which value switches the cluster into manual mode. The
570	last argument will optionally set V4L2_CTRL_FLAG_VOLATILE for the non-auto controls.
571	If it is false, then the manual controls are never volatile. You would typically
572	use that if the hardware does not give you the option to read back to values as
573	determined by the auto mode (e.g. if autogain is on, the hardware doesn't allow
574	you to obtain the current gain value).
575	
576	The first control of the cluster is assumed to be the 'auto' control.
577	
578	Using this function will ensure that you don't need to handle all the complex
579	flag and volatile handling.
580	
581	
582	VIDIOC_LOG_STATUS Support
583	=========================
584	
585	This ioctl allow you to dump the current status of a driver to the kernel log.
586	The v4l2_ctrl_handler_log_status(ctrl_handler, prefix) can be used to dump the
587	value of the controls owned by the given handler to the log. You can supply a
588	prefix as well. If the prefix didn't end with a space, then ': ' will be added
589	for you.
590	
591	
592	Different Handlers for Different Video Nodes
593	============================================
594	
595	Usually the V4L2 driver has just one control handler that is global for
596	all video nodes. But you can also specify different control handlers for
597	different video nodes. You can do that by manually setting the ctrl_handler
598	field of struct video_device.
599	
600	That is no problem if there are no subdevs involved but if there are, then
601	you need to block the automatic merging of subdev controls to the global
602	control handler. You do that by simply setting the ctrl_handler field in
603	struct v4l2_device to NULL. Now v4l2_device_register_subdev() will no longer
604	merge subdev controls.
605	
606	After each subdev was added, you will then have to call v4l2_ctrl_add_handler
607	manually to add the subdev's control handler (sd->ctrl_handler) to the desired
608	control handler. This control handler may be specific to the video_device or
609	for a subset of video_device's. For example: the radio device nodes only have
610	audio controls, while the video and vbi device nodes share the same control
611	handler for the audio and video controls.
612	
613	If you want to have one handler (e.g. for a radio device node) have a subset
614	of another handler (e.g. for a video device node), then you should first add
615	the controls to the first handler, add the other controls to the second
616	handler and finally add the first handler to the second. For example:
617	
618		v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_VOLUME, ...);
619		v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
620		v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
621		v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
622		v4l2_ctrl_add_handler(&video_ctrl_handler, &radio_ctrl_handler, NULL);
623	
624	The last argument to v4l2_ctrl_add_handler() is a filter function that allows
625	you to filter which controls will be added. Set it to NULL if you want to add
626	all controls.
627	
628	Or you can add specific controls to a handler:
629	
630		volume = v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_AUDIO_VOLUME, ...);
631		v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_BRIGHTNESS, ...);
632		v4l2_ctrl_new_std(&video_ctrl_handler, &ops, V4L2_CID_CONTRAST, ...);
633		v4l2_ctrl_add_ctrl(&radio_ctrl_handler, volume);
634	
635	What you should not do is make two identical controls for two handlers.
636	For example:
637	
638		v4l2_ctrl_new_std(&radio_ctrl_handler, &radio_ops, V4L2_CID_AUDIO_MUTE, ...);
639		v4l2_ctrl_new_std(&video_ctrl_handler, &video_ops, V4L2_CID_AUDIO_MUTE, ...);
640	
641	This would be bad since muting the radio would not change the video mute
642	control. The rule is to have one control for each hardware 'knob' that you
643	can twiddle.
644	
645	
646	Finding Controls
647	================
648	
649	Normally you have created the controls yourself and you can store the struct
650	v4l2_ctrl pointer into your own struct.
651	
652	But sometimes you need to find a control from another handler that you do
653	not own. For example, if you have to find a volume control from a subdev.
654	
655	You can do that by calling v4l2_ctrl_find:
656	
657		struct v4l2_ctrl *volume;
658	
659		volume = v4l2_ctrl_find(sd->ctrl_handler, V4L2_CID_AUDIO_VOLUME);
660	
661	Since v4l2_ctrl_find will lock the handler you have to be careful where you
662	use it. For example, this is not a good idea:
663	
664		struct v4l2_ctrl_handler ctrl_handler;
665	
666		v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_BRIGHTNESS, ...);
667		v4l2_ctrl_new_std(&ctrl_handler, &video_ops, V4L2_CID_CONTRAST, ...);
668	
669	...and in video_ops.s_ctrl:
670	
671		case V4L2_CID_BRIGHTNESS:
672			contrast = v4l2_find_ctrl(&ctrl_handler, V4L2_CID_CONTRAST);
673			...
674	
675	When s_ctrl is called by the framework the ctrl_handler.lock is already taken, so
676	attempting to find another control from the same handler will deadlock.
677	
678	It is recommended not to use this function from inside the control ops.
679	
680	
681	Inheriting Controls
682	===================
683	
684	When one control handler is added to another using v4l2_ctrl_add_handler, then
685	by default all controls from one are merged to the other. But a subdev might
686	have low-level controls that make sense for some advanced embedded system, but
687	not when it is used in consumer-level hardware. In that case you want to keep
688	those low-level controls local to the subdev. You can do this by simply
689	setting the 'is_private' flag of the control to 1:
690	
691		static const struct v4l2_ctrl_config ctrl_private = {
692			.ops = &ctrl_custom_ops,
693			.id = V4L2_CID_...,
694			.name = "Some Private Control",
695			.type = V4L2_CTRL_TYPE_INTEGER,
696			.max = 15,
697			.step = 1,
698			.is_private = 1,
699		};
700	
701		ctrl = v4l2_ctrl_new_custom(&foo->ctrl_handler, &ctrl_private, NULL);
702	
703	These controls will now be skipped when v4l2_ctrl_add_handler is called.
704	
705	
706	V4L2_CTRL_TYPE_CTRL_CLASS Controls
707	==================================
708	
709	Controls of this type can be used by GUIs to get the name of the control class.
710	A fully featured GUI can make a dialog with multiple tabs with each tab
711	containing the controls belonging to a particular control class. The name of
712	each tab can be found by querying a special control with ID <control class | 1>.
713	
714	Drivers do not have to care about this. The framework will automatically add
715	a control of this type whenever the first control belonging to a new control
716	class is added.
717	
718	
719	Adding Notify Callbacks
720	=======================
721	
722	Sometimes the platform or bridge driver needs to be notified when a control
723	from a sub-device driver changes. You can set a notify callback by calling
724	this function:
725	
726	void v4l2_ctrl_notify(struct v4l2_ctrl *ctrl,
727		void (*notify)(struct v4l2_ctrl *ctrl, void *priv), void *priv);
728	
729	Whenever the give control changes value the notify callback will be called
730	with a pointer to the control and the priv pointer that was passed with
731	v4l2_ctrl_notify. Note that the control's handler lock is held when the
732	notify function is called.
733	
734	There can be only one notify function per control handler. Any attempt
735	to set another notify function will cause a WARN_ON.
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