[PATCH libinput] touchpad: use the fuzz value (if any) for the hysteresis margin

[Peter Hutterer]

We currently used 0.5mm on touchpads as hysteresis value. This causes pointer
movement delays, it is likely too high. Reduce it to a kernel-set fuzz value
(if any) and see how we go with that. On many touchpads, the fuzz is 8 which
would be closer to 0.2mm on e.g. a T440.

Note that the does some defuzzing anyway, but the response of that function is
nonlinear, e.g. for a fuzz of 8, the physical deltas map to:

phys 0..3  → delta 0
phys 4..7  → delta 1
phys 8..15 → delta 4, 5, 6, 7
phys 16..N → delta 16..N

In other words, we never see some logical deltas 2 and 3. While this shouldn't
matter given the average touchpad resolution, reducing the hysteresis margin
is likely to provide some better response. We never see values 8-15 either
which could be the cause of some pointer jumps we've been seeing.

Devices with a fuzz of 0 still see the hysteresis enabled, but reduced to
0.25mm (from 0.5mm). The problem with a 0 fuzz is that the distinction between
"does not have fuzz" and "too lazy to set it" is not clear. Long-term we
should probably punish those that don't set it though...

https://bugs.freedesktop.org/show_bug.cgi?id=105108

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
 src/evdev-mt-touchpad.c | 20 +++++++++++++++-----
 1 file changed, 15 insertions(+), 5 deletions(-)

diff --git a/src/evdev-mt-touchpad.c b/src/evdev-mt-touchpad.c
index b3ed6d56..e0da7f05 100644
--- a/src/evdev-mt-touchpad.c
+++ b/src/evdev-mt-touchpad.c
@@ -2947,12 +2947,22 @@ tp_init_default_resolution(struct tp_dispatch *tp,
 static inline void
 tp_init_hysteresis(struct tp_dispatch *tp)
 {
-	int res_x, res_y;
+	int xmargin, ymargin;
+	const struct input_absinfo *ax = tp->device->abs.absinfo_x,
+				   *ay = tp->device->abs.absinfo_y;
 
-	res_x = tp->device->abs.absinfo_x->resolution;
-	res_y = tp->device->abs.absinfo_y->resolution;
-	tp->hysteresis.margin.x = res_x/2;
-	tp->hysteresis.margin.y = res_y/2;
+	if (ax->fuzz)
+		xmargin = ax->fuzz;
+	else
+		xmargin = ax->resolution/4;
+
+	if (ay->fuzz)
+		ymargin = ay->fuzz;
+	else
+		ymargin = ay->resolution/4;
+
+	tp->hysteresis.margin.x = xmargin;
+	tp->hysteresis.margin.y = ymargin;
 	tp->hysteresis.enabled = true;
 }
 
-- 
2.14.3