Hi,

It depends. We do not have a clear way to see if new ioctls are
supported (and I do not consider "try new ioctl and see if data sticks"
being a good way) so that facilitated protocol version rev-up.

Yea, EVIOCGKEYCODE_V2 on a old kernel returns EINVAL.  Not good.  There 
is another one which should have been used to signal "unknown ioctl", 
ENOTTY IIRC (a bit silly for historical reasons), so you can figure 
whenever your input data is invalid or whenever the ioctl isn't 
supported in the first place (in which case you could just fallback to 
the old version).

So keymap
manipulating tools might be forced to check protocol version.

Guess that is the best way indeed.

cheers,
   Gerd.