202 lines
6.7 KiB
XML
202 lines
6.7 KiB
XML
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<refentry id="vidioc-g-fmt">
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<refmeta>
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<refentrytitle>ioctl VIDIOC_G_FMT, VIDIOC_S_FMT,
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VIDIOC_TRY_FMT</refentrytitle>
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&manvol;
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</refmeta>
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<refnamediv>
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<refname>VIDIOC_G_FMT</refname>
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<refname>VIDIOC_S_FMT</refname>
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<refname>VIDIOC_TRY_FMT</refname>
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<refpurpose>Get or set the data format, try a format</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcprototype>
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<funcdef>int <function>ioctl</function></funcdef>
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<paramdef>int <parameter>fd</parameter></paramdef>
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<paramdef>int <parameter>request</parameter></paramdef>
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<paramdef>struct v4l2_format
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*<parameter>argp</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Arguments</title>
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<variablelist>
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<varlistentry>
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<term><parameter>fd</parameter></term>
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<listitem>
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<para>&fd;</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>request</parameter></term>
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<listitem>
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<para>VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><parameter>argp</parameter></term>
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<listitem>
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<para></para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1>
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<title>Description</title>
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<para>These ioctls are used to negotiate the format of data
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(typically image format) exchanged between driver and
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application.</para>
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<para>To query the current parameters applications set the
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<structfield>type</structfield> field of a struct
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<structname>v4l2_format</structname> to the respective buffer (stream)
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type. For example video capture devices use
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<constant>V4L2_BUF_TYPE_VIDEO_CAPTURE</constant>. When the application
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calls the <constant>VIDIOC_G_FMT</constant> ioctl with a pointer to
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this structure the driver fills the respective member of the
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<structfield>fmt</structfield> union. In case of video capture devices
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that is the &v4l2-pix-format; <structfield>pix</structfield> member.
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When the requested buffer type is not supported drivers return an
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&EINVAL;.</para>
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<para>To change the current format parameters applications
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initialize the <structfield>type</structfield> field and all
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fields of the respective <structfield>fmt</structfield>
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union member. For details see the documentation of the various devices
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types in <xref linkend="devices" />. Good practice is to query the
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current parameters first, and to
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modify only those parameters not suitable for the application. When
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the application calls the <constant>VIDIOC_S_FMT</constant> ioctl
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with a pointer to a <structname>v4l2_format</structname> structure
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the driver checks
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and adjusts the parameters against hardware abilities. Drivers
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should not return an error code unless the input is ambiguous, this is
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a mechanism to fathom device capabilities and to approach parameters
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acceptable for both the application and driver. On success the driver
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may program the hardware, allocate resources and generally prepare for
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data exchange.
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Finally the <constant>VIDIOC_S_FMT</constant> ioctl returns the
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current format parameters as <constant>VIDIOC_G_FMT</constant> does.
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Very simple, inflexible devices may even ignore all input and always
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return the default parameters. However all V4L2 devices exchanging
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data with the application must implement the
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<constant>VIDIOC_G_FMT</constant> and
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<constant>VIDIOC_S_FMT</constant> ioctl. When the requested buffer
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type is not supported drivers return an &EINVAL; on a
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<constant>VIDIOC_S_FMT</constant> attempt. When I/O is already in
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progress or the resource is not available for other reasons drivers
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return the &EBUSY;.</para>
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<para>The <constant>VIDIOC_TRY_FMT</constant> ioctl is equivalent
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to <constant>VIDIOC_S_FMT</constant> with one exception: it does not
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change driver state. It can also be called at any time, never
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returning <errorcode>EBUSY</errorcode>. This function is provided to
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negotiate parameters, to learn about hardware limitations, without
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disabling I/O or possibly time consuming hardware preparations.
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Although strongly recommended drivers are not required to implement
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this ioctl.</para>
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<table pgwide="1" frame="none" id="v4l2-format">
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<title>struct <structname>v4l2_format</structname></title>
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<tgroup cols="4">
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<colspec colname="c1" />
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<colspec colname="c2" />
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<colspec colname="c3" />
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<colspec colname="c4" />
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<tbody valign="top">
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<row>
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<entry>&v4l2-buf-type;</entry>
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<entry><structfield>type</structfield></entry>
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<entry></entry>
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<entry>Type of the data stream, see <xref
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linkend="v4l2-buf-type" />.</entry>
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</row>
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<row>
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<entry>union</entry>
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<entry><structfield>fmt</structfield></entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-pix-format;</entry>
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<entry><structfield>pix</structfield></entry>
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<entry>Definition of an image format, see <xref
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linkend="pixfmt" />, used by video capture and output
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devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-window;</entry>
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<entry><structfield>win</structfield></entry>
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<entry>Definition of an overlaid image, see <xref
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linkend="overlay" />, used by video overlay devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-vbi-format;</entry>
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<entry><structfield>vbi</structfield></entry>
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<entry>Raw VBI capture or output parameters. This is
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discussed in more detail in <xref linkend="raw-vbi" />. Used by raw VBI
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capture and output devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>&v4l2-sliced-vbi-format;</entry>
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<entry><structfield>sliced</structfield></entry>
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<entry>Sliced VBI capture or output parameters. See
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<xref linkend="sliced" /> for details. Used by sliced VBI
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capture and output devices.</entry>
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</row>
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<row>
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<entry></entry>
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<entry>__u8</entry>
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<entry><structfield>raw_data</structfield>[200]</entry>
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<entry>Place holder for future extensions and custom
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(driver defined) formats with <structfield>type</structfield>
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<constant>V4L2_BUF_TYPE_PRIVATE</constant> and higher.</entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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</refsect1>
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<refsect1>
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&return-value;
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<variablelist>
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<varlistentry>
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<term><errorcode>EBUSY</errorcode></term>
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<listitem>
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<para>The data format cannot be changed at this
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time, for example because I/O is already in progress.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><errorcode>EINVAL</errorcode></term>
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<listitem>
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<para>The &v4l2-format; <structfield>type</structfield>
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field is invalid, the requested buffer type not supported, or
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<constant>VIDIOC_TRY_FMT</constant> was called and is not
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supported with this buffer type.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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</refentry>
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<!--
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Local Variables:
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mode: sgml
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sgml-parent-document: "v4l2.sgml"
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indent-tabs-mode: nil
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End:
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-->
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