linux/Documentation/DocBook/v4l/vidioc-subdev-g-fmt.xml

<refentry id="vidioc-subdev-g-fmt">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_SUBDEV_G_FMT</refname>
    <refname>VIDIOC_SUBDEV_S_FMT</refname>
    <refpurpose>Get or set the data format on a subdev pad</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_subdev_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_SUBDEV_G_FMT, VIDIOC_SUBDEV_S_FMT</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

    <note>
      <title>Experimental</title>
      <para>This is an <link linkend="experimental">experimental</link>
      interface and may change in the future.</para>
    </note>

    <para>These ioctls are used to negotiate the frame format at specific
    subdev pads in the image pipeline.</para>

    <para>To retrieve the current format applications set the
    <structfield>pad</structfield> field of a &v4l2-subdev-format; to the
    desired pad number as reported by the media API and the
    <structfield>which</structfield> field to
    <constant>V4L2_SUBDEV_FORMAT_ACTIVE</constant>. When they call the
    <constant>VIDIOC_SUBDEV_G_FMT</constant> ioctl with a pointer to this
    structure the driver fills the members of the <structfield>format</structfield>
    field.</para>

    <para>To change the current format applications set both the
    <structfield>pad</structfield> and <structfield>which</structfield> fields
    and all members of the <structfield>format</structfield> field. When they
    call the <constant>VIDIOC_SUBDEV_S_FMT</constant> ioctl with a pointer to this
    structure the driver verifies the requested format, adjusts it based on the
    hardware capabilities and configures the device. Upon return the
    &v4l2-subdev-format; contains the current format as would be returned by a
    <constant>VIDIOC_SUBDEV_G_FMT</constant> call.</para>

    <para>Applications can query the device capabilities by setting the
    <structfield>which</structfield> to
    <constant>V4L2_SUBDEV_FORMAT_TRY</constant>. When set, 'try' formats are not
    applied to the device by the driver, but are changed exactly as active
    formats and stored in the sub-device file handle. Two applications querying
    the same sub-device would thus not interact with each other.</para>

    <para>For instance, to try a format at the output pad of a sub-device,
    applications would first set the try format at the sub-device input with the
    <constant>VIDIOC_SUBDEV_S_FMT</constant> ioctl. They would then either
    retrieve the default format at the output pad with the
    <constant>VIDIOC_SUBDEV_G_FMT</constant> ioctl, or set the desired output
    pad format with the <constant>VIDIOC_SUBDEV_S_FMT</constant> ioctl and check
    the returned value.</para>

    <para>Try formats do not depend on active formats, but can depend on the
    current links configuration or sub-device controls value. For instance, a
    low-pass noise filter might crop pixels at the frame boundaries, modifying
    its output frame size.</para>

    <para>Drivers must not return an error solely because the requested format
    doesn't match the device capabilities. They must instead modify the format
    to match what the hardware can provide. The modified format should be as
    close as possible to the original request.</para>

    <table pgwide="1" frame="none" id="v4l2-subdev-format">
      <title>struct <structname>v4l2_subdev_format</structname></title>
      <tgroup cols="3">
        &cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>pad</structfield></entry>
	    <entry>Pad number as reported by the media controller API.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>which</structfield></entry>
	    <entry>Format to modified, from &v4l2-subdev-format-whence;.</entry>
	  </row>
	  <row>
	    <entry>&v4l2-mbus-framefmt;</entry>
	    <entry><structfield>format</structfield></entry>
	    <entry>Definition of an image format, see <xref
	    linkend="v4l2-mbus-framefmt" /> for details.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[8]</entry>
	    <entry>Reserved for future extensions. Applications and drivers must
	    set the array to zero.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>

    <table pgwide="1" frame="none" id="v4l2-subdev-format-whence">
      <title>enum <structname>v4l2_subdev_format_whence</structname></title>
      <tgroup cols="3">
        &cs-def;
	<tbody valign="top">
	  <row>
	    <entry>V4L2_SUBDEV_FORMAT_TRY</entry>
	    <entry>0</entry>
	    <entry>Try formats, used for querying device capabilities.</entry>
	  </row>
	  <row>
	    <entry>V4L2_SUBDEV_FORMAT_ACTIVE</entry>
	    <entry>1</entry>
	    <entry>Active formats, applied to the hardware.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <varlistentry>
	<term><errorcode>EBUSY</errorcode></term>
	<listitem>
	  <para>The format can't be changed because the pad is currently busy.
	  This can be caused, for instance, by an active video stream on the
	  pad. The ioctl must not be retried without performing another action
	  to fix the problem first. Only returned by
	  <constant>VIDIOC_SUBDEV_S_FMT</constant></para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	  <para>The &v4l2-subdev-format; <structfield>pad</structfield>
	  references a non-existing pad, or the <structfield>which</structfield>
	  field references a non-existing format.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>