Re: [PATCH v2 1/1] virtio-video: Add virtio video device specification

[Tomasz Figa <tfiga@chromium.org>]

On Thu, Dec 19, 2019 at 6:26 PM Dmitry Sepp <dmitry.sepp@opensynergy.com> wrote:
>
> Hi Gerd,
>
> On Mittwoch, 18. Dezember 2019 14:40:37 CET Gerd Hoffmann wrote:
> >   Hi,
> >
> > > +The device MUST mark the last buffer with the
> > > +VIRTIO_VIDEO_BUFFER_F_EOS flag to denote completion of the drain
> > > +sequence.
> >
> > No, that would build a race condition into the protocol.  The device
> > could complete the last buffer after the driver has sent the drain
> > command but before the device saw it.  So the flag would not be
> > reliable.
> No, then it means the device was not in drain, but, for example, hit a
> resolution change in the stream and tells us that this is the last buffer with
> the old resolution.
>
> >
> > I also can't see why the flag is needed in the first place.  The driver
> > should know which buffers are queued still and be able to figure
> > whenever the drain is complete or not without depending on that flag.
> > So I'd suggest to simply drop it.
> This flag is used not for drain only. In marks the completion of whatever
> specific buffer sequence, like a full end-of-stream, resolution change, drain
> etc. We also need this to handle nested sequences. For instance, a resolution
> change event might happen while in drain.

Good point. The resolution change event is in a different virtqueue,
so it's not serialized with the request completions.