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

Enrico Granata
2019-12-04T19:12:04Z

On Wed, Dec 4, 2019 at 1:16 AM Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> Hi,
>
> > 1. Focus on only decoder/encoder functionalities first.
> >
> > As Tomasz said earlier in this thread, it'd be too complicated to support camera
> > usage at the same time. So, I'd suggest to make it just a generic mem-to-mem
> > video processing device protocol for now.
> > If we finally decide to support camera in this protocol, we can add it later.
>
> Agree.
>
> > 2. Only one feature bit can be specified for one device.
> >
> > I'd like to have a decoder device and encoder device separately.
> > It'd be natural to assume it because a decoder and an encoder are provided as
> > different hardware.
>
> Hmm, modern GPUs support both encoding and decoding ...
>
> I don't think we should bake that restriction into the specification.
> It probably makes sense to use one virtqueue per function though, that
> will simplify dispatching in both host and guest.
>
> > 3. Separate buffer allocation functionalities from virtio-video protocol.
> >
> > To support various ways of guest/host buffer sharing, we might want to have a
> > dedicated buffer sharing device as we're discussing in another thread. Until we
> > reach consensus there, it'd be good not to have buffer allocation
> > functionalities in virtio-video.
>
> I think virtio-video should be able to work as stand-alone device,
> so we need some way to allocate buffers ...
>
> Buffer sharing with other devices can be added later.
>
> > > +The virtio video device is a virtual video streaming device that supports the
> > > +following functions: encoder, decoder, capture, output.
> > > +
> > > +\subsection{Device ID}\label{sec:Device Types / Video Device / Device ID}
> > > +
> > > +TBD.
> >
> > I'm wondering how and when we can determine and reserve this ID?
>
> Grab the next free, update the spec accordingly, submit the one-line
> patch.
>
> > > +\begin{lstlisting}
> > > +enum virtio_video_pixel_format {
> > > + VIRTIO_VIDEO_PIX_FMT_UNDEFINED = 0,
> > > +
> > > + VIRTIO_VIDEO_PIX_FMT_H264 = 0x0100,
> > > + VIRTIO_VIDEO_PIX_FMT_NV12,
> > > + VIRTIO_VIDEO_PIX_FMT_NV21,
> > > + VIRTIO_VIDEO_PIX_FMT_I420,
> > > + VIRTIO_VIDEO_PIX_FMT_I422,
> > > + VIRTIO_VIDEO_PIX_FMT_XBGR,
> > > +};
> >
> > I'm wondering if we can use FOURCC instead. So, we can avoid reinventing a
> > mapping from formats to integers.
> > Also, I suppose the word "pixel formats" means only raw (decoded) formats.
> > But, it can be encoded format like H.264. So, I guess "image format" or
> > "fourcc" is a better word choice.
>
> Use separate pixel_format (fourcc) and stream_format (H.264 etc.) enums?
>
> > > +\begin{lstlisting}
> > > +struct virtio_video_function {
> > > + struct virtio_video_desc desc;
> > > + __le32 function_type; /* One of VIRTIO_VIDEO_FUNC_* types */
> > > + __le32 function_id;
> > > + struct virtio_video_params in_params;
> > > + struct virtio_video_params out_params;
> > > + __le32 num_caps;
> > > + __u8 padding[4];
> > > + /* Followed by struct virtio_video_capability video_caps[]; */
> > > +};
> > > +\end{lstlisting}
> >
> > If one device only has one functionality, virtio_video_function's fields will be
> > no longer needed except in_params and out_params. So, we'd be able to remove
> > virtio_video_function and have in_params and out_params in
> > virtio_video_capability instead.
>
> Same goes for per-function virtqueues (used virtqueue implies function).
>
> > > +\begin{lstlisting}
> > > +struct virtio_video_resource_detach_backing {
> > > + struct virtio_video_ctrl_hdr hdr;
> > > + __le32 resource_id;
> > > + __u8 padding[4];
> > > +};
> > > +\end{lstlisting}
> > > +
> > > +\begin{description}
> > > +\item[\field{resource_id}] internal id of the resource.
> > > +\end{description}
> >
> > I suppose that it'd be better not to have the above series of T_RESOURCE
> > controls at least until we reach a conclusion in the thread of buffer-sharing
> > device. If we end up concluding this type of controls is the best way, we'll be
> > able to revisit here.
>

As an interim solution, this form of "manual resource backing-store
management" could be specified as a feature flag.
Once there is an established solution for buffer sharing, we would
then go ahead and introduce a new feature flag for "works with the
buffer sharing mechanism", as an alternative to this manual mechanism.

wdyt?

> Well. For buffer management there are a bunch of options.
>
> (1) Simply stick the buffers (well, pointers to the buffer pages) into
> the virtqueue. This is the standard virtio way.
>
> (2) Create resources, then put the resource ids into the virtqueue.
> virtio-gpu uses that model. First, because virtio-gpu needs an id
> to reference resources in the rendering command stream
> (virtio-video doesn't need this). Also because (some kinds of)
> resources are around for a long time and the guest-physical ->
> host-virtual mapping needs to be done only once that way (which
> I think would be the case for virtio-video too because v4l2
> re-uses buffers in robin-round fashion). Drawback is this
> assumes shared memory between host and guest (which is the case
> in typical use cases but it is not mandated by the virtio spec).
>
> (3) Import external resources (from virtio-gpu for example).
> Out of scope for now, will probably added as optional feature
> later.
>
> I guess long-term we want support either (1)+(3) or (2)+(3).
>
> cheers,
> Gerd
>
>
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