[PATCH xf86-input-synaptics v4 09/10] Don't use linear regression when calculating touchpad motion deltas

[Chase Douglas]

The results depend on the data rate of the device. A device with a
higher data rate, and thus lower individual deltas, will behave
differently with the regression calculation.

This can be verified on Synaptics semi-mt clickpads. The data rate is
halved when two or more touches are on the device. When trying to press
a button and drag the cursor with another touch, the motion will feel
faster than dragging with only one touch on the device.

Signed-off-by: Chase Douglas <chase.douglas at canonical.com>
---
 src/synaptics.c |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)

diff --git a/src/synaptics.c b/src/synaptics.c
index 202ae9b..e80dff8 100644
--- a/src/synaptics.c
+++ b/src/synaptics.c
@@ -2163,9 +2163,8 @@ get_delta(SynapticsPrivate *priv, const struct SynapticsHwState *hw,
     int x_edge_speed = 0;
     int y_edge_speed = 0;
 
-    /* HIST is full enough: priv->count_packet_finger > 3 */
-    *dx = estimate_delta(hw->x, HIST(0).x, HIST(1).x, HIST(2).x);
-    *dy = estimate_delta(hw->y, HIST(0).y, HIST(1).y, HIST(2).y);
+    *dx = hw->x - HIST(0).x;
+    *dy = hw->y - HIST(0).y;
 
     if ((priv->tap_state == TS_DRAG) || para->edge_motion_use_always)
         get_edge_speed(priv, hw, edge, &x_edge_speed, &y_edge_speed);
@@ -2234,7 +2233,7 @@ ComputeDeltas(SynapticsPrivate *priv, const struct SynapticsHwState *hw,
      * POLL_MS declaration. */
     delay = MIN(delay, POLL_MS);
 
-    if (priv->count_packet_finger <= 3) /* min. 3 packets, see get_delta() */
+    if (priv->count_packet_finger <= 1)
         goto out; /* skip the lot */
 
     if (priv->moving_state == MS_TRACKSTICK)
-- 
1.7.9