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=88903Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/input/input-mt.c

@@ -88,10 +88,13 @@ int input_mt_init_slots(struct input_dev *dev, unsigned int num_slots,
 			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;
 err_mem:
@@ -430,6 +433,8 @@ EXPORT_SYMBOL(input_mt_assign_slots);
  * 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)
 {
@@ -444,7 +449,7 @@ 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;
 		}