[PATCHv8 17/26] Documentation: media: description of DMABUF exporting in V4L2
Hans Verkuil
hverkuil at xs4all.nl
Wed Aug 22 04:23:52 PDT 2012
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 at samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park at samsung.com>
> CC: linux-doc at 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(&buf, 0, sizeof buf);
> + buf.type = bt;
> + buf.memory = V4L2_MEMORY_MMAP;
> + buf.index = index;
> +
> + if (ioctl (v4lfd, &VIDIOC-QUERYBUF;, &buf) == -1) {
> + perror ("VIDIOC_QUERYBUF");
> + return -1;
> + }
> +
> + memset(&expbuf, 0, sizeof expbuf);
> + expbuf.mem_offset = buf.m.offset;
> + if (ioctl (v4lfd, &VIDIOC-EXPBUF;, &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(&buf, 0, sizeof buf);
> + buf.type = bt;
> + buf.memory = V4L2_MEMORY_MMAP;
> + buf.index = index;
> + buf.m.planes = planes;
> + buf.length = n_planes;
> + memset(&planes, 0, sizeof planes);
> +
> + if (ioctl (v4lfd, &VIDIOC-QUERYBUF;, &buf) == -1) {
> + perror ("VIDIOC_QUERYBUF");
> + return -1;
> + }
> +
> + for (i = 0; i < n_planes; ++i) {
> + &v4l2-exportbuffer; expbuf;
> +
> + memset(&expbuf, 0, sizeof expbuf);
> + expbuf.mem_offset = plane[i].m.offset;
> + if (ioctl (v4lfd, &VIDIOC-EXPBUF;, &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
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