1<refentry id="vidioc-g-fmt">
2  <refmeta>
3    <refentrytitle>ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,
4VIDIOC_TRY_FMT</refentrytitle>
5    &manvol;
6  </refmeta>
7
8  <refnamediv>
9    <refname>VIDIOC_G_FMT</refname>
10    <refname>VIDIOC_S_FMT</refname>
11    <refname>VIDIOC_TRY_FMT</refname>
12    <refpurpose>Get or set the data format, try a format</refpurpose>
13  </refnamediv>
14
15  <refsynopsisdiv>
16    <funcsynopsis>
17      <funcprototype>
18	<funcdef>int <function>ioctl</function></funcdef>
19	<paramdef>int <parameter>fd</parameter></paramdef>
20	<paramdef>int <parameter>request</parameter></paramdef>
21	<paramdef>struct v4l2_format
22*<parameter>argp</parameter></paramdef>
23      </funcprototype>
24    </funcsynopsis>
25  </refsynopsisdiv>
26
27  <refsect1>
28    <title>Arguments</title>
29
30    <variablelist>
31      <varlistentry>
32	<term><parameter>fd</parameter></term>
33	<listitem>
34	  <para>&fd;</para>
35	</listitem>
36      </varlistentry>
37      <varlistentry>
38	<term><parameter>request</parameter></term>
39	<listitem>
40	  <para>VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT</para>
41	</listitem>
42      </varlistentry>
43      <varlistentry>
44	<term><parameter>argp</parameter></term>
45	<listitem>
46	  <para></para>
47	</listitem>
48      </varlistentry>
49    </variablelist>
50  </refsect1>
51
52  <refsect1>
53    <title>Description</title>
54
55    <para>These ioctls are used to negotiate the format of data
56(typically image format) exchanged between driver and
57application.</para>
58
59    <para>To query the current parameters applications set the
60<structfield>type</structfield> field of a struct
61<structname>v4l2_format</structname> to the respective buffer (stream)
62type. For example video capture devices use
63<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant> or
64<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE</constant>. When the application
65calls the <constant>VIDIOC_G_FMT</constant> ioctl with a pointer to
66this structure the driver fills the respective member of the
67<structfield>fmt</structfield> union. In case of video capture devices
68that is either the &v4l2-pix-format; <structfield>pix</structfield> or
69the &v4l2-pix-format-mplane; <structfield>pix_mp</structfield> member.
70When the requested buffer type is not supported drivers return an
71&EINVAL;.</para>
72
73    <para>To change the current format parameters applications
74initialize the <structfield>type</structfield> field and all
75fields of the respective <structfield>fmt</structfield>
76union member. For details see the documentation of the various devices
77types in <xref linkend="devices" />. Good practice is to query the
78current parameters first, and to
79modify only those parameters not suitable for the application. When
80the application calls the <constant>VIDIOC_S_FMT</constant> ioctl
81with a pointer to a <structname>v4l2_format</structname> structure
82the driver checks
83and adjusts the parameters against hardware abilities. Drivers
84should not return an error code unless the <structfield>type</structfield> field is invalid, this is
85a mechanism to fathom device capabilities and to approach parameters
86acceptable for both the application and driver. On success the driver
87may program the hardware, allocate resources and generally prepare for
88data exchange.
89Finally the <constant>VIDIOC_S_FMT</constant> ioctl returns the
90current format parameters as <constant>VIDIOC_G_FMT</constant> does.
91Very simple, inflexible devices may even ignore all input and always
92return the default parameters. However all V4L2 devices exchanging
93data with the application must implement the
94<constant>VIDIOC_G_FMT</constant> and
95<constant>VIDIOC_S_FMT</constant> ioctl. When the requested buffer
96type is not supported drivers return an &EINVAL; on a
97<constant>VIDIOC_S_FMT</constant> attempt. When I/O is already in
98progress or the resource is not available for other reasons drivers
99return the &EBUSY;.</para>
100
101    <para>The <constant>VIDIOC_TRY_FMT</constant> ioctl is equivalent
102to <constant>VIDIOC_S_FMT</constant> with one exception: it does not
103change driver state. It can also be called at any time, never
104returning <errorcode>EBUSY</errorcode>. This function is provided to
105negotiate parameters, to learn about hardware limitations, without
106disabling I/O or possibly time consuming hardware preparations.
107Although strongly recommended drivers are not required to implement
108this ioctl.</para>
109
110    <para>The format as returned by <constant>VIDIOC_TRY_FMT</constant>
111must be identical to what <constant>VIDIOC_S_FMT</constant> returns for
112the same input or output.</para>
113
114    <table pgwide="1" frame="none" id="v4l2-format">
115      <title>struct <structname>v4l2_format</structname></title>
116      <tgroup cols="4">
117	<colspec colname="c1" />
118	<colspec colname="c2" />
119	<colspec colname="c3" />
120	<colspec colname="c4" />
121	<tbody valign="top">
122	  <row>
123	    <entry>__u32</entry>
124	    <entry><structfield>type</structfield></entry>
125	    <entry></entry>
126	    <entry>Type of the data stream, see <xref
127		linkend="v4l2-buf-type" />.</entry>
128	  </row>
129	  <row>
130	    <entry>union</entry>
131	    <entry><structfield>fmt</structfield></entry>
132	  </row>
133	  <row>
134	    <entry></entry>
135	    <entry>&v4l2-pix-format;</entry>
136	    <entry><structfield>pix</structfield></entry>
137	    <entry>Definition of an image format, see <xref
138		linkend="pixfmt" />, used by video capture and output
139devices.</entry>
140	  </row>
141	  <row>
142	    <entry></entry>
143	    <entry>&v4l2-pix-format-mplane;</entry>
144	    <entry><structfield>pix_mp</structfield></entry>
145	    <entry>Definition of an image format, see <xref
146		linkend="pixfmt" />, used by video capture and output
147devices that support the <link linkend="planar-apis">multi-planar
148version of the API</link>.</entry>
149	  </row>
150	  <row>
151	    <entry></entry>
152	    <entry>&v4l2-window;</entry>
153	    <entry><structfield>win</structfield></entry>
154	    <entry>Definition of an overlaid image, see <xref
155	    linkend="overlay" />, used by video overlay devices.</entry>
156	  </row>
157	  <row>
158	    <entry></entry>
159	    <entry>&v4l2-vbi-format;</entry>
160	    <entry><structfield>vbi</structfield></entry>
161	    <entry>Raw VBI capture or output parameters. This is
162discussed in more detail in <xref linkend="raw-vbi" />. Used by raw VBI
163capture and output devices.</entry>
164	  </row>
165	  <row>
166	    <entry></entry>
167	    <entry>&v4l2-sliced-vbi-format;</entry>
168	    <entry><structfield>sliced</structfield></entry>
169	    <entry>Sliced VBI capture or output parameters. See
170<xref linkend="sliced" /> for details. Used by sliced VBI
171capture and output devices.</entry>
172	  </row>
173	  <row>
174	    <entry></entry>
175	    <entry>&v4l2-sdr-format;</entry>
176	    <entry><structfield>sdr</structfield></entry>
177	    <entry>Definition of a data format, see
178<xref linkend="pixfmt" />, used by SDR capture devices.</entry>
179	  </row>
180	  <row>
181	    <entry></entry>
182	    <entry>__u8</entry>
183	    <entry><structfield>raw_data</structfield>[200]</entry>
184	    <entry>Place holder for future extensions.</entry>
185	  </row>
186	</tbody>
187      </tgroup>
188    </table>
189  </refsect1>
190
191  <refsect1>
192    &return-value;
193
194    <variablelist>
195      <varlistentry>
196	<term><errorcode>EINVAL</errorcode></term>
197	<listitem>
198	  <para>The &v4l2-format; <structfield>type</structfield>
199field is invalid or the requested buffer type not supported.</para>
200	</listitem>
201      </varlistentry>
202    </variablelist>
203  </refsect1>
204</refentry>
205