Re: [PATCH RFC 2/6] media: uapi: h264: Add the concept of decoding mode

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

+Maxime

Oops, just realized Maxime was not Cc-ed on this series.

On Mon, 3 Jun 2019 14:30:20 +0200
Thierry Reding <thierry.reding@gmail.com> wrote:

> On Mon, Jun 03, 2019 at 01:09:42PM +0200, Boris Brezillon wrote:
> > Some stateless decoders don't support per-slice decoding (or at least
> > not in a way that would make them efficient or easy to use).
> > Let's expose a menu to control and expose the supported decoding modes.
> > Drivers are allowed to support only one decoding but they can support
> > both too.
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > ---
> >  .../media/uapi/v4l/ext-ctrls-codec.rst        | 42 ++++++++++++++++++-
> >  drivers/media/v4l2-core/v4l2-ctrls.c          |  9 ++++
> >  include/media/h264-ctrls.h                    | 13 ++++++
> >  3 files changed, 63 insertions(+), 1 deletion(-)
> > 
> > diff --git a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > index 82547d5de250..188f625acb7c 100644
> > --- a/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > +++ b/Documentation/media/uapi/v4l/ext-ctrls-codec.rst
> > @@ -1748,6 +1748,14 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> >      * - __u32
> >        - ``size``
> >        -
> > +    * - __u32
> > +      - ``start_byte_offset``
> > +      - Where the slice payload starts in the output buffer. Useful when
> > +        operating in per frame decoding mode and decoding multi-slice content.
> > +        In this case, the output buffer will contain more than one slice and
> > +        some codecs need to know where each slice starts. Note that this
> > +        offsets points to the beginning of the slice which is supposed to
> > +        contain an ANNEX B start code
> >      * - __u32
> >        - ``header_bit_size``
> >        -
> > @@ -1931,7 +1939,10 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> >        -
> >      * - __u16
> >        - ``num_slices``
> > -      - Number of slices needed to decode the current frame
> > +      - Number of slices needed to decode the current frame/field. When
> > +        operating in per-slice decoding mode (see
> > +        :c:type:`v4l2_mpeg_video_h264_decoding_mode`), this field
> > +        should always be set to one
> >      * - __u16
> >        - ``nal_ref_idc``
> >        - NAL reference ID value coming from the NAL Unit header
> > @@ -2022,6 +2033,35 @@ enum v4l2_mpeg_video_h264_hierarchical_coding_type -
> >        - 0x00000004
> >        - The DPB entry is a long term reference frame
> >  
> > +``V4L2_CID_MPEG_VIDEO_H264_DECODING_MODE (enum)``
> > +    Specifies the decoding mode to use. Currently exposes per slice and per
> > +    frame decoding but new modes might be added later on.
> > +
> > +    .. note::
> > +
> > +       This menu control is not yet part of the public kernel API and
> > +       it is expected to change.
> > +
> > +.. c:type:: v4l2_mpeg_video_h264_decoding_mode
> > +
> > +.. cssclass:: longtable
> > +
> > +.. flat-table::
> > +    :header-rows:  0
> > +    :stub-columns: 0
> > +    :widths:       1 1 2
> > +
> > +    * - ``V4L2_MPEG_VIDEO_H264_DECODING_PER_SLICE``
> > +      - 0
> > +      - The decoding is done per slice. v4l2_ctrl_h264_decode_params->num_slices
> > +        must be set to 1 and the output buffer should contain only one slice.  
> 
> I wonder if we need to be that strict. Wouldn't it be possible for
> drivers to just iterate over a number of slices and decode each in turn
> if userspace passed more than one?
> 
> Or perhaps a decoder can batch queue a couple of slices. I'm not sure
> how frequent this is, but consider something like a spike in activity on
> your system, causing some slices to get delayed so you actually get a
> few buffered up before you get a chance to hand them to the V4L2 device.
> Processing them all at once may help conceal the lag.

Hm, so we'd be in some kind of slice-batch mode, which means we could
trigger a decode operation with more than one slice, but not
necessarily all the slices needed to decode a frame. TBH, supporting
per-frame (or the batch mode you suggest) on a HW that supports
per-slice decoding should be pretty simple and has not real impact on
perfs (as you said, it's just a matter of iterating over all the slices
attached to a decode operation), so I'm fine relaxing the rule here and
patching the cedrus driver accordingly (I can't really test the
changes though). Paul, Maxime, what's your opinion?