1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2.. c:namespace:: V4L
3
4.. _VIDIOC_G_PARM:
5
6**********************************
7ioctl VIDIOC_G_PARM, VIDIOC_S_PARM
8**********************************
9
10Name
11====
12
13VIDIOC_G_PARM - VIDIOC_S_PARM - Get or set streaming parameters
14
15Synopsis
16========
17
18.. c:macro:: VIDIOC_G_PARM
19
20``int ioctl(int fd, VIDIOC_G_PARM, v4l2_streamparm *argp)``
21
22.. c:macro:: VIDIOC_S_PARM
23
24``int ioctl(int fd, VIDIOC_S_PARM, v4l2_streamparm *argp)``
25
26Arguments
27=========
28
29``fd``
30    File descriptor returned by :c:func:`open()`.
31
32``argp``
33    Pointer to struct :c:type:`v4l2_streamparm`.
34
35Description
36===========
37
38Applications can request a different frame interval. The capture or
39output device will be reconfigured to support the requested frame
40interval if possible. Optionally drivers may choose to skip or
41repeat frames to achieve the requested frame interval.
42
43For stateful encoders (see :ref:`encoder`) this represents the
44frame interval that is typically embedded in the encoded video stream.
45
46Changing the frame interval shall never change the format. Changing the
47format, on the other hand, may change the frame interval.
48
49Further these ioctls can be used to determine the number of buffers used
50internally by a driver in read/write mode. For implications see the
51section discussing the :c:func:`read()` function.
52
53To get and set the streaming parameters applications call the
54:ref:`VIDIOC_G_PARM <VIDIOC_G_PARM>` and
55:ref:`VIDIOC_S_PARM <VIDIOC_G_PARM>` ioctl, respectively. They take a
56pointer to a struct :c:type:`v4l2_streamparm` which contains a
57union holding separate parameters for input and output devices.
58
59.. tabularcolumns:: |p{3.5cm}|p{3.5cm}|p{3.5cm}|p{7.0cm}|
60
61.. c:type:: v4l2_streamparm
62
63.. flat-table:: struct v4l2_streamparm
64    :header-rows:  0
65    :stub-columns: 0
66    :widths:       1 1 2
67
68    * - __u32
69      - ``type``
70      - The buffer (stream) type, same as struct
71	:c:type:`v4l2_format` ``type``, set by the
72	application. See :c:type:`v4l2_buf_type`.
73    * - union {
74      - ``parm``
75    * - struct :c:type:`v4l2_captureparm`
76      - ``capture``
77      - Parameters for capture devices, used when ``type`` is
78	``V4L2_BUF_TYPE_VIDEO_CAPTURE`` or
79	``V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE``.
80    * - struct :c:type:`v4l2_outputparm`
81      - ``output``
82      - Parameters for output devices, used when ``type`` is
83	``V4L2_BUF_TYPE_VIDEO_OUTPUT`` or ``V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE``.
84    * - __u8
85      - ``raw_data``\ [200]
86      - A place holder for future extensions.
87    * - }
88      -
89
90
91.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
92
93.. c:type:: v4l2_captureparm
94
95.. flat-table:: struct v4l2_captureparm
96    :header-rows:  0
97    :stub-columns: 0
98    :widths:       1 1 2
99
100    * - __u32
101      - ``capability``
102      - See :ref:`parm-caps`.
103    * - __u32
104      - ``capturemode``
105      - Set by drivers and applications, see :ref:`parm-flags`.
106    * - struct :c:type:`v4l2_fract`
107      - ``timeperframe``
108      - This is the desired period between successive frames captured by
109	the driver, in seconds.
110    * - :cspan:`2`
111
112	This will configure the speed at which the video source (e.g. a sensor)
113	generates video frames. If the speed is fixed, then the driver may
114	choose to skip or repeat frames in order to achieve the requested
115	frame rate.
116
117	For stateful encoders (see :ref:`encoder`) this represents the
118	frame interval that is typically embedded in the encoded video stream.
119
120	Applications store here the desired frame period, drivers return
121	the actual frame period.
122
123	Changing the video standard (also implicitly by switching
124	the video input) may reset this parameter to the nominal frame
125	period. To reset manually applications can just set this field to
126	zero.
127
128	Drivers support this function only when they set the
129	``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.
130    * - __u32
131      - ``extendedmode``
132      - Custom (driver specific) streaming parameters. When unused,
133	applications and drivers must set this field to zero. Applications
134	using this field should check the driver name and version, see
135	:ref:`querycap`.
136    * - __u32
137      - ``readbuffers``
138      - Applications set this field to the desired number of buffers used
139	internally by the driver in :c:func:`read()` mode.
140	Drivers return the actual number of buffers. When an application
141	requests zero buffers, drivers should just return the current
142	setting rather than the minimum or an error code. For details see
143	:ref:`rw`.
144    * - __u32
145      - ``reserved``\ [4]
146      - Reserved for future extensions. Drivers and applications must set
147	the array to zero.
148
149
150.. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.7cm}|
151
152.. c:type:: v4l2_outputparm
153
154.. flat-table:: struct v4l2_outputparm
155    :header-rows:  0
156    :stub-columns: 0
157    :widths:       1 1 2
158
159    * - __u32
160      - ``capability``
161      - See :ref:`parm-caps`.
162    * - __u32
163      - ``outputmode``
164      - Set by drivers and applications, see :ref:`parm-flags`.
165    * - struct :c:type:`v4l2_fract`
166      - ``timeperframe``
167      - This is the desired period between successive frames output by the
168	driver, in seconds.
169    * - :cspan:`2`
170
171	The field is intended to repeat frames on the driver side in
172	:c:func:`write()` mode (in streaming mode timestamps
173	can be used to throttle the output), saving I/O bandwidth.
174
175	For stateful encoders (see :ref:`encoder`) this represents the
176	frame interval that is typically embedded in the encoded video stream
177	and it provides a hint to the encoder of the speed at which raw
178	frames are queued up to the encoder.
179
180	Applications store here the desired frame period, drivers return
181	the actual frame period.
182
183	Changing the video standard (also implicitly by switching
184	the video output) may reset this parameter to the nominal frame
185	period. To reset manually applications can just set this field to
186	zero.
187
188	Drivers support this function only when they set the
189	``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.
190    * - __u32
191      - ``extendedmode``
192      - Custom (driver specific) streaming parameters. When unused,
193	applications and drivers must set this field to zero. Applications
194	using this field should check the driver name and version, see
195	:ref:`querycap`.
196    * - __u32
197      - ``writebuffers``
198      - Applications set this field to the desired number of buffers used
199	internally by the driver in :c:func:`write()` mode. Drivers
200	return the actual number of buffers. When an application requests
201	zero buffers, drivers should just return the current setting
202	rather than the minimum or an error code. For details see
203	:ref:`rw`.
204    * - __u32
205      - ``reserved``\ [4]
206      - Reserved for future extensions. Drivers and applications must set
207	the array to zero.
208
209
210.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
211
212.. _parm-caps:
213
214.. flat-table:: Streaming Parameters Capabilities
215    :header-rows:  0
216    :stub-columns: 0
217    :widths:       3 1 4
218
219    * - ``V4L2_CAP_TIMEPERFRAME``
220      - 0x1000
221      - The frame period can be modified by setting the ``timeperframe``
222	field.
223
224
225.. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.7cm}|
226
227.. _parm-flags:
228
229.. flat-table:: Capture Parameters Flags
230    :header-rows:  0
231    :stub-columns: 0
232    :widths:       3 1 4
233
234    * - ``V4L2_MODE_HIGHQUALITY``
235      - 0x0001
236      - High quality imaging mode. High quality mode is intended for still
237	imaging applications. The idea is to get the best possible image
238	quality that the hardware can deliver. It is not defined how the
239	driver writer may achieve that; it will depend on the hardware and
240	the ingenuity of the driver writer. High quality mode is a
241	different mode from the regular motion video capture modes. In
242	high quality mode:
243
244	-  The driver may be able to capture higher resolutions than for
245	   motion capture.
246
247	-  The driver may support fewer pixel formats than motion capture
248	   (eg; true color).
249
250	-  The driver may capture and arithmetically combine multiple
251	   successive fields or frames to remove color edge artifacts and
252	   reduce the noise in the video data.
253
254	-  The driver may capture images in slices like a scanner in order
255	   to handle larger format images than would otherwise be
256	   possible.
257
258	-  An image capture operation may be significantly slower than
259	   motion capture.
260
261	-  Moving objects in the image might have excessive motion blur.
262
263	-  Capture might only work through the :c:func:`read()` call.
264
265Return Value
266============
267
268On success 0 is returned, on error -1 and the ``errno`` variable is set
269appropriately. The generic error codes are described at the
270:ref:`Generic Error Codes <gen-errors>` chapter.
271