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

[Nicolas Dufresne]

Le mardi 29 janvier 2019 Ã 16:44 +0900, Alexandre Courbot a Ãcrit :
> On Fri, Jan 25, 2019 at 10:04 PM Paul Kocialkowski
> <paul.kocialkowski@xxxxxxxxxxx> 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@xxxxxxxxxxx> 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 wonât track the previous result, but I donât
> > > 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 doesnât 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?

At maximum allowed bitrate for let's say HEVC (940MB/s iirc), yes, it
would be really really bad. In GStreamer project we have discussed for
a while (but have never done anything about) adding the ability through
a bitmask to select which part of the stream need to be parsed, as
parsing itself was causing some overhead. Maybe similar thing applies,
though as per our new design, it's the fourcc that dictate the driver
behaviour, we'd need yet another fourcc for drivers that wants the full
bitstream (which seems odd if you have already parsed everything, I
think this need some clarification).

> 
> 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?

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