[vlc-devel] Software decoding in Hardware buffers

Steve Lhomme robux4 at ycbcr.xyz
Fri Aug 9 11:57:53 CEST 2019

On 2019-08-09 10:16, Rémi Denis-Courmont wrote:
> Hi,
> The MMAL plugins are unmaintained. By definition, if the implementation that actually sees users and updates is another one, then ours is unmaintained.
> And the point is that I don't want to change the core for a misdesigned plugin. I have not seen any technically valid justification for adding yet another way to allocate pictures, nor how this would work.
> You cannot expect software decoders and filters to allocate pictures from decoder device or video context. That's complete denial of everything that was agreed upon, and reintroduces a whole lot of problems that push was supposed to fix.

That's your opinion and I don't share it. We made some design choices 
but until they were implemented we had no idea if all use cases were 
covered. And it turns out not all use cases are covered. With MMAL (be 
it the old module or the new module) the design we have is not good 
enough. We are forcing copies where they didn't exist before.

And what I propose is still push design. It's still the decoder that 
creates a video context and pushes it forward. It just may not be aware 
it's using it at all.

And as I experienced with this idea, for D3D11 it would make perfect 
sense to allow the decoder device be the creator of video context. They 
are highly related and one doesn't really exist without the other.

> Le 9 août 2019 08:50:43 GMT+03:00, Steve Lhomme <robux4 at ycbcr.xyz> a écrit :
>> On 2019-08-08 18:27, Rémi Denis-Courmont wrote:
>>> Le torstaina 8. elokuuta 2019, 15.29.30 EEST Steve Lhomme a écrit :
>>>> Any opinion ?
>>> I don't see why we should mess the architecture for a
>> hardware-specific
>>> implementation-specific unmaintained module.
>> It's not unmaintained, I was planning to revive it to make sure that
>> the
>> default player on Raspberry Pi remains VLC when we release 4.0. It
>> seems
>> there's a different implementation so I'll adapt that one.
>> One reason for that is to make sure our new push architecture is sound
>> and can adapt to many use cases. Supporting SoC architectures should
>> still be possible with the new architecture. Allocating all buffers
>> once
>> in the display was making this easy and efficient (in terms of copy,
>> not
>> memory usage). We should aim for the same level of efficiency.
>> Also let me remind you the VLC motto: "VLC plays everything and runs
>> everywhere".
>>> Even when the GPU uses the same RAM as the CPU, it typically uses
>> different
>>> pixel format, tile format and/or memory coherency protocol, or it
>> might simply
>>> not have a suitable IOMMU. As such, VLC cannot render directly in it.
>>> And if it could, then by definition, it implies that the decoder and
>> filters can
>>> allocate and *reference* picture buffers as they see fit, regardless
>> of the
>>> hardware. Which means the software on CPU side is doing the
>> allocation. If so,
>>> then there are no good technical reasons why push cannot work -
>> misdesigning
>>> the display plugin is not a good reason.
>> I haven't proposed any design change to the display plugin, other than
>> already discussed. What I proposed is a way to allocate CPU pictures
>>from the GPU. My current solution involves creating a video context 
>> optionally when the decoder doesn't provide one.
>> It could even be used on desktop. For example on Intel platform it's
>> possible to do it without much performance penalty. I used to do it in
>> D3D11 until I realized it sucked for separate GPU memory. But I had no
>> way to know exactly the impact of the switch because the code was quite
>> different. Now it might be possible to tell. I have a feeling on Intel
>> it may actually be better to decode in "GPU" buffers directly. The
>> driver can take shortcuts that we can't. It may do the copy more
>> efficiently if it needs one (or maybe it doesn't need one). It can do
>> the copy asynchronously (as every command sent to a
>> ID3D11DeviceContext)
>> as long as it's ready when it needs to be displayed.
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