RFC: drop frontend support for relative pointer

[Markus Armbruster <armbru@redhat.com>]

The protocol between PVFB frontend and backend supports relative and
absolute pointer.

In theory, support for absolute pointer in the backend is optional.  In
practice, our backend has always supported it.

Absolute pointers provide a superior user experience, and our frontend
always enabled them.

However, because the backend could theoretically not offer the absolute
pointer option, the frontend still supports both.  This has worked fine
so far, but it's starting to cause trouble now.

We support both relative and absolute pointer by setting both EV_REL and
EV_ABS in input device, then use whatever the backend sends us.  The
backend either sends only relative or only absolute events.

Nothing in the kernel suggests setting both EV_REL and EV_ABS is bad.
In fact, drivers/hid/hid-wacom.c and drivers/input/tablet/aiptek.c seem
to do the same.

Unfortunately, X is having difficulties with it.  It worked only because
of a bug in evdev.  This bug was fixed recently, and the fix broke the
Xen PV pointer.  Impact: pointer doesn't work at all with Fedora Rawhide
guests.  See [*] for the gory details.

X will eventually be fixed, but waiting for that isn't practical.  We
need to work around the problem in xen-kbdfront, or possibly evdev.

My preferred solution is dropping support for relative pointer in the
frontend, then set only EV_ABS.  It's easy, safe, and sends user space
down well-trodden paths.  Requires a backend supporting absolute
pointers, but as mentioned above, they all do.

Opinions?


[*] https://bugzilla.redhat.com/show_bug.cgi?id=523914