[PATCH] HID: multitouch: handle faulty Elo touch device

Since kernel v5.0, one single win8 touchscreen device failed.
And it turns out this is because it reports 2 InRange usage per touch.

It's a first, and I *really* wonder how this was allowed by Microsoft in
the first place. But IIRC, Breno told me this happened *after* a firmware
upgrade...

Anyway, better be safe for those crappy devices, and make sure we have
a full slot before jumping to the next.
This won't prevent all crappy devices to fail here, but at least we will
have a safeguard as long as the contact ID and the X and Y coordinates
are placed in the report after the grabage.

Reported-and-tested-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Benjamin Tissoires <redacted>
---
 drivers/hid/hid-multitouch.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/hid/hid-multitouch.c b/drivers/hid/hid-multitouch.c
index ef635964cfe1..a344a3cf5398 100644
--- a/drivers/hid/hid-multitouch.c
+++ b/drivers/hid/hid-multitouch.c

@@ -642,6 +642,13 @@ static void mt_store_field(struct hid_device *hdev,
 	if (*target != DEFAULT_TRUE &&
 	    *target != DEFAULT_FALSE &&
 	    *target != DEFAULT_ZERO) {
+		if (usage->contactid == DEFAULT_ZERO ||
+		    usage->x == DEFAULT_ZERO ||
+		    usage->y == DEFAULT_ZERO) {
+			hid_dbg(hdev,
+				"ignoring duplicate usage on incomplete");
+			return;
+		}
 		usage = mt_allocate_usage(hdev, application);
 		if (!usage)
 			return;

-- 
2.19.2