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

[Nicolas Dufresne <nicolas@ndufresne.ca>]

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Le mercredi 22 mai 2019 à 10:20 +0200, Boris Brezillon a écrit :
> 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.

If the flag is gone, you can schedule immediatly, otherwise you'll know
by the timestamp change on the following slice.

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

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