Input - mt: Fix input_mt_get_slot_by_key

The case occurred recently with a touchscreen using twice a slot during a
single EV_SYN event:

E: 0.288415 0000 0000 0000 # ------------ SYN_REPORT (0) ----------
E: 0.296207 0003 002f 0000 # EV_ABS / ABS_MT_SLOT 0
E: 0.296207 0003 0039 -001 # EV_ABS / ABS_MT_TRACKING_ID -1
E: 0.296207 0003 002f 0001 # EV_ABS / ABS_MT_SLOT 1
E: 0.296207 0003 0035 0908 # EV_ABS / ABS_MT_POSITION_X 908
E: 0.296207 0003 0036 1062 # EV_ABS / ABS_MT_POSITION_Y 1062
E: 0.296207 0003 002f 0000 # EV_ABS / ABS_MT_SLOT 0
E: 0.296207 0003 0039 8787 # EV_ABS / ABS_MT_TRACKING_ID 8787
E: 0.296207 0003 0035 1566 # EV_ABS / ABS_MT_POSITION_X 1566
E: 0.296207 0003 0036 0861 # EV_ABS / ABS_MT_POSITION_Y 861
E: 0.296207 0003 0000 0908 # EV_ABS / ABS_X 908
E: 0.296207 0003 0001 1062 # EV_ABS / ABS_Y 1062
E: 0.296207 0000 0000 0000 # ------------ SYN_REPORT (0) ----------

This occurred because while having already slots 0 and 1 assigned, the
touchscreen sent:

0.293377 Tip Switch: 0 | Contact Id: 0 | X: 539 | Y: 1960 | Contact Count: 3
0.294783 Tip Switch: 1 | Contact Id: 1 | X: 908 | Y: 1062 | Contact Count: 0
0.296187 Tip Switch: 1 | Contact Id: 2 | X: 1566 | Y: 861 | Contact Count: 0

Slot 0 is released correclty, but when we look for Contact ID 2, the slot
0 is then picked up again because it is marked as inactive (trackingID < 0).

This is wrong, and we should not reuse a slot in the same frame.
The test should also check for input_mt_is_used().

In addition, we need to initialize mt->frame to an other value than 0.
With mt->frame being 0, all slots are tags as currently used, and so
input_mt_get_slot_by_key() would return -1 for all requests.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=88903

Signed-off-by: Benjamin Tissoires <[e-mail]>
Acked-by: Dmitry Torokhov <[e-mail]>
Signed-off-by: Jiri Kosina <[e-mail]>

drivers/input/input-mt.c

···
 			goto err_mem;
 	}
 
-	/* Mark slots as 'unused' */
+	/* Mark slots as 'inactive' */
 	for (i = 0; i < num_slots; i++)
 		input_mt_set_value(&mt->slots[i], ABS_MT_TRACKING_ID, -1);
+
+	/* Mark slots as 'unused' */
+	mt->frame = 1;
 
 	dev->mt = mt;
 	return 0;
···
  * set the key on the first unused slot and return.
  *
  * If no available slot can be found, -1 is returned.
+ * Note that for this function to work properly, input_mt_sync_frame() has
+ * to be called at each frame.
  */
 int input_mt_get_slot_by_key(struct input_dev *dev, int key)
 {
···
 			return s - mt->slots;
 
 	for (s = mt->slots; s != mt->slots + mt->num_slots; s++)
-		if (!input_mt_is_active(s)) {
+		if (!input_mt_is_active(s) && !input_mt_is_used(mt, s)) {
 			s->key = key;
 			return s - mt->slots;
 		}