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

[Thierry Reding <thierry.reding@gmail.com>]

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On Mon, Jun 03, 2019 at 02:51:13PM +0200, Boris Brezillon wrote:
> +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?

We could perhaps have a test program to orchestrate such a scenario. I
think the assumption should obviously still be that we don't cross the
frame boundary using slices in one batch. Just that if a frame was made
up of, say, 4 slices and you first pass 3 slices, then 1, that it'd be
nice if the driver would be able to cope with that. It's something that
could probably even be implemented in the framework as a helper, though
I suspect it'd be just a couple of lines of extra code to wrap a loop
around everything.

Thierry

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