On Tue, Jan 29, 2019 at 04:44:35PM +0900, Alexandre Courbot wrote:

On Fri, Jan 25, 2019 at 10:04 PM Paul Kocialkowski

quoted

On Thu, 2019-01-24 at 20:23 +0800, Ayaka wrote:

quoted

Sent from my iPad

quoted

On Jan 24, 2019, at 6:27 PM, Paul Kocialkowski [off-list ref] wrote:

Hi,

quoted

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?

I mean, that argument can be made for the rockchip driver as well. If
reconstructing the bitstream is something we can do, and if we don't
care about being suboptimal for one particular hardware, then why the
rockchip driver doesn't just recreate the bitstream from that API?

After all, this is just a hardware specific header that happens to be
identical to the original bitstream format

Maxime

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

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