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

[Maxime Ripard]

On Wed, Jan 30, 2019 at 12:35:41PM +0900, Tomasz Figa wrote:
> On Wed, Jan 30, 2019 at 11:29 AM Alexandre Courbot
> <acourbot@xxxxxxxxxxxx> wrote:
> >
> > On Wed, Jan 30, 2019 at 6:41 AM Nicolas Dufresne <nicolas@xxxxxxxxxxxx> wrote:
> > >
> > > 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).
> >
> > Note that I am not proposing to rebuild the *entire* bitstream
> > in-kernel. What I am saying is that if the hardware interprets some
> > structures (like SPS/PPS) in their raw format, this raw format could
> > be reconstructed from the structures passed by userspace at negligible
> > cost. Such manipulation would only happen on a small amount of data.
> >
> > Exposing finer-grained driver requirements through a bitmask may
> > deserve more exploring. Maybe we could end with a spectrum of
> > capabilities that would allow us to cover the range from fully
> > stateless to fully stateful IPs more smoothly. Right now we have two
> > specifications that only consider the extremes of that range.
> 
> I gave it a bit more thought and if we combine what Nicolas suggested
> about the bitmask control with the userspace providing the full
> bitstream in the OUTPUT buffers, split into some logical units and
> "tagged" with their type (e.g. SPS, PPS, slice, etc.), we could
> potentially get an interface that would work for any kind of decoder I
> can think of, actually eliminating the boundary between stateful and
> stateless decoders.
> 
> For example, a fully stateful decoder would have the bitmask control
> set to 0 and accept data from all the OUTPUT buffers as they come. A
> decoder that doesn't do any parsing on its own would have all the
> valid bits in the bitmask set and ignore the data in OUTPUT buffers
> tagged as any kind of metadata. And then, we could have any cases in
> between, including stateful decoders which just can't parse the stream
> on their own, but still manage anything else themselves, or stateless
> ones which can parse parts of the stream, like the rk3399 vdec can
> parse the H.264 slice headers on its own.
> 
> That could potentially let us completely eliminate the distinction
> between the stateful and stateless interfaces and just have one that
> covers both.
> 
> Thoughts?

If we have to provide the whole bitstream in the buffers, then it
entirely breaks the sole software stack we have running and working
currently, for a use case and a driver that hasn't seen a single line
of code.

Seriously, this is a *private* API that we did that way so that we can
change it and only make it public. Why not do just that?

Maxime

-- 
Maxime Ripard, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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