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

[Keiichi Watanabe <keiichiw@chromium.org>]

On Tue, Dec 10, 2019 at 10:16 PM Dmitry Sepp
<dmitry.sepp@opensynergy.com> wrote:
>
> Hi,
>
> Just to start, let's consider this v4l2 control:
> V4L2_CID_MPEG_VIDEO_FRAME_RC_ENABLE.
> As I can see, this control is referenced as a mandatory one in the Chromium
> sources [1].
>
> So could someone from the Chromium team please explain why it is mandatory?
> (YouTube?) In fact, almost no encoders implement this control. Do we need it
> in virtio-video?

That control is used to encode videos in constant bitrate (CBR) mode,
which is critical for
real-time use case like video conferencing.

However, that Chromium source code just requires *host-side* drivers
to have the v4l2
control. Also, I guess it's rare that a guest app wants to use CQP
instead of CBR from our
experience.
So, I suppose we can omit this control in virtio spec for now by
assuming that guests
always use CBR mode.

Best regards,
Keiichi

>
> [1] https://chromium.googlesource.com/chromium/src/media/+/refs/heads/master/
> gpu/v4l2/v4l2_video_encode_accelerator.cc#1500
>
> Regards,
> Dmitry.
>
> On Montag, 9. Dezember 2019 22:12:28 CET Enrico Granata wrote:
> > +Changyeon Jo <changyeon@google.com> for his awareness
> >
> > Thanks,
> > - Enrico
> >
> >
> > On Mon, Dec 9, 2019 at 6:20 AM Dmitry Sepp <dmitry.sepp@opensynergy.com>
> >
> > wrote:
> > > Hello,
> > >
> > > I'd like to invite everyone to share ideas regarding required encoder
> > > features
> > > in this separate sub-tree.
> > >
> > > In general, encoder devices are more complex compared to decoders. So the
> > > question I'd like to rise is in what way we define the minimal subset of
> > > features to be implemented by the virtio-video.
> > >
> > > We may look at the following to define the set of features:
> > > 1. USB video, 2.3.6 Encoding Unit [1].
> > > 2. Android 10 Compatibility Definition [2].
> > >
> > > Would be nice to hear about any specific requirements from the Chromium
> > > team as
> > > well.
> > >
> > > [1] https://www.usb.org/sites/default/files/USB_Video_Class_1_5.zip
> > > [2]
> > > https://source.android.com/compatibility/android-cdd#5_2_video_encoding
> > >
> > > Thank you.
> > >
> > > Best regards,
> > > Dmitry.
> > >
> > > On Mittwoch, 4. Dezember 2019 10:16:20 CET Gerd Hoffmann 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.
> > > >
> > > > 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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