Re: [RFC PATCH v2] media: docs-rst: Document m2m stateless video decoder interface

[Hans Verkuil]

On 10/05/2018 07:10 PM, Paul Kocialkowski wrote:
> Hi,
> 
> Le jeudi 04 octobre 2018 Ã 14:10 -0400, Nicolas Dufresne a Ãcrit :
>> Le jeudi 04 octobre 2018 Ã 14:47 +0200, Paul Kocialkowski a Ãcrit :
>>>> +    Instance of struct v4l2_ctrl_h264_scaling_matrix, containing the scaling
>>>> +    matrix to use when decoding the next queued frame. Applicable to the H.264
>>>> +    stateless decoder.
>>>> +
>>>> +``V4L2_CID_MPEG_VIDEO_H264_SLICE_PARAM``
>>>
>>> Ditto with "H264_SLICE_PARAMS".
>>>
>>>> +    Array of struct v4l2_ctrl_h264_slice_param, containing at least as many
>>>> +    entries as there are slices in the corresponding ``OUTPUT`` buffer.
>>>> +    Applicable to the H.264 stateless decoder.
>>>> +
>>>> +``V4L2_CID_MPEG_VIDEO_H264_DECODE_PARAM``
>>>> +    Instance of struct v4l2_ctrl_h264_decode_param, containing the high-level
>>>> +    decoding parameters for a H.264 frame. Applicable to the H.264 stateless
>>>> +    decoder.
>>>
>>> Since we require all the macroblocks to decode one frame to be held in
>>> the same OUTPUT buffer, it probably doesn't make sense to keep
>>> DECODE_PARAM and SLICE_PARAM distinct.
>>>
>>> I would suggest merging both in "SLICE_PARAMS", similarly to what I
>>> have proposed for H.265: https://patchwork.kernel.org/patch/10578023/
>>>
>>> What do you think?
>>
>> I don't understand why we add this arbitrary restriction of "all the
>> macroblocks to decode one frame". The bitstream may contain multiple
>> NALs per frame (e.g. slices), and stateless API shall pass each NAL
>> separately imho. The driver can then decide to combine them if needed,
>> or to keep them seperate. I would expect most decoder to decode each
>> slice independently from each other, even though they write into the
>> same frame.
> 
> Well, we sort of always assumed that there is a 1:1 correspondency
> between request and output frame when implemeting the software for
> cedrus, which simplified both userspace and the driver. The approach we
> have taken is to use one of the slice parameters for the whole series
> of slices and just append the slice data.
> 
> Now that you bring it up, I realize this is an unfortunate decision.
> This may have been the cause of bugs and limitations with our driver
> because the slice parameters may very well be distinct for each slice.
> Moreover, I suppose that just appending the slices data implies that
> they are coded in the same order as the picture, which is probably
> often the case but certainly not anything guaranteed. 
> 
> So I think we should change our software to associate one request per
> slice, not per frame and drop this limitation that all the macroblocks
> for the frame must be included.
> 
> This will require a number of changes to our driver and userspace, but
> also to the MPEG-2 controls where I don't think we have the macroblock
> position specified.
> 
> So it certainly makes sense to keep SLICE_PARAMS separate from
> DECODE_PARAMS for H.264. I should probably also rework the H.265
> controls to reflect this. Still, all controls must be passed per slice
> (and the hardware decoding pipeline is fully reconfigured then), so I
> guess it doesn't make such a big difference in practice.
> 
> Thanks for pointing this out, it should help bring the API closer to
> what is represented in the bitstream.

One concern I have with this:

If we support slices with one slice per buffer, then I think our
current max of 32 buffers will be insufficient, right? So that will
have to be fixed. That's a fair amount of work since we want to do this
right.

Regards,

	Hans