Input: MT - add support for balanced slot assignment

Some devices are not fast enough to differentiate between a fast-moving
contact and a new contact. This problem cannot be fully resolved because
information is truly missing, but it is possible to safe-guard against
obvious mistakes by restricting movement with a maximum displacement.

The new problem formulation for dmax > 0 cannot benefit from the speedup
for positive definite matrices, but since the convergence is faster, the
result is about the same. For a handful of contacts, the latency difference
is truly negligible.

Suggested-by: Benjamin Tissoires <[e-mail]>
Tested-by: Benjamin Tissoires <[e-mail]>
Signed-off-by: Henrik Rydberg <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/input-mt.c

···
 }
 EXPORT_SYMBOL(input_mt_sync_frame);
 
-static int adjust_dual(int *begin, int step, int *end, int eq)
+static int adjust_dual(int *begin, int step, int *end, int eq, int mu)
 {
 	int f, *p, s, c;
 
···
 			s = *p;
 
 	c = (f + s + 1) / 2;
-	if (c == 0 || (c > 0 && !eq))
+	if (c == 0 || (c > mu && (!eq || mu > 0)))
 		return 0;
-	if (s < 0)
+	/* Improve convergence for positive matrices by penalizing overcovers */
+	if (s < 0 && mu <= 0)
 		c *= 2;
 
 	for (p = begin; p != end; p += step)
···
 	return (c < s && s <= 0) || (f >= 0 && f < c);
 }
 
-static void find_reduced_matrix(int *w, int nr, int nc, int nrc)
+static void find_reduced_matrix(int *w, int nr, int nc, int nrc, int mu)
 {
 	int i, k, sum;
 
 	for (k = 0; k < nrc; k++) {
 		for (i = 0; i < nr; i++)
-			adjust_dual(w + i, nr, w + i + nrc, nr <= nc);
+			adjust_dual(w + i, nr, w + i + nrc, nr <= nc, mu);
 		sum = 0;
 		for (i = 0; i < nrc; i += nr)
-			sum += adjust_dual(w + i, 1, w + i + nr, nc <= nr);
+			sum += adjust_dual(w + i, 1, w + i + nr, nc <= nr, mu);
 		if (!sum)
 			break;
 	}
 }
 
 static int input_mt_set_matrix(struct input_mt *mt,
-			       const struct input_mt_pos *pos, int num_pos)
+			       const struct input_mt_pos *pos, int num_pos,
+			       int mu)
 {
 	const struct input_mt_pos *p;
 	struct input_mt_slot *s;
···
 		y = input_mt_get_value(s, ABS_MT_POSITION_Y);
 		for (p = pos; p != pos + num_pos; p++) {
 			int dx = x - p->x, dy = y - p->y;
-			*w++ = dx * dx + dy * dy;
+			*w++ = dx * dx + dy * dy - mu;
 		}
 	}
 
···
  * @slots: the slot assignment to be filled
  * @pos: the position array to match
  * @num_pos: number of positions
+ * @dmax: maximum ABS_MT_POSITION displacement (zero for infinite)
  *
  * Performs a best match against the current contacts and returns
  * the slot assignment list. New contacts are assigned to unused
  * slots.
  *
+ * The assignments are balanced so that all coordinate displacements are
+ * below the euclidian distance dmax. If no such assignment can be found,
+ * some contacts are assigned to unused slots.
+ *
  * Returns zero on success, or negative error in case of failure.
  */
 int input_mt_assign_slots(struct input_dev *dev, int *slots,
-			  const struct input_mt_pos *pos, int num_pos)
+			  const struct input_mt_pos *pos, int num_pos,
+			  int dmax)
 {
 	struct input_mt *mt = dev->mt;
+	int mu = 2 * dmax * dmax;
 	int nrc;
 
 	if (!mt || !mt->red)
···
 	if (num_pos < 1)
 		return 0;
 
-	nrc = input_mt_set_matrix(mt, pos, num_pos);
-	find_reduced_matrix(mt->red, num_pos, nrc / num_pos, nrc);
+	nrc = input_mt_set_matrix(mt, pos, num_pos, mu);
+	find_reduced_matrix(mt->red, num_pos, nrc / num_pos, nrc, mu);
 	input_mt_set_slots(mt, slots, num_pos);
 
 	return 0;

drivers/input/mouse/alps.c

···
 	struct alps_fields *f = &priv->f;
 	int i, slot[MAX_TOUCHES];
 
-	input_mt_assign_slots(dev, slot, f->mt, n);
+	input_mt_assign_slots(dev, slot, f->mt, n, 0);
 	for (i = 0; i < n; i++)
 		alps_set_slot(dev, slot[i], f->mt[i].x, f->mt[i].y);
 

drivers/input/mouse/bcm5974.c

···
 		dev->index[n++] = &f[i];
 	}
 
-	input_mt_assign_slots(input, dev->slots, dev->pos, n);
+	input_mt_assign_slots(input, dev->slots, dev->pos, n, 0);
 
 	for (i = 0; i < n; i++)
 		report_finger_data(input, dev->slots[i],

drivers/input/mouse/cypress_ps2.c

···
 		pos[i].y = contact->y;
 	}
 
-	input_mt_assign_slots(input, slots, pos, n);
+	input_mt_assign_slots(input, slots, pos, n, 0);
 
 	for (i = 0; i < n; i++) {
 		contact = &report_data.contacts[i];

drivers/input/mouse/synaptics.c

···
 		pos[i].y = synaptics_invert_y(hw[i]->y);
 	}
 
-	input_mt_assign_slots(dev, slot, pos, nsemi);
+	input_mt_assign_slots(dev, slot, pos, nsemi, 0);
 
 	for (i = 0; i < nsemi; i++) {
 		input_mt_slot(dev, slot[i]);

drivers/input/touchscreen/pixcir_i2c_ts.c

···
 			pos[i].y = touch->y;
 		}
 
-		input_mt_assign_slots(ts->input, slots, pos, n);
+		input_mt_assign_slots(ts->input, slots, pos, n, 0);
 	}
 
 	for (i = 0; i < n; i++) {

include/linux/input/mt.h

···
 };
 
 int input_mt_assign_slots(struct input_dev *dev, int *slots,
-			  const struct input_mt_pos *pos, int num_pos);
+			  const struct input_mt_pos *pos, int num_pos,
+			  int dmax);
 
 int input_mt_get_slot_by_key(struct input_dev *dev, int key);