[media] Add multi-planar API documentation
Add DocBook documentation for the new multi-planar API extensions to the
Video for Linux 2 API DocBook.
Signed-off-by: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/Documentation/DocBook/v4l/planar-apis.xml b/Documentation/DocBook/v4l/planar-apis.xml
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+<section id="planar-apis">
+ <title>Single- and multi-planar APIs</title>
+
+ <para>Some devices require data for each input or output video frame
+ to be placed in discontiguous memory buffers. In such cases one
+ video frame has to be addressed using more than one memory address, i.e. one
+ pointer per "plane". A plane is a sub-buffer of current frame. For examples
+ of such formats see <xref linkend="pixfmt" />.</para>
+
+ <para>Initially, V4L2 API did not support multi-planar buffers and a set of
+ extensions has been introduced to handle them. Those extensions constitute
+ what is being referred to as the "multi-planar API".</para>
+
+ <para>Some of the V4L2 API calls and structures are interpreted differently,
+ depending on whether single- or multi-planar API is being used. An application
+ can choose whether to use one or the other by passing a corresponding buffer
+ type to its ioctl calls. Multi-planar versions of buffer types are suffixed with
+ an `_MPLANE' string. For a list of available multi-planar buffer types
+ see &v4l2-buf-type;.
+ </para>
+
+ <section>
+ <title>Multi-planar formats</title>
+ <para>Multi-planar API introduces new multi-planar formats. Those formats
+ use a separate set of FourCC codes. It is important to distinguish between
+ the multi-planar API and a multi-planar format. Multi-planar API calls can
+ handle all single-planar formats as well, while the single-planar API cannot
+ handle multi-planar formats. Applications do not have to switch between APIs
+ when handling both single- and multi-planar devices and should use the
+ multi-planar API version for both single- and multi-planar formats.
+ Drivers that do not support multi-planar API can still be handled with it,
+ utilizing a compatibility layer built into standard V4L2 ioctl handling.
+ </para>
+ </section>
+
+ <section>
+ <title>Single and multi-planar API compatibility layer</title>
+ <para>In most cases<footnote><para>The compatibility layer does not cover
+ drivers that do not use video_ioctl2() call.</para></footnote>, applications
+ can use the multi-planar API with older drivers that support
+ only its single-planar version and vice versa. Appropriate conversion is
+ done seamlessly for both applications and drivers in the V4L2 core. The
+ general rule of thumb is: as long as an application uses formats that
+ a driver supports, it can use either API (although use of multi-planar
+ formats is only possible with the multi-planar API). The list of formats
+ supported by a driver can be obtained using the &VIDIOC-ENUM-FMT; call.
+ It is possible, but discouraged, for a driver or an application to support
+ and use both versions of the API.</para>
+ </section>
+
+ <section>
+ <title>Calls that distinguish between single and multi-planar APIs</title>
+ <variablelist>
+ <varlistentry>
+ <term>&VIDIOC-QUERYCAP;</term>
+ <listitem>Two additional multi-planar capabilities are added. They can
+ be set together with non-multi-planar ones for devices that handle
+ both single- and multi-planar formats.</listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>&VIDIOC-G-FMT;, &VIDIOC-S-FMT;, &VIDIOC-TRY-FMT;</term>
+ <listitem>New structures for describing multi-planar formats are added:
+ &v4l2-pix-format-mplane; and &v4l2-plane-pix-format;. Drivers may
+ define new multi-planar formats, which have distinct FourCC codes from
+ the existing single-planar ones.
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>&VIDIOC-QBUF;, &VIDIOC-DQBUF;, &VIDIOC-QUERYBUF;</term>
+ <listitem>A new &v4l2-plane; structure for describing planes is added.
+ Arrays of this structure are passed in the new
+ <structfield>m.planes</structfield> field of &v4l2-buffer;.
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>&VIDIOC-REQBUFS;</term>
+ <listitem>Will allocate multi-planar buffers as requested.</listitem>
+ </varlistentry>
+ </variablelist>
+ </section>
+</section>