Re: [linux-sunxi] [PATCH v2 1/2] media: v4l: Add definitions for the HEVC slice format and controls

Alexandre Courbot
2019-01-29T07:45:05Z

On Fri, Jan 25, 2019 at 10:04 PM Paul Kocialkowski
<paul.kocialkowski@bootlin.com> wrote:
>
> Hi,
>
> On Thu, 2019-01-24 at 20:23 +0800, Ayaka wrote:
> >
> > Sent from my iPad
> >
> > > On Jan 24, 2019, at 6:27 PM, Paul Kocialkowski <paul.kocialkowski@bootlin.com> wrote:
> > >
> > > Hi,
> > >
> > > > On Thu, 2019-01-10 at 21:32 +0800, ayaka wrote:
> > > > I forget a important thing, for the rkvdec and rk hevc decoder, it would
> > > > requests cabac table, scaling list, picture parameter set and reference
> > > > picture storing in one or various of DMA buffers. I am not talking about
> > > > the data been parsed, the decoder would requests a raw data.
> > > >
> > > > For the pps and rps, it is possible to reuse the slice header, just let
> > > > the decoder know the offset from the bitstream bufer, I would suggest to
> > > > add three properties(with sps) for them. But I think we need a method to
> > > > mark a OUTPUT side buffer for those aux data.
> > >
> > > I'm quite confused about the hardware implementation then. From what
> > > you're saying, it seems that it takes the raw bitstream elements rather
> > > than parsed elements. Is it really a stateless implementation?
> > >
> > > The stateless implementation was designed with the idea that only the
> > > raw slice data should be passed in bitstream form to the decoder. For
> > > H.264, it seems that some decoders also need the slice header in raw
> > > bitstream form (because they take the full slice NAL unit), see the
> > > discussions in this thread:
> > > media: docs-rst: Document m2m stateless video decoder interface
> >
> > Stateless just mean it won’t track the previous result, but I don’t
> > think you can define what a date the hardware would need. Even you
> > just build a dpb for the decoder, it is still stateless, but parsing
> > less or more data from the bitstream doesn’t stop a decoder become a
> > stateless decoder.
>
> Yes fair enough, the format in which the hardware decoder takes the
> bitstream parameters does not make it stateless or stateful per-se.
> It's just that stateless decoders should have no particular reason for
> parsing the bitstream on their own since the hardware can be designed
> with registers for each relevant bitstream element to configure the
> decoding pipeline. That's how GPU-based decoder implementations are
> implemented (VAAPI/VDPAU/NVDEC, etc).
>
> So the format we have agreed on so far for the stateless interface is
> to pass parsed elements via v4l2 control structures.
>
> If the hardware can only work by parsing the bitstream itself, I'm not
> sure what the best solution would be. Reconstructing the bitstream in
> the kernel is a pretty bad option, but so is parsing in the kernel or
> having the data both in parsed and raw forms. Do you see another
> possibility?

Is reconstructing the bitstream so bad? The v4l2 controls provide a
generic interface to an encoded format which the driver needs to
convert into a sequence that the hardware can understand. Typically
this is done by populating hardware-specific structures. Can't we
consider that in this specific instance, the hardware-specific
structure just happens to be identical to the original bitstream
format?

I agree that this is not strictly optimal for that particular
hardware, but such is the cost of abstractions, and in this specific
case I don't believe the cost would be particularly high?