linux/Documentation/DocBook/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> or <link linkend="userp">user pointer</link>
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.</para>

    <para>To allocate device buffers applications initialize three
fields of a <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, and <structfield>memory</structfield>
must be set to <constant>V4L2_MEMORY_MMAP</constant>. When the ioctl
is called with a pointer to this structure the driver attempts to
allocate the requested number of buffers and 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 possible when the driver requires
more buffers to function correctly.<footnote>
	<para>For example video output requires at least two buffers,
one displayed and one filled by the application.</para>
	</footnote> When memory mapping I/O 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>

    <para>To negotiate user pointer I/O, applications initialize only
the <structfield>type</structfield> field and set
<structfield>memory</structfield> to
<constant>V4L2_MEMORY_USERPTR</constant>. When the ioctl is called
with a pointer to this structure the driver prepares for user pointer
I/O, when this I/O method is not supported the ioctl returns an
&EINVAL;.</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. This
field is only used when <structfield>memory</structfield> is set to
<constant>V4L2_MEMORY_MMAP</constant>.</entry>
	  </row>
	  <row>
	    <entry>&v4l2-buf-type;</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>&v4l2-memory;</entry>
	    <entry><structfield>memory</structfield></entry>
	    <entry>Applications set this field to
<constant>V4L2_MEMORY_MMAP</constant> or
<constant>V4L2_MEMORY_USERPTR</constant>.</entry>
	  </row>
	  <row>
	    <entry>__u32</entry>
	    <entry><structfield>reserved</structfield>[2]</entry>
	    <entry>A place holder for future extensions and custom
(driver defined) buffer types <constant>V4L2_BUF_TYPE_PRIVATE</constant> and
higher.</entry>
	  </row>
	</tbody>
      </tgroup>
    </table>
  </refsect1>

  <refsect1>
    &return-value;

    <variablelist>
      <varlistentry>
	<term><errorcode>EBUSY</errorcode></term>
	<listitem>
	  <para>The driver supports multiple opens and I/O is already
in progress, or reallocation of buffers was attempted although one or
more are still mapped.</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>

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