linux/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 or DMA Buffer
    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>

    <para>This ioctl is used to create buffers for <link linkend="mmap">memory
mapped</link> or <link linkend="userp">user pointer</link> or <link
linkend="dmabuf">DMA buffer</link> I/O. It can be used as an alternative or in
addition to the &VIDIOC-REQBUFS; 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 the device buffers applications must initialize the
relevant fields of the <structname>v4l2_create_buffers</structname> structure.
The <structfield>count</structfield> field must be set to the number of
requested buffers, the <structfield>memory</structfield> field specifies the
requested I/O method and the <structfield>reserved</structfield> array must be
zeroed.</para>

    <para>The <structfield>format</structfield> field specifies the image format
that the buffers must be able to handle. The application has to fill in this
&v4l2-format;. Usually this will be done using the &VIDIOC-TRY-FMT; or &VIDIOC-G-FMT; ioctls
to ensure that the requested format is supported by the driver.
Based on the format's <structfield>type</structfield> field the requested buffer
size (for single-planar) or plane sizes (for multi-planar formats) will be
used for the allocated buffers. The driver may return an error if the size(s)
are not supported by the hardware (usually because they are too small).</para>

    <para>The buffers created by this ioctl will have as minimum size the size
defined by the <structfield>format.pix.sizeimage</structfield> field (or the
corresponding fields for other format types). Usually if the
<structfield>format.pix.sizeimage</structfield> field is less than the minimum
required for the given format, then an error will be returned since drivers will
typically not allow this. If it is larger, then the value will be used as-is.
In other words, the driver may reject the requested size, but if it is accepted
the driver will use it unchanged.</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.</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. If count == 0, then
	    <constant>VIDIOC_CREATE_BUFS</constant> will set <structfield>index</structfield>
	    to the current number of created buffers, and it will check the validity of
	    <structfield>memory</structfield> and <structfield>format.type</structfield>.
	    If those are invalid -1 is returned and errno is set to &EINVAL;,
	    otherwise <constant>VIDIOC_CREATE_BUFS</constant> returns 0. It will
	    never set errno to &EBUSY; in this particular case.</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>&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. Drivers and applications
must set the array to zero.</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>format.type</structfield> field),
requested I/O method (<structfield>memory</structfield>) or format
(<structfield>format</structfield> field) is not valid.</para>
	</listitem>
      </varlistentry>
    </variablelist>
  </refsect1>
</refentry>