Re: [PATCH v3 17/24] media: rkvdec: h264: Fix reference frame_num wrap for second field

[Nicolas Dufresne <nicolas.dufresne@collabora.com>]

Le lundi 25 avril 2022 à 14:55 -0400, Nicolas Dufresne a écrit :
> Le vendredi 22 avril 2022 à 09:43 +0200, Hans Verkuil a écrit :
> > On 05/04/2022 22:44, Nicolas Dufresne wrote:
> > > From: Jonas Karlman <jonas@kwiboo.se>
> > > 
> > > When decoding the second field in a complementary field pair the second
> > > field is sharing the same frame_num with the first field.
> > > 
> > > Currently the frame_num for the first field is wrapped when it matches the
> > > field being decoded, this cause issues to decode the second field in a
> > 
> > cause issues to decode -> caused issues decoding
> > 
> > > complementary field pair.
> > > 
> > > Fix this by using inclusive comparison, less than or equal.
> > 
> > I would change this last sentence to:
> > 
> > 	Fix this by using inclusive comparison: 'less than or equal'.
> > 
> > It makes it a bit easier to parse.
> > 
> > > 
> > > Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
> > > Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
> > > Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
> > > Reviewed-by: Sebastian Fricke <sebastian.fricke@collabora.com>
> > > ---
> > >  drivers/staging/media/rkvdec/rkvdec-h264.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/drivers/staging/media/rkvdec/rkvdec-h264.c b/drivers/staging/media/rkvdec/rkvdec-h264.c
> > > index f081b476340f..60eaf06b6e25 100644
> > > --- a/drivers/staging/media/rkvdec/rkvdec-h264.c
> > > +++ b/drivers/staging/media/rkvdec/rkvdec-h264.c
> > > @@ -781,7 +781,7 @@ static void assemble_hw_rps(struct rkvdec_ctx *ctx,
> > >  			continue;
> > >  
> > >  		if (dpb[i].flags & V4L2_H264_DPB_ENTRY_FLAG_LONG_TERM ||
> > > -		    dpb[i].frame_num < dec_params->frame_num) {
> > > +		    dpb[i].frame_num <= dec_params->frame_num) {
> > 
> > I wonder if a comment should be added here, explaining the reason for '<='.
> > 
> > It doesn't seem obvious to me. Up to you, though.
> 
> I guess I could, the algo for wrapping in the spec is (formula 8-27):
> 
>     if( FrameNum > frame_num )
>         FrameNumWrap = FrameNum − MaxFrameNum
>     else
>         FrameNumWrap = FrameNum
> 
> Our implementation has the branch condition flip over, and the flipped version of that is:
> 
>     if( FrameNum <= frame_num )
>         FrameNumWrap = FrameNum
>     else
>         FrameNumWrap = FrameNum − MaxFrameNum
> 
> There is no deeper rationale since we simply follow the recipe described in the
> spec. This is done so that we can share that condition with that long term
> reference handling.

Now I come to realize that in patch "[v3,19/24] media: rkvdec-h264: Add field
decoding support" all this code is removed. This is because the wrapping is
already done by the ref-builder, so while enabling field decoding, I now use the
wrapped value from the ref builder. I'm mostly keeping the patch so that this
fix is well documented. I will leave it like this then.

> 
> > 
> > >  			p[i] = dpb[i].frame_num;
> > >  			continue;
> > >  		}
> > 
> > Regards,
> > 
> > 	Hans
>