[PATCH v2 1/3] media: uapi: h264: Clarify our expectations regarding NAL header format

[Boris Brezillon <boris.brezillon@collabora.com>]

Looks like some stateless decoders expect slices to be prefixed with
ANNEX B start codes (they most likely do some kind of bitstream parsing
and/or need that to delimit slices when doing per frame decoding).
Since skipping those start codes for dummy stateless decoders (those
expecting all params to be passed through controls) should be pretty
easy, let's mandate that all slices be prepended with ANNEX B start
codes.

If we ever need to support AVC headers, we can add a new menu control
to select the type of NAL header to use.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
---
Changes in v2:
* None

Note: we might want to add a nal_header_size field to allow drivers to
quickly find where the actual payload start. That'd be particularly
useful here since ANNEX B start codes can be of arbitrary size
(2+(0x00 bytes) + 1(0x01 byte)). The other option would be to enforce
the number of leading 0x00 bytes (a minimum of 2 is required).
---
 Documentation/media/uapi/v4l/ext-ctrls-codec.rst | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
index d6ea2ffd65c5..82547d5de250 100644
--- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
+++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
@@ -1726,6 +1726,7 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
     :ref:`h264`, section 7.4.3 "Slice Header Semantics". For further
     documentation, refer to the above specification, unless there is
     an explicit comment stating otherwise.
+    All slices should be prepended with an ANNEX B start code.
 
     .. note::
 
-- 
2.20.1