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[deliverable/linux.git] / Documentation / DocBook / media / v4l / vidioc-create-bufs.xml

<refentry id="vidioc-create-bufs">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_CREATE_BUFS</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_CREATE_BUFS</refname>
    <refpurpose>Create buffers for Memory Mapped or User Pointer I/O</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_create_buffers *<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_CREATE_BUFS</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>This ioctl is used to create buffers for <link linkend="mmap">memory
mapped</link> or <link linkend="userp">user pointer</link>
I/O. It can be used as an alternative or in addition to the
<constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter control over buffers
is required. This ioctl can be called multiple times to create buffers of
different sizes.</para>

    <para>To allocate device buffers applications initialize relevant fields of
the <structname>v4l2_create_buffers</structname> structure. They set the
<structfield>type</structfield> field in the
<structname>v4l2_format</structname> structure, embedded in this
structure, to the respective stream or buffer type.
<structfield>count</structfield> must be set to the number of required buffers.
<structfield>memory</structfield> specifies the required I/O method. The
<structfield>format</structfield> field shall typically be filled in using
either the <constant>VIDIOC_TRY_FMT</constant> or
<constant>VIDIOC_G_FMT</constant> ioctl(). Additionally, applications can adjust
<structfield>sizeimage</structfield> fields to fit their specific needs. The
<structfield>reserved</structfield> array must be zeroed.</para>

    <para>When the ioctl is called with a pointer to this structure the driver
will attempt to allocate up to the requested number of buffers and store the
actual number allocated and the starting index in the
<structfield>count</structfield> and the <structfield>index</structfield> fields
respectively. On return <structfield>count</structfield> can be smaller than
the number requested. The driver may also increase buffer sizes if required,
however, it will not update <structfield>sizeimage</structfield> field values.
The user has to use <constant>VIDIOC_QUERYBUF</constant> to retrieve that
information.</para>

    <table pgwide="1" frame="none" id="v4l2-create-buffers">
      <title>struct <structname>v4l2_create_buffers</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>index</structfield></entry>
	    <entry>The starting buffer index, returned by the driver.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>count</structfield></entry>
	    <entry>The number of buffers requested or granted.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>memory</structfield></entry>
	    <entry>Applications set this field to
<constant>V4L2_MEMORY_MMAP</constant> or
<constant>V4L2_MEMORY_USERPTR</constant>. See <xref linkend="v4l2-memory"
/></entry>
	  </row>
	  <row>
	    <entry>struct&nbsp;v4l2_format</entry>
	    <entry><structfield>format</structfield></entry>
	    <entry>Filled in by the application, preserved by the driver.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[8]</entry>
	    <entry>A place holder for future extensions.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <varlistentry>
	<term><errorcode>ENOMEM</errorcode></term>
	<listitem>
	  <para>No memory to allocate buffers for <link linkend="mmap">memory
mapped</link> I/O.</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><errorcode>EINVAL</errorcode></term>
	<listitem>
	  <para>The buffer type (<structfield>type</structfield> field) or the
requested I/O method (<structfield>memory</structfield>) is not
supported.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>