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

[Alexander Gordeev <alexander.gordeev@opensynergy.com>]

Hi BartÅomiej,

On 21.04.23 13:49, BartÅomiej Grzesik wrote:
> +CC chromeos-arc-video-eng team that also works on virtio-video
>
> Hi everyone!
>
>  From the experience of working on virtio-video I can definitely agree
> with Alex Courbot, that moving to virtio-v4l2 will be a great move.
> This move will not only simply things a lot but also allow us things like
> vhost-net like implementation for some devices (however this is
> thinking way ahead and linux only).
>
> One added benefit of this move that I'd like to point out now, that
> probably
> haven't been mentioned before, is moving to asynchronous resource queue
> call. Previously `VIRTIO_VIDEO_CMD_RESOURCE_QUEUE` have been
> synchronous and caused one hard to debug bug caused by this flawed
> design. During the command execution the virtio queue descriptors are
> blocked, potentially leading to dead locking the device. The implementation
> of virtio-v4l2 (even as is - btw nice work Alex!) eliminates this issue
> by moving to asynchronous response of the resource queue (VIDIOC_QBUF).

Thanks for your valuable feedback! Could you please share some details
about the bug? That would be very helpful. I'm working on the next
version of the virtio-video draft, so I can change it there. I like the
idea to use V4L2 as a reference, so we should probably do it like it is
done there, only simpler. Still it would be interesting to know the
details, because we didn't have issues with the current design.

> I also agree that in this case v4l2 would only be a protocol similar to how
> virtio-video was, that has the benefit of allowing very simple guest
> drivers.

--
Alexander Gordeev
Senior Software Engineer

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