Re: Proposed updates and guidelines for MPEG-2, H.264 and H.265 stateless support

[Boris Brezillon <boris.brezillon@collabora.com>]

On Wed, 22 May 2019 09:29:24 +0200
Boris Brezillon <boris.brezillon@collabora.com> wrote:

> On Wed, 22 May 2019 15:39:37 +0900
> Tomasz Figa <tfiga@chromium.org> wrote:
> 
> > > It would be premature to state that we are excluding. We are just
> > > trying to find one format to get things upstream, and make sure we have
> > > a plan how to extend it. Trying to support everything on the first try
> > > is not going to work so well.
> > >
> > > What is interesting to provide is how does you IP achieve multi-slice
> > > decoding per frame. That's what we are studying on the RK/Hantro chip.
> > > Typical questions are:
> > >
> > >   1. Do all slices have to be contiguous in memory
> > >   2. If 1., do you place start-code, AVC header or pass a seperate index to let the HW locate the start of each NAL ?
> > >   3. Does the HW do support single interrupt per frame (RK3288 as an example does not, but RK3399 do)    
> > 
> > AFAICT, the bit about RK3288 isn't true. At least in our downstream
> > driver that was created mostly by RK themselves, we've been assuming
> > that the interrupt is for the complete frame, without any problems.  
> 
> I confirm that's what happens when all slices forming a frame are packed
> in a single output buffer: you only get one interrupt at the end of the
> decoding process (in that case, when the frame is decoded). Of course,
> if you split things up and do per-slice decoding instead (one slice per
> buffer) you get an interrupt per slice, though I didn't manage to make
> that work.
> I get a DEC_BUFFER interrupt (AKA, "buffer is empty but frame is not
> fully decoded") on the first slice and an ASO (Arbitrary Slice Ordering)
> interrupt on the second slice, which makes me think some states are
> reset between the 2 operations leading the engine to think that the
> second slice is part of a new frame.
> 
> Anyway, it doesn't sound like a crazy idea to support both per-slice
> and per-frame decoding and maybe have a way to expose what a
> specific codec can do (through an extra cap mechanism).
> The other option would be to support only per-slice decoding with a
> mandatory START_FRAME/END_FRAME sequence to let drivers for HW that
> only support per-frame decoding know when they should trigger the
> decoding operation.

Just to clarify, we can use Hans' V4L2_BUF_FLAG_M2M_HOLD_CAPTURE_BUF
work to identify start/end frame boundaries, the only problem I see is
that users are not required to clear the flag on the last slice of a
frame, so there's no way for the driver to know when it should trigger
the decode-frame operation. I guess we could trigger this decode
operation when v4l2_m2m_release_capture_buf() returns true, but I
wonder if it's not too late to do that.

> The downside is that it implies having a bounce
> buffer where the driver can pack slices to be decoded on the END_FRAME
> event.
>