linux-2.6/Documentation/DocBook/media/v4l/vidioc-reqbufs.xml

<refentry id="vidioc-reqbufs">
  <refmeta>
    <refentrytitle>ioctl VIDIOC_REQBUFS</refentrytitle>
    &manvol;
  </refmeta>

  <refnamediv>
    <refname>VIDIOC_REQBUFS</refname>
    <refpurpose>Initiate Memory Mapping 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_requestbuffers *<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_REQBUFS</para>
	</listitem>
      </varlistentry>
      <varlistentry>
	<term><parameter>argp</parameter></term>
	<listitem>
	  <para></para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1>
    <title>Description</title>

<para>This ioctl is used to initiate <link linkend="mmap">memory mapped</link>,
<link linkend="userp">user pointer</link> or <link
linkend="dmabuf">DMABUF</link> based I/O.  Memory mapped buffers are located in
device memory and must be allocated with this ioctl before they can be mapped
into the application's address space. User buffers are allocated by
applications themselves, and this ioctl is merely used to switch the driver
into user pointer I/O mode and to setup some internal structures.
Similarly, DMABUF buffers are allocated by applications through a device
driver, and this ioctl only configures the driver into DMABUF I/O mode without
performing any direct allocation.</para>

    <para>To allocate device buffers applications initialize all fields of the
<structname>v4l2_requestbuffers</structname> structure.  They set the
<structfield>type</structfield> field to the respective stream or buffer type,
the <structfield>count</structfield> field to the desired number of buffers,
<structfield>memory</structfield> must be set to the requested I/O method and
the <structfield>reserved</structfield> array must be zeroed. When the ioctl is
called with a pointer to this structure the driver will attempt to allocate the
requested number of buffers and it stores the actual number allocated in the
<structfield>count</structfield> field. It can be smaller than the number
requested, even zero, when the driver runs out of free memory. A larger number
is also possible when the driver requires more buffers to function correctly.
For example video output requires at least two buffers, one displayed and one
filled by the application.</para>
    <para>When the I/O method is not supported the ioctl
returns an &EINVAL;.</para>

    <para>Applications can call <constant>VIDIOC_REQBUFS</constant>
again to change the number of buffers, however this cannot succeed
when any buffers are still mapped. A <structfield>count</structfield>
value of zero frees all buffers, after aborting or finishing any DMA
in progress, an implicit &VIDIOC-STREAMOFF;. <!-- mhs: I see no
reason why munmap()ping one or even all buffers must imply
streamoff.--></para>

    <table pgwide="1" frame="none" id="v4l2-requestbuffers">
      <title>struct <structname>v4l2_requestbuffers</structname></title>
      <tgroup cols="3">
	&cs-str;
	<tbody valign="top">
	  <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>type</structfield></entry>
	    <entry>Type of the stream or buffers, this is the same
as the &v4l2-format; <structfield>type</structfield> field. See <xref
		linkend="v4l2-buf-type" /> for valid values.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>memory</structfield></entry>
	    <entry>Applications set this field to
<constant>V4L2_MEMORY_MMAP</constant>,
<constant>V4L2_MEMORY_DMABUF</constant> or
<constant>V4L2_MEMORY_USERPTR</constant>. See <xref linkend="v4l2-memory"
/>.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[2]</entry>
	    <entry>A place holder for future extensions. This array should
be zeroed by applications.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <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>