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

[Dmitry Sepp <dmitry.sepp@opensynergy.com>]

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.

Regards,
Dmitry.

> 
> That is the only issue I've spotted in the protocol on a first quick
> look.  There are a few places where the spec text could be improved.
> I'll try to set aside some time to go over this in detail, but I can't
> promise I'll find the time to do that before xmas and new year.
> 
> cheers,
>   Gerd