[PATCH synaptics] Limit the movement to 20 mm per event

[Peter Hutterer]

Touchpads are limited by a fixed sampling rate (usually 80Hz). Some finger
changes may happen too fast for this sampling rate, resulting in two distinct
event sequences:
* finger 1 up and finger 2 down in the same EV_SYN frame. Synaptics sees one
  finger down before and after and the changed coordinates
* finger 1 up and finger 2 down _between_ two EV_SYN frames. Synaptics sees one
  touchpoint move from f1 position to f2 position.

That move causes a large cursor jump. The former could be solved (with
difficulty) by adding fake EV_SYN handling after releasing touchpoints but
that won't fix the latter case.

So as a solution for now limit the finger movement to 20mm per event.
Tests on a T440 and an x220 showed that this is just above what a reasonable
finger movement would trigger. If a movement is greater than that limit, reset
it to 0/0.

On devices without resolution, use 0.25 the touchpad's diagonal instead.

Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
---
See the git repo below for a simple script to verify valid movement deltas
on your device if you feel like it:
https://github.com/whot/input-data-analysis/tree/master/touchpad-max-delta
Max I got was 9mm on the x220 and 17mm on the T440s but the latter was
really ripping the cursor around the screen beyond what I'd deem useful.

 src/synaptics.c    | 33 +++++++++++++++++++++++++++++++++
 src/synapticsstr.h |  2 ++
 2 files changed, 35 insertions(+)

diff --git a/src/synaptics.c b/src/synaptics.c
index 2e3ad0c..756751d 100644
--- a/src/synaptics.c
+++ b/src/synaptics.c
@@ -780,6 +780,23 @@ set_default_parameters(InputInfoPtr pInfo)
         pars->resolution_vert = 1;
     }
 
+    /* Touchpad sampling rate is too low to detect all movements.
+       A user may lift one finger and put another one down within the same
+       EV_SYN or even between samplings so the driver doesn't notice at all.
+
+       We limit the movement to 20 mm within one event, that is more than
+       recordings showed is needed (17mm on a T440).
+      */
+    if (pars->resolution_horiz > 1 &&
+        pars->resolution_vert > 1)
+        pars->maxDeltaMM = 20;
+    else {
+        /* on devices without resolution set the vector length to 0.25 of
+           the touchpad diagonal */
+        pars->maxDeltaMM = diag * 0.25;
+    }
+
+
     /* Warn about (and fix) incorrectly configured TopEdge/BottomEdge parameters */
     if (pars->top_edge > pars->bottom_edge) {
         int tmp = pars->top_edge;
@@ -2229,6 +2246,13 @@ get_delta(SynapticsPrivate *priv, const struct SynapticsHwState *hw,
     *dy = integral;
 }
 
+/* Vector length, but not sqrt'ed, we only need it for comparison */
+static inline double
+vlenpow2(double x, double y)
+{
+    return x * x + y * y;
+}
+
 /**
  * Compute relative motion ('deltas') including edge motion.
  */
@@ -2238,6 +2262,7 @@ ComputeDeltas(SynapticsPrivate * priv, const struct SynapticsHwState *hw,
 {
     enum MovingState moving_state;
     double dx, dy;
+    double vlen;
     int delay = 1000000000;
 
     dx = dy = 0;
@@ -2283,6 +2308,14 @@ ComputeDeltas(SynapticsPrivate * priv, const struct SynapticsHwState *hw,
  out:
     priv->prevFingers = hw->numFingers;
 
+    vlen = vlenpow2(dx/priv->synpara.resolution_horiz,
+                    dy/priv->synpara.resolution_vert);
+
+    if (vlen > priv->synpara.maxDeltaMM * priv->synpara.maxDeltaMM) {
+        dx = 0;
+        dy = 0;
+    }
+
     *dxP = dx;
     *dyP = dy;
 
diff --git a/src/synapticsstr.h b/src/synapticsstr.h
index 4bd32ac..75f52d5 100644
--- a/src/synapticsstr.h
+++ b/src/synapticsstr.h
@@ -226,6 +226,8 @@ typedef struct _SynapticsParameters {
     int area_left_edge, area_right_edge, area_top_edge, area_bottom_edge;       /* area coordinates absolute */
     int softbutton_areas[4][4]; /* soft button area coordinates, 0 => right, 1 => middle , 2 => secondary right, 3 => secondary middle button */
     int hyst_x, hyst_y;         /* x and y width of hysteresis box */
+
+    int maxDeltaMM;               /* maximum delta movement (vector length) in mm */
 } SynapticsParameters;
 
 struct _SynapticsPrivateRec {
-- 
1.9.3