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

["Alex Bennée" <alex.bennee@linaro.org>]

Cornelia Huck <cohuck@redhat.com> writes:

> On Fri, Apr 28 2023, Alexander Gordeev <alexander.gordeev@opensynergy.com> wrote:
>
>> On 27.04.23 15:16, Alexandre Courbot wrote:
>>> But in any case, that's irrelevant to the guest-host interface, and I
>>> think a big part of the disagreement stems from the misconception that
>>> V4L2 absolutely needs to be used on either the guest or the host,
>>> which is absolutely not the case.
>>
>> I understand this, of course. I'm arguing, that it is harder to
>> implement it, get it straight and then maintain it over years. Also it
>> brings limitations, that sometimes can be workarounded in the virtio
>> spec, but this always comes at a cost of decreased readability and
>> increased complexity. Overall it looks clearly as a downgrade compared
>> to virtio-video for our use-case. And I believe it would be the same for
>> every developer, that has to actually implement the spec, not just do
>> the pass through. So if we think of V4L2 UAPI pass through as a
>> compatibility device (which I believe it is), then it is fine to have
>> both and keep improving the virtio-video, including taking the best
>> ideas from the V4L2 and overall using it as a reference to make writing
>> the driver simpler.
>
> Let me jump in here and ask another question:
>
> Imagine that, some years in the future, somebody wants to add a virtio
> device for handling video encoding/decoding to their hypervisor.
>
> Option 1: There are different devices to chose from. How is the person
> implementing this supposed to pick a device? They might have a narrow
> use case, where it is clear which of the devices is the one that needs to
> be supported; but they also might have multiple, diverse use cases, and
> end up needing to implement all of the devices.
>
> Option 2: There is one device with various optional features. The person
> implementing this can start off with a certain subset of features
> depending on their expected use cases, and add to it later, if needed;
> but the upfront complexity might be too high for specialized use cases.
>
> Leaving concrete references to V4L2 out of the picture, we're currently
> trying to decide whether our future will be more like Option 1 or Option
> 2, with their respective trade-offs.
>
> I'm slightly biased towards Option 2; does it look feasible at all, or
> am I missing something essential here? (I had the impression that some
> previous confusion had been cleared up; apologies in advance if I'm
> misrepresenting things.)
>
> I'd really love to see some kind of consensus for 1.3, if at all
> possible :)

I think feature discovery and extensibility is a key part of the VirtIO
paradigm which is why I find the virtio-v4l approach limiting. By
pegging the device to a Linux API we effectively limit the growth of the
device specification to as fast as the Linux API changes. I'm not fully
immersed in v4l but I don't think it is seeing any additional features
developed for it and its limitations for camera are one of the reasons
stuff is being pushed to userspace in solutions like libcamera:

  How is libcamera different from V4L2?

  We see libcamera as a continuation of V4L2. One that can more easily
  handle the recent advances in hardware design. As embedded cameras have
  developed, all of the complexity has been pushed on to the developers.
  With libcamera, all of that complexity is simplified and a single model
  is presented to application developers.

That said its not totally our experience to have virtio devices act as
simple pipes for some higher level protocol. The virtio-gpu spec says
very little about the details of how 3D devices work and simply offers
an opaque pipe to push a (potentially propriety) command stream to the
back end. As far as I'm aware the proposals for Vulkan and Wayland
device support doesn't even offer a feature bit but simply changes the
graphics stream type in the command packets.

We could just offer a VIRTIO_VIDEO_F_V4L feature bit, document it as
incompatible with other feature bits and make that the baseline
implementation but it's not really in the spirit of what VirtIO is
trying to achieve.

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro

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