Input: alps - v7: sometimes a single touch is reported in mt[1]

The v7 proto differentiates between a primary touch (with high precision)
and a secondary touch (with lower precision). Normally when 2 fingers are
down and one is lifted the still present touch becomes the primary touch,
but some traces have shown that this does not happen always.

This commit deals with this by making alps_get_mt_count() not stop at the
first empty mt slot, and if a touch is present in mt[1] and not mt[0]
moving the data to mt[0] (for input_mt_assign_slots).

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=86338
Cc: stable@vger.kernel.org # 3.17
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/mouse/alps.c

@@ -919,12 +919,14 @@ static void alps_get_finger_coordinate_v7(struct input_mt_pos *mt,
 
 static int alps_get_mt_count(struct input_mt_pos *mt)
 {
-	int i;
+	int i, fingers = 0;
 
-	for (i = 0; i < MAX_TOUCHES && mt[i].x != 0 && mt[i].y != 0; i++)
-		/* empty */;
+	for (i = 0; i < MAX_TOUCHES; i++) {
+		if (mt[i].x != 0 || mt[i].y != 0)
+			fingers++;
+	}
 
-	return i;
+	return fingers;
 }
 
 static int alps_decode_packet_v7(struct alps_fields *f,
@@ -970,6 +972,14 @@ static int alps_decode_packet_v7(struct alps_fields *f,
 	else /* pkt_id == V7_PACKET_ID_MULTI */
 		f->fingers = 3 + (p[5] & 0x03);
 
+	/* Sometimes a single touch is reported in mt[1] rather then mt[0] */
+	if (f->fingers == 1 && f->mt[0].x == 0 && f->mt[0].y == 0) {
+		f->mt[0].x = f->mt[1].x;
+		f->mt[0].y = f->mt[1].y;
+		f->mt[1].x = 0;
+		f->mt[1].y = 0;
+	}
+
 	return 0;
 }