[vlc-devel] Software decoding in Hardware buffers

Rémi Denis-Courmont remi at remlab.net
Fri Aug 9 13:55:03 CEST 2019

No, it is not my opinion. It is what was agreed collectively. Unlike your opinion, which engages only you.

I am very fed up with people misconstruing earlier decisions as my opinion. You can not have it both ways.

Le 9 août 2019 12:57:53 GMT+03:00, Steve Lhomme <robux4 at ycbcr.xyz> a écrit :
>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
>> 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
>>> 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
>>>> And if it could, then by definition, it implies that the decoder
>>> filters can
>>>> allocate and *reference* picture buffers as they see fit,
>>> 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
>>> 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
>>> D3D11 until I realized it sucked for separate GPU memory. But I had
>>> way to know exactly the impact of the switch because the code was
>>> different. Now it might be possible to tell. I have a feeling on
>>> 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
>>> 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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