Peter Hutterer authored Jun 01, 2016 and Adam Jackson committed Jun 08, 2016

A new --with-fallback-input-driver=foo option allows selecting a
fallback driver for the server if the driver configured for the device
is not found.  Note that this only applies when the device has a driver
assigned and that module fails to load, devices without a driver are
ignored as usual.

This avoids the situation where a configuration assigns e.g. the
synaptics driver but that driver is not available on the system,
resulting in a dead device. A fallback driver can at least provides some
functionality.

This becomes more important as we move towards making other driver true
leaf nodes that can be installed/uninstalled as requested. Specifically,
wacom and synaptics, a config that assigns either driver should be
viable even when the driver itself is not (yet) installed on the system.

It is up to the distributions to make sure that the fallback driver is
always installed. The fallback driver can be disabled with
--without-fallback-input-driver and is disabled by default on non-Linux
systems because we don't have generic drivers on those platforms.
Default driver on Linux is libinput, evdev is the only other serious
candidate here.

Sample log output:
[  3274.421] (II) config/udev: Adding input device SynPS/2 Synaptics TouchPad (/dev/input/event4)
[  3274.421] (**) SynPS/2 Synaptics TouchPad: Applying InputClass "touchpad weird driver"
[  3274.421] (II) LoadModule: "banana"
[  3274.422] (WW) Warning, couldn't open module banana
[  3274.422] (II) UnloadModule: "banana"
[  3274.422] (II) Unloading banana
[  3274.422] (EE) Failed to load module "banana" (module does not exist, 0)
[  3274.422] (EE) No input driver matching `banana'
[  3274.422] (II) Falling back to input driver `libinput'
.. server proceeds to assign libinput, init the device, world peace and rainbows
everywhere, truly what a sight. Shame about the banana though.

Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>