> I guess the question is if it is allowed to have .master_xfer_atomic
> but not .master_xfer (and similarly for .smbus_xfer{,_atomic})? Maybe
> that decision should be made explicit? And perhaps enforced?

xfer_atomic callbacks are optional. One xfer callback is mandatory. I
did a check for falling back to master_xfer_atomic if there is no
suitable smbus_xfer_atomic. I will think about the vice-versa case you
mentioned. Yet, this is indeed a super corner case, so I prefer to fix
this incrementally.

> I don't care deeply about the above though, so feel free to do
> something about it, or
> 
> Reviewed-by: Peter Rosin <peda@axentia.se>

Thanks for the review!