High-resolution scroll wheel support

Starting with kernel v5.0 two new axes are available for high-resolution wheel
scrolling: REL_WHEEL_HI_RES and REL_HWHEEL_HI_RES. Both axes send data in
fractions of 120 where each multiple of 120 amounts to one logical scroll
event. Fractions of 120 indicate a wheel movement less than one detent.

This commit adds a new API for scroll events. Three new event types that encode
the axis source in the event type name and a new API to get a normalized-to-120
value that also used by Windows and the kernel (each multiple of 120 represents
a logical scroll click).

This addresses a main shortcoming with the existing API - it was unreliable to
calculate the click angle based on the axis value+discrete events and thus any
caller using the axis value alone would be left with some ambiguity. With the
v120 API it's now possible to (usually) calculate the click angle, but more
importantly it provides the simplest hw-independent way of scrolling by a
click or a fraction of a click.

A new event type is required, the only way to integrate the v120 value
otherwise was to start sending events with a discrete value of 0. This
would break existing xf86-input-libinput (divide by zero, fixed in 0.28.2) and
weston (general confusion). mutter, kwin are unaffected.

With the new API, the old POINTER_AXIS event are deprecated - callers should use
the new API where available and discard any POINTER_AXIS events.

Notable: REL_WHEEL/REL_HWHEEL are emulated by the kernel but there's no
guarantee that they'll come every accumulated 120 values, e.g. Logitech mice
often send events that don't add up to 120 per detent.

We use the kernel's wheel click emulation instead of doing our own.

libinput guarantees high-resolution events even on pre-5.0 kernels.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/test/litest-device-mouse-low-dpi.c b/test/litest-device-mouse-low-dpi.c
index 9525058e..a01b89f1 100644
--- a/test/litest-device-mouse-low-dpi.c
+++ b/test/litest-device-mouse-low-dpi.c
@@ -39,6 +39,9 @@ static int events[] = {
 	EV_REL, REL_X,
 	EV_REL, REL_Y,
 	EV_REL, REL_WHEEL,
+	EV_REL, REL_WHEEL_HI_RES,
+	EV_REL, REL_HWHEEL,
+	EV_REL, REL_HWHEEL_HI_RES,
 	-1 , -1,
 };