[PATCH v4 libinput 3/3] touchpad: add wobbling detection

[Peter Hutterer]

From: Konstantin Kharlamov <Hi-Angel at yandex.ru>

The details are explained in comment in the code. That aside, I shall
mention the check is so light, that it shouldn't influence CPU
performance even a bit, and can blindly be kept always enabled.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104828

v2: rename the function, change comment style, and add calculation of
x_diff (the one that was used is an absolute coordinate).

FWIW, the algorithm don't care that now added "prev_x" is unintialized,
because that happening means the time threshold won't get satisfied
either. It's not like I can't default-init it — it's just that asking
oneself a question "what default value should it have" results in "none".

v3:
* style fixes (empty line after a block, declaration at top)
* s/x_diff/dx
* s/x_motion_in_threashold/x_motion_history
* compare with r_l_r instead of &
* store whole point instead of just x
* move x_motion_history and prev_coords into tp_touch
* move everything around tp_detect_wobbling() call into the function,
use goto for that purpose.
* per request: add a comparison for when previous coordinates don't
actually have the prev. coordinates yet.
* increased timeout from 20ms to 40ms just in case — it's still out of
human ability anyway, and in fact can be increased even further
* ignore Y-only changes, again just in case — it could happen that Y and
X events would be sent separately, and break the heuristic.

v4:
* some more style fixes

Signed-off-by: Konstantin Kharlamov <Hi-Angel at yandex.ru>
Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
Changes to v3:
- more style fixes
- ajudsted for patch 1/3

 src/evdev-mt-touchpad.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++++-
 src/evdev-mt-touchpad.h |  2 ++
 2 files changed, 59 insertions(+), 1 deletion(-)

diff --git a/src/evdev-mt-touchpad.c b/src/evdev-mt-touchpad.c
index 0c3e6a2b..cec4ba34 100644
--- a/src/evdev-mt-touchpad.c
+++ b/src/evdev-mt-touchpad.c
@@ -135,6 +135,61 @@ tp_motion_history_push(struct tp_touch *t)
 	t->history.index = motion_index;
 }
 
+/* Idea: if we got a tuple of *very* quick moves like {Left, Right,
+ * Left}, or {Right, Left, Right}, it means touchpad jitters since no
+ * human can move like that within thresholds.
+ *
+ * We encode left moves as zeroes, and right as ones. We also drop
+ * the array to all zeroes when contraints are not satisfied. Then we
+ * search for the pattern {1,0,1}. It can't match {Left, Right, Left},
+ * but it does match {Left, Right, Left, Right}, so it's okay.
+ *
+ * This only looks at x changes, y changes are ignored.
+ */
+static inline void
+tp_detect_wobbling(struct tp_dispatch *tp,
+		   struct tp_touch *t,
+		   uint64_t time)
+{
+	int dx, dy;
+	uint64_t dtime;
+
+	if (!(tp->queued & TOUCHPAD_EVENT_MOTION) || tp->hysteresis.enabled)
+		return;
+
+	if (t->last_point.x == 0) { /* first invocation */
+		dx = 0;
+		dy = 0;
+	} else {
+		dx = t->last_point.x - t->point.x;
+		dy = t->last_point.y - t->point.y;
+	}
+
+	dtime = time - tp->hysteresis.last_motion_time;
+
+	tp->hysteresis.last_motion_time = time;
+	t->last_point = t->point;
+
+	if (dx == 0 && dy != 0) /* ignore y-only changes */
+		return;
+
+	if (dtime > ms2us(40)) {
+		t->hysteresis.x_motion_history = 0;
+		return;
+	}
+
+	t->hysteresis.x_motion_history <<= 1;
+	if (dx > 0) { /* right move */
+		static const char r_l_r = 0x5; /* {Right, Left, Right} */
+
+		t->hysteresis.x_motion_history |= 0x1;
+		if (t->hysteresis.x_motion_history == r_l_r) {
+			tp->hysteresis.enabled = true;
+			evdev_log_debug(tp->device, "hysteresis enabled\n");
+		}
+	}
+}
+
 static inline void
 tp_motion_hysteresis(struct tp_dispatch *tp,
 		     struct tp_touch *t)
@@ -265,6 +320,7 @@ tp_new_touch(struct tp_dispatch *tp, struct tp_touch *t, uint64_t time)
 	t->time = time;
 	t->speed.last_speed = 0;
 	t->speed.exceeded_count = 0;
+	t->hysteresis.x_motion_history = 0;
 	tp->queued |= TOUCHPAD_EVENT_MOTION;
 }
 
@@ -1443,7 +1499,7 @@ tp_process_state(struct tp_dispatch *tp, uint64_t time)
 
 		tp_thumb_detect(tp, t, time);
 		tp_palm_detect(tp, t, time);
-
+		tp_detect_wobbling(tp, t, time);
 		tp_motion_hysteresis(tp, t);
 		tp_motion_history_push(t);
 
diff --git a/src/evdev-mt-touchpad.h b/src/evdev-mt-touchpad.h
index 3ce893d2..1fd8f1e3 100644
--- a/src/evdev-mt-touchpad.h
+++ b/src/evdev-mt-touchpad.h
@@ -149,6 +149,7 @@ struct tp_touch {
 	bool has_ended;				/* TRACKING_ID == -1 */
 	bool dirty;
 	struct device_coords point;
+	struct device_coords last_point;
 	uint64_t time;
 	int pressure;
 	bool is_tool_palm; /* MT_TOOL_PALM */
@@ -177,6 +178,7 @@ struct tp_touch {
 
 	struct {
 		struct device_coords center;
+		uint8_t x_motion_history;
 	} hysteresis;
 
 	/* A pinned touchpoint is the one that pressed the physical button
-- 
2.14.3