Some tablets such as those in the XP-PEN PRO series use "dials" which
are actually scrollwheels and emit EV_REL events. These should not be
emulated as rings (which are absolute) so we must expose them as a new
tablet event.
Adds LIBINPUT_EVENT_TABLET_PAD_DIAL that work largely identical as our
high-resolution wheel events (i.e. the values are in multiples or
fractions of of 120). Currently supports two dials.
This is a lot of copy/paste from the ring axes because the interface is
virtually identical. The main difference is that dials give us a v120
value in the same manner as our scroll axes.
Notes:
- REL_DIAL is mutually exclusive with REL_WHEEL, we assume the kernel
doesn't (at this point) give us devices with both. If this changes for
devices with three dials (wheel + hwheel + dial) we need to add code
for that.
- REL_DIAL does not have a high-resolution axis and we assume that any
device with REL_WHEEL_HI_RES will also have REL_HWHEEL_HI_RES (if the
second wheel exists).
- With dials being REL_DIAL or REL_WHEEL there is no possibility of
detecting a finger release (the kernel does not route EV_RELs with a
value of zero). Unless this is implemented via a side-channel - and it
doesn't look like any hardware that supports dials does that - we
cannot forward any information here. So unlike absolute rings we
cannot provide a source information here.
Closes#600
Co-authored-by: Peter Hutterer <peter.hutterer@who-t.net>
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/967>
