[FFmpeg-devel] [PATCH] dxva2: add AV1 decode support

[Hendrik Leppkes]

On Thu, Nov 5, 2020 at 5:45 PM James Almer <jamrial at gmail.com> wrote:
> > +    pp->coding.reference_frame_update       = !(frame_header->show_existing_frame == 1 && frame_header->frame_type == AV1_FRAME_KEY);
>
> hwaccel->start_frame() is not called for
> frame_header->show_existing_frame == 1 frames (Those are essentially
> just a header telling the decoder to output a previously decoded frame,
> and maybe update the reference frame state), so that check is
> superfluous, and by extension the whole thing. Just hardcode it to 1.
>
> Is this field documented anywhere?
>

DXVA Spec:
"Indicates that the reference frame update process as specified by
section 7.20 of the specification should be performed after decoding
this frame.  Otherwise section 7.21 should be performed."

AV1 Spec (7.4):
"Otherwise (show_existing_frame is equal to 1), if frame_type is equal
to KEY_FRAME, the reference frame loading process as specified in
section 7.21 is invoked"

That was my interpretation of that flag, and also matches the
implementation that was provided to dav1d a few months ago by one of
the authors of the DXVA AV1 spec.
I can hardcode it, but I also don't see the harm in keeping it as-is
for documentary purposes.

> > +
> > +    if (ctx_pic->bitstream_size > dxva_size) {
> > +        av_log(avctx, AV_LOG_ERROR, "Bitstream size exceeds hardware buffer");
> > +        return -1;
> > +    }
> > +
> > +    memcpy(dxva_data, ctx_pic->bitstream, ctx_pic->bitstream_size);
>
> This is a memcpy after a (potential) memcpy in dxva2_av1_decode_slice().
> Is there no way to avoid it?
>

That depends on the behavior of av1dec. Are all tile groups guaranteed
to be available at the same time, so that I can simply save pointers
to them, and copy them into the final buffer later?
Or could the first one become invalid? In which case I need to copy
its data. The call order is not strict enough to allow the buffer
management to keep the hardware buffer mapped persistently.

- Hendrik