Re: [virtio-dev] Re: [RFC PATCH v6] virtio-video: Add virtio video device specification

[Alexandre Courbot <acourbot@chromium.org>]

On Mon, Feb 6, 2023 at 11:13 PM Cornelia Huck <cohuck@redhat.com> wrote:
>
> On Thu, Jan 19 2023, Alexander Gordeev <alexander.gordeev@opensynergy.com> wrote:
>
> > Hi Alexandre,
> >
> > On 12.01.23 07:39, Alexandre Courbot wrote:
> >> On Thu, Jan 12, 2023 at 3:42 AM Alexander Gordeev
> >> <alexander.gordeev@opensynergy.com> wrote:
>
> >>> Well, on the one hand mimicking v4l2 looks like an easy solution from
> >>> virtio-video spec writing perspective. (But the implementers will have
> >>> to read the V4L2 API instead AFAIU, which is probably longer...)
> >> It should not necessarily be much longer as the parts we are
> >> interested in have their own dedicated pages:
> >>
> >> https://docs.kernel.org/userspace-api/media/v4l/dev-decoder.html  <https://docs.kernel.org/userspace-api/media/v4l/dev-decoder.html>https://docs.kernel.org/userspace-api/media/v4l/dev-encoder.html  <https://docs.kernel.org/userspace-api/media/v4l/dev-encoder.html>
> >>
> >> Besides, the decoding and encoding processes are described with more
> >> precision, not that we couldn't do that here but it would make the
> >> spec grow longer than I am comfortable with...
> >
> > I read the references carefully, thanks. I am somewhat familiar with the
> > stateful decoder API, but the stateless one still needs exploring.
> >
> > There is one serious issue with these references IMO: they represent
> > guest user-space <-> v4l2 subsystem API, not v4l2 subsystem <->
> > virtio-video driver API. Just to make sure we're on the same page:
> >
> > guest user-space <-> v4l2 kernel subsystem <-> virtio-video driver <-
> > virtio-video protocol -> virtio-video device.
> >
> > I believe this is how it is supposed to work, right? So I thought, that
> > your intention is to simplify virtio-video driver and virtio-video
> > protocol by reusing the v4l2 subsystem <-> v4l2 driver API. But having
> > these references I can assume, that you want to use user-space <-> v4l2
> > subsystem API, right? Well, I think this cannot happen and therefore
> > these references cannot be used directly unless:
> >
> > 1. You suggest that virtio-video driver should not use v4l2 subsystem,
> > but should mimic its user-space API in every detail. Probably not a good
> > idea.
> >
> > 2. There is already a way to bypass the subsystem completely. I'm not
> > aware of that.
> >
> > 3. user-space <-> v4l2 subsystem API is already the same or very close
> > to v4l2 subsystem <-> v4l2 driver API. I believe this is not the case
> > even with stateful decoder/encoder. Even more with stateless decoders
> > because I can see, that v4l2 subsystem actually stores some state in
> > this case as well. Which is quite reasonable I think.
> >
> > So I think what we need to reference here is v4l2 subsystem <-> v4l2
> > driver API. Do you have this reference? Well, I know there is some
> > documentation, but still I doubt that. AFAIR kernel internal APIs are
> > never fixed. Right?
>
> So, I'm not that familiar with v4l2, but if that's indeed the case,
> depending on some kernel internal APIs is a no-go. First, because
> in-kernel APIs are not stable, and second, because we want something
> that's BSD-licenced (as opposed to GPLv2-licenced) to point to. The
> kernel<->userspace API would work (BSD-licenced header and stable); I
> had the impression that we wanted to reuse the various #defines in
> there -- did I misunderstand?

Sorry, I should have replied earlier to lift any misunderstanding. I
am not suggesting to use any kernel internal API as reference. My
suggestion is to stick strictly to the UAPI which is stable (as in,
guaranteed to be backward-compatible) and well documented. Here is for
instance the part documenting buffer queuing/dequeuing:
https://www.kernel.org/doc/html/v4.9/media/uapi/v4l/vidioc-qbuf.html

Changing anything in this documentation in a backward-incompatible way
would break user-space, and everyone familiar with the kernel
community knows what happens when someone breaks user-space. ;) So
this can be a reliable source for virtio-video (and if you look
closely, you will also notice many similarities between the two).

>
> (...)
>
> >> Let me try to summarize the case for using V4L2 over Virtio (I'll call
> >> it virtio-v4l2 to differentiate it from the current spec).
> >>
> >> There is the argument that virtio-video turns out to be a recreation
> >> of the stateful V4L2 decoder API, which itself works similarly to
> >> other high-level decoder APIs. So it's not like we could or should
> >> come with something very different. In parallel, virtio-camera is also
> >> currently using V4L2 as its model. While this is subject to change, I
> >> am starting to see a pattern here. :)
> >>
> >> Transporting V4L2 over virtio would considerably shorten the length of
> >> this spec, as we would just need to care about the transport aspect
> >> and minor amendments to the meaning of some V4L2 structure members,
> >> and leave the rest to V4L2 which is properly documented and for which
> >> there is a large collection of working examples.
> >>
> >> This would work very well for codec devices, but as a side-effect
> >> would also enable other kinds of devices that may be useful to
> >> virtualize, like image processors, DVB cards, and cameras. This
> >> doesn't mean virtio-v4l2 should be the *only* way to support cameras
> >> over virtio. It is a nice bonus of encapsulating V4L2, it may be
> >> sufficient for simple (most?) use-cases, but also doesn't forbid more
> >> specialized virtual devices for complex camera pipelines to be added
> >> later. virtio-v4l2 would just be the generic virtual video device that
> >> happens to be sufficient for our accelerated video needs - and if your
> >> host camera is a USB UVC one, well feel free to use that too.
> >>
> >> In other words, I see an opportunity to enable a whole class of
> >> devices instead of a single type for the same effort and think we
> >> should seriously consider this.
> >>
> >> I have started to put down what a virtio-v4l2 transport might look
> >> like, and am also planning on putting together a small
> >> proof-of-concept. If I can get folks here to warm up to the idea, I
> >> believe we should be able to share a spec and prototype in a month or
> >> so.
> >
> > Thanks for the detailed explanation. Please check my comments above. I'd
> > like to resolve the mentioned issue first.
>
> I hope we can sort this out soon -- I guess I'm not the only one who is
> anxious about this spec moving forward :) Please let me know if I can
> help in any way.

I'll try to address Alexander's points in more detail, but I am not
seeing any blocking issue with using the V4L2 UAPI as the basis for
virtio-video (we are working on a small proof-of-concept and things
are going smoothly so far).

Cheers,
Alex.