[vlc-devel] Software decoding in Hardware buffers

Thomas Guillem thomas at gllm.fr
Thu Aug 8 15:07:11 CEST 2019

On Thu, Aug 8, 2019, at 14:29, Steve Lhomme wrote:
> Hi,
> I'm looking at the display pool in the MMAL (Raspberry Pi) code and it 
> seems that we currently decode in "hardware" buffers all the time. 
> Either the opaque decoder output, or when the decoder outputs I420.
> In push we don't to use this pool anymore. The decoder will have its own 
> pool and the display just deals with what it receives. In most cases 
> that means copy from CPU memory to GPU memory. This doesn't work with a 
> SoC like on the Raspberry Pi where the memory is the same and can be 
> used directly from both sides.
> The idea was that current decoders continue to use decoder_NewPicture() 
> as they used to. The pictures will come from the decoder video context, 
> if there's one (hardware decoding) or from picture_NewFromFormat() if 
> there's none. That means for MMAL we would need to copy this CPU 
> allocated memory to the "port allocated" memory (the mechanism to get 
> buffers from the display). Given the limited resources that's something 
> we should avoid.
> I think we should have a third way to provide pictures: from the decoder 
> device. In case of software decoding there is no video context, but 
> there is a decoder device (A MMAL one in this case).

Small reminder: There is ONE decoder device for several decoder hw + video_context.
A decoder device hold the X11/WL and Vaapi Display instance for example.
I don't see how we can put a pool on it.

Why not using a video context for the MMAL case too ?

> So I suggests the decoders (and filters) get their output picture from:
> - the video context if there is one
> - the decoder device if there is one and it has an allocator
> - picture_NewFromFormat() otherwise
> Any opinion ?
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