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