[vlc-devel] [RFC 0/1] Let decoders decide over DPB size.

[Rafaël Carré]

Hello,

Le 23/08/2013 17:38, Julian Scheel a écrit :
> Hi,
> 
> while working at the direct rendering code in the omxil module I stumbled into
> the following problem: For direct rendering the video output core required a
> given amount of frames in the picture pool, which is calculated as:
> 
> private_picture = 4;
> decoder_picture = 1 + sys->dpb_size;
> kept_picture = 1;
> reserved_picture = DISPLAY_PICTURE_COUNT + private_picture + kept_picture;
> toal_size = reserved_picture + decoder_picture;
> 
> This is taken from src/video_output/vout_wrapper.c and modified slightly for
> readability. Now this seems to make a lot of sense, when the decoder running
> is actually utilizing the picture pool to store it's dpb. In fact this seems
> to be the case for one decoder only, which is libmpeg2, as of now. All other
> decoders care about a required dpb internally and hidden from VLC. So in the
> end the pictures for dpb will be allocated in the pool and a second time in
> the decoder itself. While this is probably not much of a problem on high end
> systems it is a problem on embedded systems.
> Taking into account that the dpb_size for H264 video is set to 18 in
> src/input/decoder.c you will have to provide a picture pool with at least 24
> frames to allow direct rendering. Doing this on a embedded system with few
> memory being available this is likely to fail for high resolution videos.
> While working with the omx modules I ran into this problem on Tegra 2 as well
> as Raspberry Pi platforms, because both did not have enough memory to store 24
> or more full 1080p frames in the GPU memory. But as they do not require the
> dpb to be stored in the picture pool, but deal with it internally it is in
> fact possible to remove the dpb_size form the picture pool and run with a much
> smaller picture pool without any issues.
> 
> So to address this issue I propose the attached patch which shifts the
> resposibility for announcing the required dpb_size to the decoder modules. I
> have not yet tested all decoders with this patch applied, but all I tested
> (libmpeg2, avcoded for mpeg2 and h264, omxil) seemed fine.

Thanks for looking at this

> Does anyone else see issues with this approach?

What about simply removing libmpeg2 once for all?

We could also remove the decoder synchro code in the core that is used
only by libmpeg2 as well.

> Cheers,
> Julian
> 
> Julian Scheel (1):
>   decoder: Let decoder modules set dpb size.
> 
>  include/vlc_codec.h      |  1 +
>  modules/codec/libmpeg2.c |  1 +
>  src/input/decoder.c      | 19 +------------------
>  3 files changed, 3 insertions(+), 18 deletions(-)
>