[vlc-devel] Software decoding in Hardware buffers

Mon Aug 12 08:57:25 CEST 2019

On 2019-08-11 13:45, Rémi Denis-Courmont wrote:
> Hi,
> 
> Indeed we did agree that some old and crappy API's may require memory copying if they have no sane ways to allocate and reference picture buffers, including but not limited to old OpenGL versions.

Yes we said that for OpenGL there were cases where some copies could be 
needed. But there was already a copy from CPU to GPU in that case, be it 
in our code or in the driver. (we're talking about software decoding) So 
it doesn't really matter who does it.

In the case of MMAL (or a SoC architecture in general) the case wasn't 
raised AFAIK. Here we introduce an extra copy that didn't exist before.

In the case of OpenGL that's on rare/old OpenGL implementations that we 
handle the copy ourself. In the case of MMAL that's the default 
implementation for everyone.

> Le 9 août 2019 16:11:55 GMT+03:00, Steve Lhomme <robux4 at ycbcr.xyz> a écrit :
>> Did we agree that MMAL will get extra copies due to our design
>> decisions ?
>>
>> On 2019-08-09 13:55, Rémi Denis-Courmont wrote:
>>> 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
>>>> 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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>>>>>
>>>>> -- 
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>>>>>
>>>>>
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