[PATCH v3 3/4] input: alps: For protocol V3, do not process data when last packet's bit7 is set

["Pali Rohár" <pali.rohar@gmail.com>]

Sometimes on Dell Latitude laptops psmouse/alps driver receive invalid ALPS
protocol V3 packets with bit7 set in last byte. More often it can be reproduced
on Dell Latitude E6440 or E7440 with closed lid and pushing cover above touchpad.

If bit7 in last packet byte is set then it is not valid ALPS packet. I was told
that ALPS devices never send these packets. It is not know yet who send those
packets, it could be Dell EC, bug in BIOS and also bug in touchpad firmware...

With this patch alps driver does not process those invalid packets and drops it
with PSMOUSE_FULL_PACKET so psmouse driver does not enter to out of sync state.

This patch fix problem when psmouse driver still resetting ALPS device when
laptop lid is closed because of receiving invalid packets in out of sync state.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Tested-by: Pali Rohár <pali.rohar@gmail.com>
Cc: stable@vger.kernel.org
---
 drivers/input/mouse/alps.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/input/mouse/alps.c b/drivers/input/mouse/alps.c
index 7c47e97..e802d28 100644
--- a/drivers/input/mouse/alps.c
+++ b/drivers/input/mouse/alps.c
@@ -1181,6 +1181,16 @@ static psmouse_ret_t alps_process_byte(struct psmouse *psmouse)
 		return PSMOUSE_BAD_DATA;
 	}
 
+	if (priv->proto_version == ALPS_PROTO_V3 && psmouse->pktcnt == psmouse->pktsize) {
+		// For protocol V3, do not process data when last packet's bit7 is set
+		if (psmouse->packet[psmouse->pktcnt - 1] & 0x80) {
+			psmouse_dbg(psmouse, "v3 discard packet[%i] = %x\n",
+				    psmouse->pktcnt - 1,
+				    psmouse->packet[psmouse->pktcnt - 1]);
+			return PSMOUSE_FULL_PACKET;
+		}
+	}
+
 	/* Bytes 2 - pktsize should have 0 in the highest bit */
 	if ((priv->proto_version < ALPS_PROTO_V5) &&
 	    psmouse->pktcnt >= 2 && psmouse->pktcnt <= psmouse->pktsize &&
-- 
1.7.9.5

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