[PATCHv8,17/26] Documentation: media: description of DMABUF exporting in V4L2

This patch adds description and usage examples for exporting
DMABUF file descriptor in V4L2.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: linux-doc@vger.kernel.org
---
 Documentation/DocBook/media/v4l/compat.xml        |    3 +
 Documentation/DocBook/media/v4l/io.xml            |    3 +
 Documentation/DocBook/media/v4l/v4l2.xml          |    1 +
 Documentation/DocBook/media/v4l/vidioc-expbuf.xml |  223 +++++++++++++++++++++
 4 files changed, 230 insertions(+)
 create mode 100644 Documentation/DocBook/media/v4l/vidioc-expbuf.xml

On Tue August 14 2012 17:34:47 Tomasz Stanislawski wrote:
> This patch adds description and usage examples for exporting
> DMABUF file descriptor in V4L2.
> 
> Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> CC: linux-doc@vger.kernel.org
> ---
>  Documentation/DocBook/media/v4l/compat.xml        |    3 +
>  Documentation/DocBook/media/v4l/io.xml            |    3 +
>  Documentation/DocBook/media/v4l/v4l2.xml          |    1 +
>  Documentation/DocBook/media/v4l/vidioc-expbuf.xml |  223 +++++++++++++++++++++
>  4 files changed, 230 insertions(+)
>  create mode 100644 Documentation/DocBook/media/v4l/vidioc-expbuf.xml
> 
> diff --git a/Documentation/DocBook/media/v4l/compat.xml b/Documentation/DocBook/media/v4l/compat.xml
> index ff45330..802c1ab 100644
> --- a/Documentation/DocBook/media/v4l/compat.xml
> +++ b/Documentation/DocBook/media/v4l/compat.xml
> @@ -2609,6 +2609,9 @@ ioctls.</para>
>  	  <para>Importing DMABUF file descriptors as a new IO method described
>  	  in <xref linkend="dmabuf" />.</para>
>          </listitem>
> +        <listitem>
> +	  <para>Exporting DMABUF files using &VIDIOC-EXPBUF; ioctl.</para>
> +        </listitem>
>        </itemizedlist>
>      </section>
>  
> diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
> index 98253ee..c27e59b 100644
> --- a/Documentation/DocBook/media/v4l/io.xml
> +++ b/Documentation/DocBook/media/v4l/io.xml
> @@ -488,6 +488,9 @@ buffer from userspace using a file descriptor previously exported for a
>  different or the same device (known as the importer role), or both. This
>  section describes the DMABUF importer role API in V4L2.</para>
>  
> +    <para>Refer to <link linked="vidioc-expbuf"> DMABUF exporting </link> for
> +details about exporting a V4L2 buffers as DMABUF file descriptors.</para>
> +
>  <para>Input and output devices support the streaming I/O method when the
>  <constant>V4L2_CAP_STREAMING</constant> flag in the
>  <structfield>capabilities</structfield> field of &v4l2-capability; returned by
> diff --git a/Documentation/DocBook/media/v4l/v4l2.xml b/Documentation/DocBook/media/v4l/v4l2.xml
> index 0292ed1..874c085 100644
> --- a/Documentation/DocBook/media/v4l/v4l2.xml
> +++ b/Documentation/DocBook/media/v4l/v4l2.xml
> @@ -568,6 +568,7 @@ and discussions on the V4L mailing list.</revremark>
>      &sub-overlay;
>      &sub-prepare-buf;
>      &sub-qbuf;
> +    &sub-expbuf;

This list is sorted alphabetically, so sub-expbuf should go after sub-enumstd.

>      &sub-querybuf;
>      &sub-querycap;
>      &sub-queryctrl;
> diff --git a/Documentation/DocBook/media/v4l/vidioc-expbuf.xml b/Documentation/DocBook/media/v4l/vidioc-expbuf.xml
> new file mode 100644
> index 0000000..30ebf67
> --- /dev/null
> +++ b/Documentation/DocBook/media/v4l/vidioc-expbuf.xml
> @@ -0,0 +1,223 @@
> +<refentry id="vidioc-expbuf">
> +
> +  <refmeta>
> +    <refentrytitle>ioctl VIDIOC_EXPBUF</refentrytitle>
> +    &manvol;
> +  </refmeta>
> +
> +  <refnamediv>
> +    <refname>VIDIOC_EXPBUF</refname>
> +    <refpurpose>Export a buffer as a DMABUF file descriptor.</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_exportbuffer *<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_EXPBUF</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 an extension to the <link linkend="mmap">memory
> +mapping</link> I/O method therefore it is available only for
> +<constant>V4L2_MEMORY_MMAP</constant> buffers.  It can be used to export a
> +buffer as DMABUF file at any time after buffers have been allocated with the
> +&VIDIOC-REQBUFS; ioctl.</para>
> +
> +<para>Prior to exporting an application calls <link
> +linkend="vidioc-querybuf">VIDIOC_QUERYBUF</link> to obtain memory offsets. When
> +using the <link linkend="planar-apis">multi-planar API</link> every plane has
> +own offset.</para>
> +
> +<para>To export a buffer, the application fills &v4l2-exportbuffer;.  The
> +<structfield> mem_offset </structfield> field is set to the offset obtained
> +from <constant> VIDIOC_QUERYBUF </constant>.  Additional flags may be posted in
> +the <structfield> flags </structfield> field.  Refer to manual for open syscall

Better IMHO: 'Refer to the manual for open()'

> +for details. Currently only O_CLOEXEC is guaranteed to be supported.  All other
> +fields must be set to zero.  In a case of multi-planar API, every plane is
> +exported separately using multiple <constant> VIDIOC_EXPBUF </constant>
> +calls.</para>
> +
> +<para> After calling <constant>VIDIOC_EXPBUF</constant> the <structfield> fd
> +</structfield> field will be set by a driver.  This is a DMABUF file
> +descriptor. The application may pass it to other API. Refer to <link
> +linkend="dmabuf">DMABUF importing</link> for details about importing DMABUF
> +files into V4L2 nodes. A developer is encouraged to close a DMABUF file when it
> +is no longer used.  </para>

Some explanation of why this is recommended would be useful.

> +
> +  </refsect1>
> +  <refsect1>
> +   <section>
> +      <title>Examples</title>
> +
> +      <example>
> +	<title>Exporting a buffer.</title>
> +	<programlisting>
> +int buffer_export(int v4lfd, &v4l2-buf-type; bt, int index, int *dmafd)
> +{
> +	&v4l2-buffer; buf;
> +	&v4l2-exportbuffer; expbuf;
> +
> +	memset(&amp;buf, 0, sizeof buf);
> +	buf.type = bt;
> +	buf.memory = V4L2_MEMORY_MMAP;
> +	buf.index = index;
> +
> +	if (ioctl (v4lfd, &VIDIOC-QUERYBUF;, &amp;buf) == -1) {
> +		perror ("VIDIOC_QUERYBUF");
> +		return -1;
> +	}
> +
> +	memset(&amp;expbuf, 0, sizeof expbuf);
> +	expbuf.mem_offset = buf.m.offset;
> +	if (ioctl (v4lfd, &VIDIOC-EXPBUF;, &amp;expbuf) == -1) {
> +		perror ("VIDIOC_EXPBUF");
> +		return -1;
> +	}

Same as in the review of the previous documentation patch: use the kernel
coding style in these examples, so no space before '('.

> +
> +	*dmafd = expbuf.fd;
> +
> +	return 0;
> +}
> +        </programlisting>
> +      </example>
> +
> +      <example>
> +	<title>Exporting a buffer using multi plane API.</title>
> +	<programlisting>
> +int buffer_export_mp(int v4lfd, &v4l2-buf-type; bt, int index,
> +	int dmafd[], int n_planes)
> +{
> +	&v4l2-buffer; buf;
> +	&v4l2-plane; planes[VIDEO_MAX_PLANES];
> +	int i;
> +
> +	memset(&amp;buf, 0, sizeof buf);
> +	buf.type = bt;
> +	buf.memory = V4L2_MEMORY_MMAP;
> +	buf.index = index;
> +	buf.m.planes = planes;
> +	buf.length = n_planes;
> +	memset(&amp;planes, 0, sizeof planes);
> +
> +	if (ioctl (v4lfd, &VIDIOC-QUERYBUF;, &amp;buf) == -1) {
> +		perror ("VIDIOC_QUERYBUF");
> +		return -1;
> +	}
> +
> +	for (i = 0; i &lt; n_planes; ++i) {
> +		&v4l2-exportbuffer; expbuf;
> +
> +		memset(&amp;expbuf, 0, sizeof expbuf);
> +		expbuf.mem_offset = plane[i].m.offset;
> +		if (ioctl (v4lfd, &VIDIOC-EXPBUF;, &amp;expbuf) == -1) {
> +			perror ("VIDIOC_EXPBUF");
> +			while (i)
> +				close(dmafd[--i]);
> +			return -1;
> +		}
> +		dmafd[i] = expbuf.fd;
> +	}
> +
> +	return 0;
> +}
> +        </programlisting>
> +      </example>
> +   </section>
> +  </refsect1>
> +
> +  <refsect1>
> +    <table pgwide="1" frame="none" id="v4l2-exportbuffer">
> +      <title>struct <structname>v4l2_exportbuffer</structname></title>
> +      <tgroup cols="3">
> +	&cs-str;
> +	<tbody valign="top">
> +	  <row>
> +	    <entry>__u32</entry>
> +	    <entry><structfield>fd</structfield></entry>
> +	    <entry>The DMABUF file descriptor associated with a buffer. Set by
> +		a driver.</entry>
> +	  </row>
> +	  <row>
> +	    <entry>__u32</entry>
> +	    <entry><structfield>reserved0</structfield></entry>
> +	    <entry>Reserved field for future use. Must be set to zero.</entry>
> +	  </row>
> +	  <row>
> +	    <entry>__u32</entry>
> +	    <entry><structfield>mem_offset</structfield></entry>
> +	    <entry>Buffer memory offset as returned by <constant>
> +VIDIOC_QUERYBUF </constant> in &v4l2-buffer;<structfield> ::m.offset
> +</structfield> (for single-plane formats) or &v4l2-plane;<structfield>
> +::m.offset </structfield> (for multi-planar formats)</entry>
> +	  </row>
> +	  <row>
> +	    <entry>__u32</entry>
> +	    <entry><structfield>flags</structfield></entry>
> +	    <entry>Flags for newly created file, currently only <constant>
> +O_CLOEXEC </constant> is supported, refer to manual of open syscall for more

'to the manual of open() for more'

> +details.</entry>
> +	  </row>
> +	  <row>
> +	    <entry>__u32</entry>
> +	    <entry><structfield>reserved[12]</structfield></entry>
> +	    <entry>Reserved field for future use. Must be set to zero.</entry>
> +	  </row>
> +	</tbody>
> +      </tgroup>
> +    </table>
> +
> +  </refsect1>
> +
> +  <refsect1>
> +    &return-value;
> +    <variablelist>
> +      <varlistentry>
> +	<term><errorcode>EINVAL</errorcode></term>
> +	<listitem>
> +	  <para>A queue is not in MMAP mode or DMABUF exporting is not
> +supported or <structfield> flag </structfield> or <structfield> mem_offset

flag -> flags

> +</structfield> fields are invalid.</para>
> +	</listitem>
> +      </varlistentry>
> +    </variablelist>
> +  </refsect1>
> +
> +</refentry>
> 

Regards,

	Hans