Re: [PATCH v5 04/11] media: uapi: h264: Add the concept of start code

Ezequiel Garcia
2019-08-14T14:12:33Z

On Wed, 2019-08-14 at 13:49 +0200, Paul Kocialkowski wrote:
> Hi,
>
> On Wed 14 Aug 19, 10:11, Hans Verkuil wrote:
> > On 8/12/19 9:35 PM, Ezequiel Garcia wrote:
> > > Stateless decoders have different expectations about the
> > > start code that is prepended on H264 slices. Add a
> > > menu control to express the supported start code types
> > > (including no start code).
> > >
> > > Drivers are allowed to support only one start code type,
> > > but they can support both too.
> > >
> > > Note that this is independent of the H264 decoding mode,
> > > which specifies the granularity of the decoding operations.
> > > Either in frame-based or slice-based mode, this new control
> > > will allow to define the start code expected on H264 slices.
> > >
> > > Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
> > > Tested-by: Philipp Zabel <p.zabel@pengutronix.de>
> > > ---
> > > Changes in v5:
> > > * Improve specification as suggested by Hans.
> > > Changes in v4:
> > > * New patch.
> > > ---
> > > .../media/uapi/v4l/ext-ctrls-codec.rst | 33 +++++++++++++++++++
> > > .../media/uapi/v4l/pixfmt-compressed.rst | 3 +-
> > > drivers/media/v4l2-core/v4l2-ctrls.c | 9 +++++
> > > include/media/h264-ctrls.h | 6 ++++
> > > 4 files changed, 50 insertions(+), 1 deletion(-)
> > >
> >
> > <snip>
> >
> > > diff --git a/include/media/h264-ctrls.h b/include/media/h264-ctrls.h
> > > index e6c510877f67..31555c99f64a 100644
> > > --- a/include/media/h264-ctrls.h
> > > +++ b/include/media/h264-ctrls.h
> > > @@ -27,6 +27,7 @@
> > > #define V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAMS (V4L2_CID_MPEG_BASE+1003)
> > > #define V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAMS (V4L2_CID_MPEG_BASE+1004)
> > > #define V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE (V4L2_CID_MPEG_BASE+1005)
> > > +#define V4L2_CID_MPEG_VIDEO_H264_STARTCODE (V4L2_CID_MPEG_BASE+1006)
> >
> > I almost forgot: can you change this to _START_CODE? Since it is two words?
>
> Agreed, I like it better this way too.
>

The reason to have STARTCODE instead of START_CODE was to have some
consistency with other controls (namely, V4L2_CID_MPEG_VIDEO_HEVC_WITHOUT_STARTCODE).

Consistency is really important in a API,
but at the same time, I agree START_CODE looks better.

> > Thanks!
> >
> > Hans
> >
> > >
> > > /* enum v4l2_ctrl_type type values */
> > > #define V4L2_CTRL_TYPE_H264_SPS 0x0110
> > > @@ -41,6 +42,11 @@ enum v4l2_mpeg_video_h264_decoding_mode {
> > > V4L2_MPEG_VIDEO_H264_FRAME_BASED_DECODING,
> > > };
> > >
> > > +enum v4l2_mpeg_video_h264_start_code {
> > > + V4L2_MPEG_VIDEO_H264_NO_STARTCODE,
> > > + V4L2_MPEG_VIDEO_H264_ANNEX_B_STARTCODE,
>
> Could we apply the same START_CODE renaming here too?
>
> I was also thinking that it would be slightly more readable put like this,
> with START_CODE as a prefix since it's common to both options and the name of
> the enum:
>
> - V4L2_MPEG_VIDEO_H264_START_CODE_NONE
> - V4L2_MPEG_VIDEO_H264_START_CODE_ANNEX_B
>

Yes, that looks much better.

Thanks,
Ezequiel