[vlc-devel] Software decoding in Hardware buffers

Steve Lhomme robux4 at ycbcr.xyz
Thu Aug 8 15:40:39 CEST 2019

On 2019-08-08 14:55, John Cox wrote:
> Hi
>> 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).
>> 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 ?
> I'm sure I should have an opinion (though I'm not quite sure what it is)
> as I've get a substantial rewrite of the entire mmal/Pi modules here,
> which I intended to upstream when it was a bit closer to finished (it is
> currently in use as the default shipped with Pis but there is still work
> to be done before I want it set in anything like stone).

Oh ! Then ours is never used unless people build it from scratch 
themselves ? If it's a complete rewrite I guess we can ditch the old code ?

In any case since that's the one currently used by most people it would 
be good to merge even if not perfect. The one we have in 4.0 won't even 
compile as it is given all the changes in 4.0.

> There are a number of buffer types that I now pass around - all
> currently declared as h/w though some have a plausible existance in CPU
> memory (though not all).  All end up as having their actual allocation
> done at source (decoder or filter) though the picture_t is allocated by
> the "display"

That's how it's supposed to work in 3.0. In 4.0 things will be radically 
different. MMAL is the last part I'm looking at for this redesign. It's 
a good example of SoC use of VLC, compared to the other more desktop 
with GPU oriented display/decoders.

Since you seem to know a bit more about MMAL than me, is it possible to 
allocate memory in heap and then wrap it inside a MMAL_BUFFER_HEADER_T ? 
It doesn't look like from what I see in mmal_buffer.h [1]. If not we 
have the problem described in my original post (to avoid a copy we 
software decoders to be able to use this hardware buffers directly).


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