1 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2 .. c:namespace:: V4L
3
4 .. _VIDIOC_G_PARM:
5
6 **********************************
7 ioctl VIDIOC_G_PARM, VIDIOC_S_PARM
8 **********************************
9
10 Name
11 ====
12
13 VIDIOC_G_PARM - VIDIOC_S_PARM - Get or set streaming parameters
14
15 Synopsis
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
26 Arguments
27 =========
28
29 ``fd``
30 File descriptor returned by :c:func:`open()`.
31
32 ``argp``
33 Pointer to struct :c:type:`v4l2_streamparm`.
34
35 Description
36 ===========
37
38 Applications can request a different frame interval. The capture or
39 output device will be reconfigured to support the requested frame
40 interval if possible. Optionally drivers may choose to skip or
41 repeat frames to achieve the requested frame interval.
42
43 For stateful encoders (see :ref:`encoder`) this represents the
44 frame interval that is typically embedded in the encoded video stream.
45
46 Changing the frame interval shall never change the format. Changing the
47 format, on the other hand, may change the frame interval.
48
49 Further these ioctls can be used to determine the number of buffers used
50 internally by a driver in read/write mode. For implications see the
51 section discussing the :c:func:`read()` function.
52
53 To 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
56 pointer to a struct :c:type:`v4l2_streamparm` which contains a
57 union holding separate parameters for input and output devices.
58
59 .. tabularcolumns:: |p{3.7cm}|p{3.5cm}|p{10.1cm}|
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 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
91
92 .. c:type:: v4l2_captureparm
93
94 .. flat-table:: struct v4l2_captureparm
95 :header-rows: 0
96 :stub-columns: 0
97 :widths: 1 1 2
98
99 * - __u32
100 - ``capability``
101 - See :ref:`parm-caps`.
102 * - __u32
103 - ``capturemode``
104 - Set by drivers and applications, see :ref:`parm-flags`.
105 * - struct :c:type:`v4l2_fract`
106 - ``timeperframe``
107 - This is the desired period between successive frames captured by
108 the driver, in seconds.
109 * - :cspan:`2`
110
111 This will configure the speed at which the video source (e.g. a sensor)
112 generates video frames. If the speed is fixed, then the driver may
113 choose to skip or repeat frames in order to achieve the requested
114 frame rate.
115
116 For stateful encoders (see :ref:`encoder`) this represents the
117 frame interval that is typically embedded in the encoded video stream.
118
119 Applications store here the desired frame period, drivers return
120 the actual frame period.
121
122 Changing the video standard (also implicitly by switching
123 the video input) may reset this parameter to the nominal frame
124 period. To reset manually applications can just set this field to
125 zero.
126
127 Drivers support this function only when they set the
128 ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.
129 * - __u32
130 - ``extendedmode``
131 - Custom (driver specific) streaming parameters. When unused,
132 applications and drivers must set this field to zero. Applications
133 using this field should check the driver name and version, see
134 :ref:`querycap`.
135 * - __u32
136 - ``readbuffers``
137 - Applications set this field to the desired number of buffers used
138 internally by the driver in :c:func:`read()` mode.
139 Drivers return the actual number of buffers. When an application
140 requests zero buffers, drivers should just return the current
141 setting rather than the minimum or an error code. For details see
142 :ref:`rw`.
143 * - __u32
144 - ``reserved``\ [4]
145 - Reserved for future extensions. Drivers and applications must set
146 the array to zero.
147
148
149 .. tabularcolumns:: |p{4.4cm}|p{4.4cm}|p{8.5cm}|
150
151 .. c:type:: v4l2_outputparm
152
153 .. flat-table:: struct v4l2_outputparm
154 :header-rows: 0
155 :stub-columns: 0
156 :widths: 1 1 2
157
158 * - __u32
159 - ``capability``
160 - See :ref:`parm-caps`.
161 * - __u32
162 - ``outputmode``
163 - Set by drivers and applications, see :ref:`parm-flags`.
164 * - struct :c:type:`v4l2_fract`
165 - ``timeperframe``
166 - This is the desired period between successive frames output by the
167 driver, in seconds.
168 * - :cspan:`2`
169
170 The field is intended to repeat frames on the driver side in
171 :c:func:`write()` mode (in streaming mode timestamps
172 can be used to throttle the output), saving I/O bandwidth.
173
174 For stateful encoders (see :ref:`encoder`) this represents the
175 frame interval that is typically embedded in the encoded video stream
176 and it provides a hint to the encoder of the speed at which raw
177 frames are queued up to the encoder.
178
179 Applications store here the desired frame period, drivers return
180 the actual frame period.
181
182 Changing the video standard (also implicitly by switching
183 the video output) may reset this parameter to the nominal frame
184 period. To reset manually applications can just set this field to
185 zero.
186
187 Drivers support this function only when they set the
188 ``V4L2_CAP_TIMEPERFRAME`` flag in the ``capability`` field.
189 * - __u32
190 - ``extendedmode``
191 - Custom (driver specific) streaming parameters. When unused,
192 applications and drivers must set this field to zero. Applications
193 using this field should check the driver name and version, see
194 :ref:`querycap`.
195 * - __u32
196 - ``writebuffers``
197 - Applications set this field to the desired number of buffers used
198 internally by the driver in :c:func:`write()` mode. Drivers
199 return the actual number of buffers. When an application requests
200 zero buffers, drivers should just return the current setting
201 rather than the minimum or an error code. For details see
202 :ref:`rw`.
203 * - __u32
204 - ``reserved``\ [4]
205 - Reserved for future extensions. Drivers and applications must set
206 the array to zero.
207
208
209 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
210
211 .. _parm-caps:
212
213 .. flat-table:: Streaming Parameters Capabilities
214 :header-rows: 0
215 :stub-columns: 0
216 :widths: 3 1 4
217
218 * - ``V4L2_CAP_TIMEPERFRAME``
219 - 0x1000
220 - The frame period can be modified by setting the ``timeperframe``
221 field.
222
223
224 .. tabularcolumns:: |p{6.6cm}|p{2.2cm}|p{8.5cm}|
225
226 .. _parm-flags:
227
228 .. flat-table:: Capture Parameters Flags
229 :header-rows: 0
230 :stub-columns: 0
231 :widths: 3 1 4
232
233 * - ``V4L2_MODE_HIGHQUALITY``
234 - 0x0001
235 - High quality imaging mode. High quality mode is intended for still
236 imaging applications. The idea is to get the best possible image
237 quality that the hardware can deliver. It is not defined how the
238 driver writer may achieve that; it will depend on the hardware and
239 the ingenuity of the driver writer. High quality mode is a
240 different mode from the regular motion video capture modes. In
241 high quality mode:
242
243 - The driver may be able to capture higher resolutions than for
244 motion capture.
245
246 - The driver may support fewer pixel formats than motion capture
247 (eg; true color).
248
249 - The driver may capture and arithmetically combine multiple
250 successive fields or frames to remove color edge artifacts and
251 reduce the noise in the video data.
252
253 - The driver may capture images in slices like a scanner in order
254 to handle larger format images than would otherwise be
255 possible.
256
257 - An image capture operation may be significantly slower than
258 motion capture.
259
260 - Moving objects in the image might have excessive motion blur.
261
262 - Capture might only work through the :c:func:`read()` call.
263
264 Return Value
265 ============
266
267 On success 0 is returned, on error -1 and the ``errno`` variable is set
268 appropriately. The generic error codes are described at the
269 :ref:`Generic Error Codes <gen-errors>` chapter.
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