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

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.

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 
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,

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