CooCenter-System-kernel/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>

    <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> or <link
linkend="dmabuf">DMA buffer</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 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
<constant>VIDIOC_TRY_FMT</constant> or <constant>VIDIOC_G_FMT</constant> ioctl()
to ensure that the requested format is supported by the driver. Unsupported
formats will result in an error.</para>

    <para>The buffers created by this ioctl will have as minimum size the size
defined by the <structfield>format.pix.sizeimage</structfield> field. If the
<structfield>format.pix.sizeimage</structfield> field is less than the minimum
required for the given format, then <structfield>sizeimage</structfield> will be
increased by the driver to that minimum to allocate the buffers. If it is
larger, then the value will be used as-is. The same applies to the
<structfield>sizeimage</structfield> field of the
<structname>v4l2_plane_pix_format</structname> structure in the case of
multiplanar formats.</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. 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>