Re: [virtio-dev] [PATCH] [RFC RESEND] vdec: Add virtio video decode device specification

[Dmitry Morozov <dmitry.morozov@opensynergy.com>]

On Montag, 14. Oktober 2019 14:34:43 CEST Gerd Hoffmann wrote:
>   Hi,
> 
> > My take on this (for a decoder) would be to allocate memory for output
> > buffers from a secure ION heap, import in the v4l2 driver, and then to
> > provide those to the device using virtio. The device side then uses the
> > dmabuf framework to make the buffers accessible for the hardware. I'm not
> > sure about that, it's just an idea.
> 
> Virtualization aside, how does the complete video decoding workflow
> work?  I assume along the lines of ...
> 
>   (1) allocate buffer for decoded video frames (from ion).
>   (2) export those buffers as dma-buf.
>   (3) import dma-buf to video decoder.
>   (4) import dma-buf to gpu.
> 
> ... to establish buffers shared between video decoder and gpu?
> 
> Then feed the video stream into the decoder, which decodes into the ion
> buffers?  Ask the gpu to scanout the ion buffers to show the video?
> 
> cheers,
>   Gerd

Yes, exactly.

[decoder]
1) Input buffers are allocated using  VIDIOC_*BUFS.
2) Output buffers are allocated in a guest specific manner (ION, gbm).
3) Both input and output buffers are exported as dma-bufs.
4) The backing storage of both inputs and outputs is made available to the 
device.
5) Decoder hardware writes to output buffers directly.
6) Back to the guest side, the output dma-bufs are used by (virtio-) gpu.

Best regards,
Dmitry