On 10/05/2018 07:10 PM, Paul Kocialkowski wrote: > Hi, > > Le jeudi 04 octobre 2018 à 14:10 -0400, Nicolas Dufresne a écrit : >> Le jeudi 04 octobre 2018 à 14:47 +0200, Paul Kocialkowski a écrit : >>>> + 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
