[PATCH] Input - mt: Fix input_mt_get_slot_by_key

Benjamin Tissoires
2015-02-03T16:55:41Z

The case occured 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 occured 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 tagged as being 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 <benjamin.tissoires@redhat.com>
---

Hi Dmitry,

I wonder if this one should go to stable as well.

Cheers,
Benjamin

drivers/input/input-mt.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/input/input-mt.c b/drivers/input/input-mt.c
index cb150a1..e02fd92 100644
--- a/drivers/input/input-mt.c
+++ b/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:
@@ -453,7 +456,7 @@ 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;
}
--
2.1.0