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Mauro Carvalho Chehab8e080c22009-09-13 22:16:04 -03001<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>. When the application
64calls the <constant>VIDIOC_G_FMT</constant> ioctl with a pointer to
65this structure the driver fills the respective member of the
66<structfield>fmt</structfield> union. In case of video capture devices
67that is the &v4l2-pix-format; <structfield>pix</structfield> member.
68When the requested buffer type is not supported drivers return an
69&EINVAL;.</para>
70
71 <para>To change the current format parameters applications
72initialize the <structfield>type</structfield> field and all
73fields of the respective <structfield>fmt</structfield>
74union member. For details see the documentation of the various devices
75types in <xref linkend="devices" />. Good practice is to query the
76current parameters first, and to
77modify only those parameters not suitable for the application. When
78the application calls the <constant>VIDIOC_S_FMT</constant> ioctl
79with a pointer to a <structname>v4l2_format</structname> structure
80the driver checks
81and adjusts the parameters against hardware abilities. Drivers
82should not return an error code unless the input is ambiguous, this is
83a mechanism to fathom device capabilities and to approach parameters
84acceptable for both the application and driver. On success the driver
85may program the hardware, allocate resources and generally prepare for
86data exchange.
87Finally the <constant>VIDIOC_S_FMT</constant> ioctl returns the
88current format parameters as <constant>VIDIOC_G_FMT</constant> does.
89Very simple, inflexible devices may even ignore all input and always
90return the default parameters. However all V4L2 devices exchanging
91data with the application must implement the
92<constant>VIDIOC_G_FMT</constant> and
93<constant>VIDIOC_S_FMT</constant> ioctl. When the requested buffer
94type is not supported drivers return an &EINVAL; on a
95<constant>VIDIOC_S_FMT</constant> attempt. When I/O is already in
96progress or the resource is not available for other reasons drivers
97return the &EBUSY;.</para>
98
99 <para>The <constant>VIDIOC_TRY_FMT</constant> ioctl is equivalent
100to <constant>VIDIOC_S_FMT</constant> with one exception: it does not
101change driver state. It can also be called at any time, never
102returning <errorcode>EBUSY</errorcode>. This function is provided to
103negotiate parameters, to learn about hardware limitations, without
104disabling I/O or possibly time consuming hardware preparations.
105Although strongly recommended drivers are not required to implement
106this ioctl.</para>
107
108 <table pgwide="1" frame="none" id="v4l2-format">
109 <title>struct <structname>v4l2_format</structname></title>
110 <tgroup cols="4">
111 <colspec colname="c1" />
112 <colspec colname="c2" />
113 <colspec colname="c3" />
114 <colspec colname="c4" />
115 <tbody valign="top">
116 <row>
117 <entry>&v4l2-buf-type;</entry>
118 <entry><structfield>type</structfield></entry>
119 <entry></entry>
120 <entry>Type of the data stream, see <xref
121 linkend="v4l2-buf-type" />.</entry>
122 </row>
123 <row>
124 <entry>union</entry>
125 <entry><structfield>fmt</structfield></entry>
126 </row>
127 <row>
128 <entry></entry>
129 <entry>&v4l2-pix-format;</entry>
130 <entry><structfield>pix</structfield></entry>
131 <entry>Definition of an image format, see <xref
132 linkend="pixfmt" />, used by video capture and output
133devices.</entry>
134 </row>
135 <row>
136 <entry></entry>
137 <entry>&v4l2-window;</entry>
138 <entry><structfield>win</structfield></entry>
139 <entry>Definition of an overlaid image, see <xref
140 linkend="overlay" />, used by video overlay devices.</entry>
141 </row>
142 <row>
143 <entry></entry>
144 <entry>&v4l2-vbi-format;</entry>
145 <entry><structfield>vbi</structfield></entry>
146 <entry>Raw VBI capture or output parameters. This is
147discussed in more detail in <xref linkend="raw-vbi" />. Used by raw VBI
148capture and output devices.</entry>
149 </row>
150 <row>
151 <entry></entry>
152 <entry>&v4l2-sliced-vbi-format;</entry>
153 <entry><structfield>sliced</structfield></entry>
154 <entry>Sliced VBI capture or output parameters. See
155<xref linkend="sliced" /> for details. Used by sliced VBI
156capture and output devices.</entry>
157 </row>
158 <row>
159 <entry></entry>
160 <entry>__u8</entry>
161 <entry><structfield>raw_data</structfield>[200]</entry>
162 <entry>Place holder for future extensions and custom
163(driver defined) formats with <structfield>type</structfield>
164<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher.</entry>
165 </row>
166 </tbody>
167 </tgroup>
168 </table>
169 </refsect1>
170
171 <refsect1>
172 &return-value;
173
174 <variablelist>
175 <varlistentry>
176 <term><errorcode>EBUSY</errorcode></term>
177 <listitem>
178 <para>The data format cannot be changed at this
179time, for example because I/O is already in progress.</para>
180 </listitem>
181 </varlistentry>
182 <varlistentry>
183 <term><errorcode>EINVAL</errorcode></term>
184 <listitem>
185 <para>The &v4l2-format; <structfield>type</structfield>
186field is invalid, the requested buffer type not supported, or
187<constant>VIDIOC_TRY_FMT</constant> was called and is not
188supported with this buffer type.</para>
189 </listitem>
190 </varlistentry>
191 </variablelist>
192 </refsect1>
193</refentry>
194
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