media: docs-rst: Document memory-to-memory video encoder interface

Due to complexity of the video encoding process, the V4L2 drivers of
stateful encoder hardware require specific sequences of V4L2 API calls
to be followed. These include capability enumeration, initialization,
encoding, encode parameters change, drain and reset.

Specifics of the above have been discussed during Media Workshops at
LinuxCon Europe 2012 in Barcelona and then later Embedded Linux
Conference Europe 2014 in Düsseldorf. The de facto Codec API that
originated at those events was later implemented by the drivers we already
have merged in mainline, such as s5p-mfc or coda.

The only thing missing was the real specification included as a part of
Linux Media documentation. Fix it now and document the encoder part of
the Codec API.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst b/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst
index 759420a872d6..e0ee2823ab1f 100644
--- a/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst
+++ b/Documentation/userspace-api/media/v4l/pixfmt-v4l2.rst
@@ -44,6 +44,11 @@ Single-planar format structure
 	inside the stream, when fed to a stateful mem2mem decoder, the fields
 	may be zero to rely on the decoder to detect the right values. For more
 	details see :ref:`decoder` and format descriptions.
+
+	For compressed formats on the CAPTURE side of a stateful mem2mem
+	encoder, the fields must be zero, since the coded size is expected to
+	be calculated internally by the encoder itself, based on the OUTPUT
+	side. For more details see :ref:`encoder` and format descriptions.
     * - __u32
       - ``pixelformat``
       - The pixel format or type of compression, set by the application.