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
