Re: [PATCH v3 1/2] virtio-video: Add virtio video device specification

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> Dmitry's virtio-video driver
> https://patchwork.linuxtv.org/patch/61717/.
> Once it becomes fully functional, I'll post a list of possible
> improvements of protocol.

Cool.  Actually implementing things can find design problems
in the protocol you didn't notice earlier.

> > > +\begin{description}
> > > +\item[\field{version}] is the protocol version that the device talks.
> > > +  The device MUST set this to 0.
> >
> > What is the intended use case for this?
> >
> > Given that virtio has feature flags to negotiate support for optional
> > features and protocol extensions between driver and device, why do you
> > think this is needed?
> 
> While feature flags work well when we "extend" the protocol with an
> optional feature, they don't when we want to "drop" or "modify"
> features.
> For example, I guess it'd be useful when we want:
> * to abandon a non-optional command,
> * to change a non-optional struct's layout,or
> * to change the order of commands in which the device expects to be sent.
> 
> Though it might be possible to handle these changes by feature flags,
> I suspect the version number allow us to transition protocols more
> smoothly.

Feature flags can be mandatory, both device and driver can fail
initialization when a specific feature is not supported by the other
end.  So in case we did screw up things so badly that we have to
effectively start over (which I hope wouldn't be the case) we can add a
VERSION_2 feature flag for a new set of commands with new structs and
new semantics.

With a feature flag both driver and device can choose whenever they want
support v1 or v2 or both.  With a version config field this is more
limited, the device can't decide to support both.  So the bonus points
for a smooth transition go to the feature flags not the version field ;)

cheers,
  Gerd