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6. vidioc-decoder-cmd.xml

<refentry id="vidioc-decoder-cmd">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_DECODER_CMD, VIDIOC_TRY_DECODER_CMD</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_DECODER_CMD</refname>
    <refname>VIDIOC_TRY_DECODER_CMD</refname>
    <refpurpose>Execute an decoder command</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_decoder_cmd *<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_DECODER_CMD, VIDIOC_TRY_DECODER_CMD</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <para>These ioctls control an audio/video (usually MPEG-) decoder.
<constant>VIDIOC_DECODER_CMD</constant> sends a command to the
decoder, <constant>VIDIOC_TRY_DECODER_CMD</constant> can be used to
try a command without actually executing it. To send a command applications
must initialize all fields of a &v4l2-decoder-cmd; and call
<constant>VIDIOC_DECODER_CMD</constant> or <constant>VIDIOC_TRY_DECODER_CMD</constant>
with a pointer to this structure.</para>

    <para>The <structfield>cmd</structfield> field must contain the
command code. Some commands use the <structfield>flags</structfield> field for
additional information.
</para>

    <para>A <function>write</function>() or &VIDIOC-STREAMON; call sends an implicit
START command to the decoder if it has not been started yet.
</para>

    <para>A <function>close</function>() or &VIDIOC-STREAMOFF; call of a streaming
file descriptor sends an implicit immediate STOP command to the decoder, and all
buffered data is discarded.</para>

    <para>These ioctls are optional, not all drivers may support
them. They were introduced in Linux 3.3.</para>

    <table pgwide="1" frame="none" id="v4l2-decoder-cmd">
      <title>struct <structname>v4l2_decoder_cmd</structname></title>
      <tgroup cols="5">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>cmd</structfield></entry>
            <entry></entry>
            <entry></entry>
	    <entry>The decoder command, see <xref linkend="decoder-cmds" />.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>flags</structfield></entry>
            <entry></entry>
            <entry></entry>
	    <entry>Flags to go with the command. If no flags are defined for
this command, drivers and applications must set this field to zero.</entry>
	  </row>
	  <row>
	    <entry>union</entry>
	    <entry>(anonymous)</entry>
            <entry></entry>
	    <entry></entry>
            <entry></entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>struct</entry>
            <entry><structfield>start</structfield></entry>
            <entry></entry>
            <entry>Structure containing additional data for the
<constant>V4L2_DEC_CMD_START</constant> command.</entry>
	  </row>
	  <row>
            <entry></entry>
            <entry></entry>
	    <entry>__s32</entry>
	    <entry><structfield>speed</structfield></entry>
            <entry>Playback speed and direction. The playback speed is defined as
<structfield>speed</structfield>/1000 of the normal speed. So 1000 is normal playback.
Negative numbers denote reverse playback, so -1000 does reverse playback at normal
speed. Speeds -1, 0 and 1 have special meanings: speed 0 is shorthand for 1000
(normal playback). A speed of 1 steps just one frame forward, a speed of -1 steps
just one frame back.
	    </entry>
	  </row>
	  <row>
            <entry></entry>
            <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>format</structfield></entry>
            <entry>Format restrictions. This field is set by the driver, not the
application. Possible values are <constant>V4L2_DEC_START_FMT_NONE</constant> if
there are no format restrictions or <constant>V4L2_DEC_START_FMT_GOP</constant>
if the decoder operates on full GOPs (<wordasword>Group Of Pictures</wordasword>).
This is usually the case for reverse playback: the decoder needs full GOPs, which
it can then play in reverse order. So to implement reverse playback the application
must feed the decoder the last GOP in the video file, then the GOP before that, etc. etc.
	    </entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>struct</entry>
            <entry><structfield>stop</structfield></entry>
            <entry></entry>
            <entry>Structure containing additional data for the
<constant>V4L2_DEC_CMD_STOP</constant> command.</entry>
	  </row>
	  <row>
            <entry></entry>
            <entry></entry>
	    <entry>__u64</entry>
	    <entry><structfield>pts</structfield></entry>
            <entry>Stop playback at this <structfield>pts</structfield> or immediately
if the playback is already past that timestamp. Leave to 0 if you want to stop after the
last frame was decoded.
	    </entry>
	  </row>
	  <row>
	    <entry></entry>
	    <entry>struct</entry>
            <entry><structfield>raw</structfield></entry>
            <entry></entry>
            <entry></entry>
	  </row>
	  <row>
            <entry></entry>
            <entry></entry>
	    <entry>__u32</entry>
	    <entry><structfield>data</structfield>[16]</entry>
	    <entry>Reserved for future extensions. Drivers and
applications must set the array to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="decoder-cmds">
      <title>Decoder Commands</title>
      <tgroup cols="3">
	&cs-def;
	<tbody valign="top">
	  <row>
	    <entry><constant>V4L2_DEC_CMD_START</constant></entry>
	    <entry>0</entry>
	    <entry>Start the decoder. When the decoder is already
running or paused, this command will just change the playback speed.
That means that calling <constant>V4L2_DEC_CMD_START</constant> when
the decoder was paused will <emphasis>not</emphasis> resume the decoder.
You have to explicitly call <constant>V4L2_DEC_CMD_RESUME</constant> for that.
This command has one flag:
<constant>V4L2_DEC_CMD_START_MUTE_AUDIO</constant>. If set, then audio will
be muted when playing back at a non-standard speed.
            </entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_DEC_CMD_STOP</constant></entry>
	    <entry>1</entry>
	    <entry>Stop the decoder. When the decoder is already stopped,
this command does nothing. This command has two flags:
if <constant>V4L2_DEC_CMD_STOP_TO_BLACK</constant> is set, then the decoder will
set the picture to black after it stopped decoding. Otherwise the last image will
repeat. mem2mem decoders will stop producing new frames altogether. They will send
a <constant>V4L2_EVENT_EOS</constant> event when the last frame has been decoded
and all frames are ready to be dequeued and will set the
<constant>V4L2_BUF_FLAG_LAST</constant> buffer flag on the last buffer of the
capture queue to indicate there will be no new buffers produced to dequeue. This
buffer may be empty, indicated by the driver setting the
<structfield>bytesused</structfield> field to 0. Once the
<constant>V4L2_BUF_FLAG_LAST</constant> flag was set, the
<link linkend="vidioc-qbuf">VIDIOC_DQBUF</link> ioctl will not block anymore,
but return an &EPIPE;.
If <constant>V4L2_DEC_CMD_STOP_IMMEDIATELY</constant> is set, then the decoder
stops immediately (ignoring the <structfield>pts</structfield> value), otherwise it
will keep decoding until timestamp >= pts or until the last of the pending data from
its internal buffers was decoded.
</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_DEC_CMD_PAUSE</constant></entry>
	    <entry>2</entry>
	    <entry>Pause the decoder. When the decoder has not been
started yet, the driver will return an &EPERM;. When the decoder is
already paused, this command does nothing. This command has one flag:
if <constant>V4L2_DEC_CMD_PAUSE_TO_BLACK</constant> is set, then set the
decoder output to black when paused.
</entry>
	  </row>
	  <row>
	    <entry><constant>V4L2_DEC_CMD_RESUME</constant></entry>
	    <entry>3</entry>
	    <entry>Resume decoding after a PAUSE command. When the
decoder has not been started yet, the driver will return an &EPERM;.
When the decoder is already running, this command does nothing. No
flags are defined for this command.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	  <para>The <structfield>cmd</structfield> field is invalid.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EPERM</errorcode></term>
	<listitem>
	  <para>The application sent a PAUSE or RESUME command when
the decoder was not running.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>