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

Nicolas Dufresne
2022-04-25T19:00:22Z

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
>