tjh.dev
Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _VIDIOC_QUERYCTRL:
5
6*******************************************************************
7ioctls VIDIOC_QUERYCTRL, VIDIOC_QUERY_EXT_CTRL and VIDIOC_QUERYMENU
8*******************************************************************
9
10Name
11====
12
13VIDIOC_QUERYCTRL - VIDIOC_QUERY_EXT_CTRL - VIDIOC_QUERYMENU - Enumerate controls and menu control items
14
15Synopsis
16========
17
18``int ioctl(int fd, int VIDIOC_QUERYCTRL, struct v4l2_queryctrl *argp)``
19
20.. c:macro:: VIDIOC_QUERY_EXT_CTRL
21
22``int ioctl(int fd, VIDIOC_QUERY_EXT_CTRL, struct v4l2_query_ext_ctrl *argp)``
23
24.. c:macro:: VIDIOC_QUERYMENU
25
26``int ioctl(int fd, VIDIOC_QUERYMENU, struct v4l2_querymenu *argp)``
27
28Arguments
29=========
30
31``fd``
32 File descriptor returned by :c:func:`open()`.
33
34``argp``
35 Pointer to struct :c:type:`v4l2_queryctrl`, :c:type:`v4l2_query_ext_ctrl`
36 or :c:type:`v4l2_querymenu` (depending on the ioctl).
37
38Description
39===========
40
41To query the attributes of a control applications set the ``id`` field
42of a struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` and call the
43``VIDIOC_QUERYCTRL`` ioctl with a pointer to this structure. The driver
44fills the rest of the structure or returns an ``EINVAL`` error code when the
45``id`` is invalid.
46
47It is possible to enumerate controls by calling ``VIDIOC_QUERYCTRL``
48with successive ``id`` values starting from ``V4L2_CID_BASE`` up to and
49exclusive ``V4L2_CID_LASTP1``. Drivers may return ``EINVAL`` if a control in
50this range is not supported. Further applications can enumerate private
51controls, which are not defined in this specification, by starting at
52``V4L2_CID_PRIVATE_BASE`` and incrementing ``id`` until the driver
53returns ``EINVAL``.
54
55In both cases, when the driver sets the ``V4L2_CTRL_FLAG_DISABLED`` flag
56in the ``flags`` field this control is permanently disabled and should
57be ignored by the application. [#f1]_
58
59When the application ORs ``id`` with ``V4L2_CTRL_FLAG_NEXT_CTRL`` the
60driver returns the next supported non-compound control, or ``EINVAL`` if
61there is none. In addition, the ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` flag
62can be specified to enumerate all compound controls (i.e. controls with
63type ≥ ``V4L2_CTRL_COMPOUND_TYPES`` and/or array control, in other words
64controls that contain more than one value). Specify both
65``V4L2_CTRL_FLAG_NEXT_CTRL`` and ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` in
66order to enumerate all controls, compound or not. Drivers which do not
67support these flags yet always return ``EINVAL``.
68
69The ``VIDIOC_QUERY_EXT_CTRL`` ioctl was introduced in order to better
70support controls that can use compound types, and to expose additional
71control information that cannot be returned in struct
72:ref:`v4l2_queryctrl <v4l2-queryctrl>` since that structure is full.
73
74``VIDIOC_QUERY_EXT_CTRL`` is used in the same way as
75``VIDIOC_QUERYCTRL``, except that the ``reserved`` array must be zeroed
76as well.
77
78Additional information is required for menu controls: the names of the
79menu items. To query them applications set the ``id`` and ``index``
80fields of struct :ref:`v4l2_querymenu <v4l2-querymenu>` and call the
81``VIDIOC_QUERYMENU`` ioctl with a pointer to this structure. The driver
82fills the rest of the structure or returns an ``EINVAL`` error code when the
83``id`` or ``index`` is invalid. Menu items are enumerated by calling
84``VIDIOC_QUERYMENU`` with successive ``index`` values from struct
85:ref:`v4l2_queryctrl <v4l2-queryctrl>` ``minimum`` to ``maximum``,
86inclusive.
87
88.. note::
89
90 It is possible for ``VIDIOC_QUERYMENU`` to return
91 an ``EINVAL`` error code for some indices between ``minimum`` and
92 ``maximum``. In that case that particular menu item is not supported by
93 this driver. Also note that the ``minimum`` value is not necessarily 0.
94
95See also the examples in :ref:`control`.
96
97.. tabularcolumns:: |p{1.2cm}|p{3.6cm}|p{12.5cm}|
98
99.. _v4l2-queryctrl:
100
101.. cssclass:: longtable
102
103.. flat-table:: struct v4l2_queryctrl
104 :header-rows: 0
105 :stub-columns: 0
106 :widths: 1 1 2
107
108 * - __u32
109 - ``id``
110 - Identifies the control, set by the application. See
111 :ref:`control-id` for predefined IDs. When the ID is ORed with
112 V4L2_CTRL_FLAG_NEXT_CTRL the driver clears the flag and
113 returns the first control with a higher ID. Drivers which do not
114 support this flag yet always return an ``EINVAL`` error code.
115 * - __u32
116 - ``type``
117 - Type of control, see :c:type:`v4l2_ctrl_type`.
118 * - __u8
119 - ``name``\ [32]
120 - Name of the control, a NUL-terminated ASCII string. This
121 information is intended for the user.
122 * - __s32
123 - ``minimum``
124 - Minimum value, inclusive. This field gives a lower bound for the
125 control. See enum :c:type:`v4l2_ctrl_type` how
126 the minimum value is to be used for each possible control type.
127 Note that this a signed 32-bit value.
128 * - __s32
129 - ``maximum``
130 - Maximum value, inclusive. This field gives an upper bound for the
131 control. See enum :c:type:`v4l2_ctrl_type` how
132 the maximum value is to be used for each possible control type.
133 Note that this a signed 32-bit value.
134 * - __s32
135 - ``step``
136 - This field gives a step size for the control. See enum
137 :c:type:`v4l2_ctrl_type` how the step value is
138 to be used for each possible control type. Note that this an
139 unsigned 32-bit value.
140
141 Generally drivers should not scale hardware control values. It may
142 be necessary for example when the ``name`` or ``id`` imply a
143 particular unit and the hardware actually accepts only multiples
144 of said unit. If so, drivers must take care values are properly
145 rounded when scaling, such that errors will not accumulate on
146 repeated read-write cycles.
147
148 This field gives the smallest change of an integer control
149 actually affecting hardware. Often the information is needed when
150 the user can change controls by keyboard or GUI buttons, rather
151 than a slider. When for example a hardware register accepts values
152 0-511 and the driver reports 0-65535, step should be 128.
153
154 Note that although signed, the step value is supposed to be always
155 positive.
156 * - __s32
157 - ``default_value``
158 - The default value of a ``V4L2_CTRL_TYPE_INTEGER``, ``_BOOLEAN``,
159 ``_BITMASK``, ``_MENU`` or ``_INTEGER_MENU`` control. Not valid
160 for other types of controls.
161
162 .. note::
163
164 Drivers reset controls to their default value only when
165 the driver is first loaded, never afterwards.
166 * - __u32
167 - ``flags``
168 - Control flags, see :ref:`control-flags`.
169 * - __u32
170 - ``reserved``\ [2]
171 - Reserved for future extensions. Drivers must set the array to
172 zero.
173
174
175.. tabularcolumns:: |p{1.2cm}|p{5.5cm}|p{10.6cm}|
176
177.. _v4l2-query-ext-ctrl:
178
179.. cssclass:: longtable
180
181.. flat-table:: struct v4l2_query_ext_ctrl
182 :header-rows: 0
183 :stub-columns: 0
184 :widths: 1 1 2
185
186 * - __u32
187 - ``id``
188 - Identifies the control, set by the application. See
189 :ref:`control-id` for predefined IDs. When the ID is ORed with
190 ``V4L2_CTRL_FLAG_NEXT_CTRL`` the driver clears the flag and
191 returns the first non-compound control with a higher ID. When the
192 ID is ORed with ``V4L2_CTRL_FLAG_NEXT_COMPOUND`` the driver clears
193 the flag and returns the first compound control with a higher ID.
194 Set both to get the first control (compound or not) with a higher
195 ID.
196 * - __u32
197 - ``type``
198 - Type of control, see :c:type:`v4l2_ctrl_type`.
199 * - char
200 - ``name``\ [32]
201 - Name of the control, a NUL-terminated ASCII string. This
202 information is intended for the user.
203 * - __s64
204 - ``minimum``
205 - Minimum value, inclusive. This field gives a lower bound for the
206 control. See enum :c:type:`v4l2_ctrl_type` how
207 the minimum value is to be used for each possible control type.
208 Note that this a signed 64-bit value.
209 * - __s64
210 - ``maximum``
211 - Maximum value, inclusive. This field gives an upper bound for the
212 control. See enum :c:type:`v4l2_ctrl_type` how
213 the maximum value is to be used for each possible control type.
214 Note that this a signed 64-bit value.
215 * - __u64
216 - ``step``
217 - This field gives a step size for the control. See enum
218 :c:type:`v4l2_ctrl_type` how the step value is
219 to be used for each possible control type. Note that this an
220 unsigned 64-bit value.
221
222 Generally drivers should not scale hardware control values. It may
223 be necessary for example when the ``name`` or ``id`` imply a
224 particular unit and the hardware actually accepts only multiples
225 of said unit. If so, drivers must take care values are properly
226 rounded when scaling, such that errors will not accumulate on
227 repeated read-write cycles.
228
229 This field gives the smallest change of an integer control
230 actually affecting hardware. Often the information is needed when
231 the user can change controls by keyboard or GUI buttons, rather
232 than a slider. When for example a hardware register accepts values
233 0-511 and the driver reports 0-65535, step should be 128.
234 * - __s64
235 - ``default_value``
236 - The default value of a ``V4L2_CTRL_TYPE_INTEGER``, ``_INTEGER64``,
237 ``_BOOLEAN``, ``_BITMASK``, ``_MENU``, ``_INTEGER_MENU``, ``_U8``
238 or ``_U16`` control. Not valid for other types of controls.
239
240 .. note::
241
242 Drivers reset controls to their default value only when
243 the driver is first loaded, never afterwards.
244 * - __u32
245 - ``flags``
246 - Control flags, see :ref:`control-flags`.
247 * - __u32
248 - ``elem_size``
249 - The size in bytes of a single element of the array. Given a char
250 pointer ``p`` to a 3-dimensional array you can find the position
251 of cell ``(z, y, x)`` as follows:
252 ``p + ((z * dims[1] + y) * dims[0] + x) * elem_size``.
253 ``elem_size`` is always valid, also when the control isn't an
254 array. For string controls ``elem_size`` is equal to
255 ``maximum + 1``.
256 * - __u32
257 - ``elems``
258 - The number of elements in the N-dimensional array. If this control
259 is not an array, then ``elems`` is 1. The ``elems`` field can
260 never be 0.
261 * - __u32
262 - ``nr_of_dims``
263 - The number of dimension in the N-dimensional array. If this
264 control is not an array, then this field is 0.
265 * - __u32
266 - ``dims[V4L2_CTRL_MAX_DIMS]``
267 - The size of each dimension. The first ``nr_of_dims`` elements of
268 this array must be non-zero, all remaining elements must be zero.
269 * - __u32
270 - ``reserved``\ [32]
271 - Reserved for future extensions. Applications and drivers must set
272 the array to zero.
273
274
275.. tabularcolumns:: |p{1.2cm}|p{3.0cm}|p{13.1cm}|
276
277.. _v4l2-querymenu:
278
279.. flat-table:: struct v4l2_querymenu
280 :header-rows: 0
281 :stub-columns: 0
282 :widths: 1 1 2
283
284 * - __u32
285 - ``id``
286 - Identifies the control, set by the application from the respective
287 struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` ``id``.
288 * - __u32
289 - ``index``
290 - Index of the menu item, starting at zero, set by the application.
291 * - union {
292 - (anonymous)
293 * - __u8
294 - ``name``\ [32]
295 - Name of the menu item, a NUL-terminated ASCII string. This
296 information is intended for the user. This field is valid for
297 ``V4L2_CTRL_TYPE_MENU`` type controls.
298 * - __s64
299 - ``value``
300 - Value of the integer menu item. This field is valid for
301 ``V4L2_CTRL_TYPE_INTEGER_MENU`` type controls.
302 * - }
303 -
304 * - __u32
305 - ``reserved``
306 - Reserved for future extensions. Drivers must set the array to
307 zero.
308
309.. c:type:: v4l2_ctrl_type
310
311.. raw:: latex
312
313 \footnotesize
314
315.. tabularcolumns:: |p{6.5cm}|p{1.5cm}|p{1.1cm}|p{1.5cm}|p{6.8cm}|
316
317.. cssclass:: longtable
318
319.. flat-table:: enum v4l2_ctrl_type
320 :header-rows: 1
321 :stub-columns: 0
322 :widths: 30 5 5 5 55
323
324 * - Type
325 - ``minimum``
326 - ``step``
327 - ``maximum``
328 - Description
329 * - ``V4L2_CTRL_TYPE_INTEGER``
330 - any
331 - any
332 - any
333 - An integer-valued control ranging from minimum to maximum
334 inclusive. The step value indicates the increment between values.
335 * - ``V4L2_CTRL_TYPE_BOOLEAN``
336 - 0
337 - 1
338 - 1
339 - A boolean-valued control. Zero corresponds to "disabled", and one
340 means "enabled".
341 * - ``V4L2_CTRL_TYPE_MENU``
342 - ≥ 0
343 - 1
344 - N-1
345 - The control has a menu of N choices. The names of the menu items
346 can be enumerated with the ``VIDIOC_QUERYMENU`` ioctl.
347 * - ``V4L2_CTRL_TYPE_INTEGER_MENU``
348 - ≥ 0
349 - 1
350 - N-1
351 - The control has a menu of N choices. The values of the menu items
352 can be enumerated with the ``VIDIOC_QUERYMENU`` ioctl. This is
353 similar to ``V4L2_CTRL_TYPE_MENU`` except that instead of strings,
354 the menu items are signed 64-bit integers.
355 * - ``V4L2_CTRL_TYPE_BITMASK``
356 - 0
357 - n/a
358 - any
359 - A bitmask field. The maximum value is the set of bits that can be
360 used, all other bits are to be 0. The maximum value is interpreted
361 as a __u32, allowing the use of bit 31 in the bitmask.
362 * - ``V4L2_CTRL_TYPE_BUTTON``
363 - 0
364 - 0
365 - 0
366 - A control which performs an action when set. Drivers must ignore
367 the value passed with ``VIDIOC_S_CTRL`` and return an ``EACCES`` error
368 code on a ``VIDIOC_G_CTRL`` attempt.
369 * - ``V4L2_CTRL_TYPE_INTEGER64``
370 - any
371 - any
372 - any
373 - A 64-bit integer valued control. Minimum, maximum and step size
374 cannot be queried using ``VIDIOC_QUERYCTRL``. Only
375 ``VIDIOC_QUERY_EXT_CTRL`` can retrieve the 64-bit min/max/step
376 values, they should be interpreted as n/a when using
377 ``VIDIOC_QUERYCTRL``.
378 * - ``V4L2_CTRL_TYPE_STRING``
379 - ≥ 0
380 - ≥ 1
381 - ≥ 0
382 - The minimum and maximum string lengths. The step size means that
383 the string must be (minimum + N * step) characters long for N ≥ 0.
384 These lengths do not include the terminating zero, so in order to
385 pass a string of length 8 to
386 :ref:`VIDIOC_S_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` you need to
387 set the ``size`` field of struct
388 :c:type:`v4l2_ext_control` to 9. For
389 :ref:`VIDIOC_G_EXT_CTRLS <VIDIOC_G_EXT_CTRLS>` you can set
390 the ``size`` field to ``maximum`` + 1. Which character encoding is
391 used will depend on the string control itself and should be part
392 of the control documentation.
393 * - ``V4L2_CTRL_TYPE_CTRL_CLASS``
394 - n/a
395 - n/a
396 - n/a
397 - This is not a control. When ``VIDIOC_QUERYCTRL`` is called with a
398 control ID equal to a control class code (see :ref:`ctrl-class`)
399 + 1, the ioctl returns the name of the control class and this
400 control type. Older drivers which do not support this feature
401 return an ``EINVAL`` error code.
402 * - ``V4L2_CTRL_TYPE_U8``
403 - any
404 - any
405 - any
406 - An unsigned 8-bit valued control ranging from minimum to maximum
407 inclusive. The step value indicates the increment between values.
408 * - ``V4L2_CTRL_TYPE_U16``
409 - any
410 - any
411 - any
412 - An unsigned 16-bit valued control ranging from minimum to maximum
413 inclusive. The step value indicates the increment between values.
414 * - ``V4L2_CTRL_TYPE_U32``
415 - any
416 - any
417 - any
418 - An unsigned 32-bit valued control ranging from minimum to maximum
419 inclusive. The step value indicates the increment between values.
420 * - ``V4L2_CTRL_TYPE_MPEG2_QUANTISATION``
421 - n/a
422 - n/a
423 - n/a
424 - A struct :c:type:`v4l2_ctrl_mpeg2_quantisation`, containing MPEG-2
425 quantisation matrices for stateless video decoders.
426 * - ``V4L2_CTRL_TYPE_MPEG2_SEQUENCE``
427 - n/a
428 - n/a
429 - n/a
430 - A struct :c:type:`v4l2_ctrl_mpeg2_sequence`, containing MPEG-2
431 sequence parameters for stateless video decoders.
432 * - ``V4L2_CTRL_TYPE_MPEG2_PICTURE``
433 - n/a
434 - n/a
435 - n/a
436 - A struct :c:type:`v4l2_ctrl_mpeg2_picture`, containing MPEG-2
437 picture parameters for stateless video decoders.
438 * - ``V4L2_CTRL_TYPE_AREA``
439 - n/a
440 - n/a
441 - n/a
442 - A struct :c:type:`v4l2_area`, containing the width and the height
443 of a rectangular area. Units depend on the use case.
444 * - ``V4L2_CTRL_TYPE_H264_SPS``
445 - n/a
446 - n/a
447 - n/a
448 - A struct :c:type:`v4l2_ctrl_h264_sps`, containing H264
449 sequence parameters for stateless video decoders.
450 * - ``V4L2_CTRL_TYPE_H264_PPS``
451 - n/a
452 - n/a
453 - n/a
454 - A struct :c:type:`v4l2_ctrl_h264_pps`, containing H264
455 picture parameters for stateless video decoders.
456 * - ``V4L2_CTRL_TYPE_H264_SCALING_MATRIX``
457 - n/a
458 - n/a
459 - n/a
460 - A struct :c:type:`v4l2_ctrl_h264_scaling_matrix`, containing H264
461 scaling matrices for stateless video decoders.
462 * - ``V4L2_CTRL_TYPE_H264_SLICE_PARAMS``
463 - n/a
464 - n/a
465 - n/a
466 - A struct :c:type:`v4l2_ctrl_h264_slice_params`, containing H264
467 slice parameters for stateless video decoders.
468 * - ``V4L2_CTRL_TYPE_H264_DECODE_PARAMS``
469 - n/a
470 - n/a
471 - n/a
472 - A struct :c:type:`v4l2_ctrl_h264_decode_params`, containing H264
473 decode parameters for stateless video decoders.
474 * - ``V4L2_CTRL_TYPE_FWHT_PARAMS``
475 - n/a
476 - n/a
477 - n/a
478 - A struct :c:type:`v4l2_ctrl_fwht_params`, containing FWHT
479 parameters for stateless video decoders.
480 * - ``V4L2_CTRL_TYPE_HEVC_SPS``
481 - n/a
482 - n/a
483 - n/a
484 - A struct :c:type:`v4l2_ctrl_hevc_sps`, containing HEVC Sequence
485 Parameter Set for stateless video decoders.
486 * - ``V4L2_CTRL_TYPE_HEVC_PPS``
487 - n/a
488 - n/a
489 - n/a
490 - A struct :c:type:`v4l2_ctrl_hevc_pps`, containing HEVC Picture
491 Parameter Set for stateless video decoders.
492 * - ``V4L2_CTRL_TYPE_HEVC_SLICE_PARAMS``
493 - n/a
494 - n/a
495 - n/a
496 - A struct :c:type:`v4l2_ctrl_hevc_slice_params`, containing HEVC
497 slice parameters for stateless video decoders.
498 * - ``V4L2_CTRL_TYPE_VP8_FRAME``
499 - n/a
500 - n/a
501 - n/a
502 - A struct :c:type:`v4l2_ctrl_vp8_frame`, containing VP8
503 frame parameters for stateless video decoders.
504 * - ``V4L2_CTRL_TYPE_HEVC_DECODE_PARAMS``
505 - n/a
506 - n/a
507 - n/a
508 - A struct :c:type:`v4l2_ctrl_hevc_decode_params`, containing HEVC
509 decoding parameters for stateless video decoders.
510
511.. raw:: latex
512
513 \normalsize
514
515.. tabularcolumns:: |p{7.3cm}|p{1.8cm}|p{8.2cm}|
516
517.. cssclass:: longtable
518
519.. _control-flags:
520
521.. flat-table:: Control Flags
522 :header-rows: 0
523 :stub-columns: 0
524 :widths: 3 1 4
525
526 * - ``V4L2_CTRL_FLAG_DISABLED``
527 - 0x0001
528 - This control is permanently disabled and should be ignored by the
529 application. Any attempt to change the control will result in an
530 ``EINVAL`` error code.
531 * - ``V4L2_CTRL_FLAG_GRABBED``
532 - 0x0002
533 - This control is temporarily unchangeable, for example because
534 another application took over control of the respective resource.
535 Such controls may be displayed specially in a user interface.
536 Attempts to change the control may result in an ``EBUSY`` error code.
537 * - ``V4L2_CTRL_FLAG_READ_ONLY``
538 - 0x0004
539 - This control is permanently readable only. Any attempt to change
540 the control will result in an ``EINVAL`` error code.
541 * - ``V4L2_CTRL_FLAG_UPDATE``
542 - 0x0008
543 - A hint that changing this control may affect the value of other
544 controls within the same control class. Applications should update
545 their user interface accordingly.
546 * - ``V4L2_CTRL_FLAG_INACTIVE``
547 - 0x0010
548 - This control is not applicable to the current configuration and
549 should be displayed accordingly in a user interface. For example
550 the flag may be set on a MPEG audio level 2 bitrate control when
551 MPEG audio encoding level 1 was selected with another control.
552 * - ``V4L2_CTRL_FLAG_SLIDER``
553 - 0x0020
554 - A hint that this control is best represented as a slider-like
555 element in a user interface.
556 * - ``V4L2_CTRL_FLAG_WRITE_ONLY``
557 - 0x0040
558 - This control is permanently writable only. Any attempt to read the
559 control will result in an ``EACCES`` error code error code. This flag
560 is typically present for relative controls or action controls
561 where writing a value will cause the device to carry out a given
562 action (e. g. motor control) but no meaningful value can be
563 returned.
564 * - ``V4L2_CTRL_FLAG_VOLATILE``
565 - 0x0080
566 - This control is volatile, which means that the value of the
567 control changes continuously. A typical example would be the
568 current gain value if the device is in auto-gain mode. In such a
569 case the hardware calculates the gain value based on the lighting
570 conditions which can change over time.
571
572 .. note::
573
574 Setting a new value for a volatile control will be ignored
575 unless
576 :ref:`V4L2_CTRL_FLAG_EXECUTE_ON_WRITE <FLAG_EXECUTE_ON_WRITE>`
577 is also set.
578 Setting a new value for a volatile control will *never* trigger a
579 :ref:`V4L2_EVENT_CTRL_CH_VALUE <ctrl-changes-flags>` event.
580 * - ``V4L2_CTRL_FLAG_HAS_PAYLOAD``
581 - 0x0100
582 - This control has a pointer type, so its value has to be accessed
583 using one of the pointer fields of struct
584 :c:type:`v4l2_ext_control`. This flag is set
585 for controls that are an array, string, or have a compound type.
586 In all cases you have to set a pointer to memory containing the
587 payload of the control.
588 * .. _FLAG_EXECUTE_ON_WRITE:
589
590 - ``V4L2_CTRL_FLAG_EXECUTE_ON_WRITE``
591 - 0x0200
592 - The value provided to the control will be propagated to the driver
593 even if it remains constant. This is required when the control
594 represents an action on the hardware. For example: clearing an
595 error flag or triggering the flash. All the controls of the type
596 ``V4L2_CTRL_TYPE_BUTTON`` have this flag set.
597 * .. _FLAG_MODIFY_LAYOUT:
598
599 - ``V4L2_CTRL_FLAG_MODIFY_LAYOUT``
600 - 0x0400
601 - Changing this control value may modify the layout of the
602 buffer (for video devices) or the media bus format (for sub-devices).
603
604 A typical example would be the ``V4L2_CID_ROTATE`` control.
605
606 Note that typically controls with this flag will also set the
607 ``V4L2_CTRL_FLAG_GRABBED`` flag when buffers are allocated or
608 streaming is in progress since most drivers do not support changing
609 the format in that case.
610
611Return Value
612============
613
614On success 0 is returned, on error -1 and the ``errno`` variable is set
615appropriately. The generic error codes are described at the
616:ref:`Generic Error Codes <gen-errors>` chapter.
617
618EINVAL
619 The struct :ref:`v4l2_queryctrl <v4l2-queryctrl>` ``id`` is
620 invalid. The struct :ref:`v4l2_querymenu <v4l2-querymenu>` ``id``
621 is invalid or ``index`` is out of range (less than ``minimum`` or
622 greater than ``maximum``) or this particular menu item is not
623 supported by the driver.
624
625EACCES
626 An attempt was made to read a write-only control.
627
628.. [#f1]
629 ``V4L2_CTRL_FLAG_DISABLED`` was intended for two purposes: Drivers
630 can skip predefined controls not supported by the hardware (although
631 returning ``EINVAL`` would do as well), or disable predefined and private
632 controls after hardware detection without the trouble of reordering
633 control arrays and indices (``EINVAL`` cannot be used to skip private
634 controls because it would prematurely end the enumeration).