[RFC PATCH] Input: elan_i2c_smbus - resend ENABLE_TP on wake from sleep (NOT TESTED, NEEDS TESTER!)

["R. Larocque" <rlarocque@disroot.org>]

elan_smbus_sleep_control() sends ETP_SMBUS_SLEEP_CMD to put the
controller to sleep, but does nothing on wake, leaving a long-standing
XXX comment questionning wether ETP_SMBUS_ENABLE_TP should be re sent.

elan_initialize() already treats sleep_control(client, false) as a
genuine "wake device up" primitive (see the ASUS special-firmware
branch and its accompanying dev_err() message), so this makes the
non-ASUS/default path resend ETP_SMBUS_ENABLE_TP symetrically with
elan_smbus_initialize() instead of being a no-op.

Signed-off-by: Raphaël Larocque <rlarocque@disroot.org>
---
RFC: I dont have the Elan SMBus protocol datasheet and consequently 
havent been able to test this on any real hardware where the previous no-op
path was actually relied upon. Sending this for testing by
someone with access to affected devices before it is considered for
merge. A bad SMBus write to a sleeping controller could leave it
in a weird state on the wrong firmware revision if unlucky.

 drivers/input/mouse/elan_i2c_smbus.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/input/mouse/elan_i2c_smbus.c b/drivers/input/mouse/elan_i2c_smbus.c
index 0287441cda46..95f5951e9549 100644
--- a/drivers/input/mouse/elan_i2c_smbus.c
+++ b/drivers/input/mouse/elan_i2c_smbus.c
@@ -93,10 +93,25 @@ static int elan_smbus_set_mode(struct i2c_client *client, u8 mode)
 
 static int elan_smbus_sleep_control(struct i2c_client *client, bool sleep)
 {
+	int error;
+
 	if (sleep)
 		return i2c_smbus_write_byte(client, ETP_SMBUS_SLEEP_CMD);
-	else
-		return 0; /* XXX should we send ETP_SMBUS_ENABLE_TP here? */
+
+	/*
+	 * The controller stops touch reporting while asleep; resend
+	 * ETP_SMBUS_ENABLE_TP on wake, mirroring what is done during
+	 * elan_smbus_initialize(), so that reporting actually resumes.
+	 */
+	error = i2c_smbus_write_byte(client, ETP_SMBUS_ENABLE_TP);
+	if (error) {
+		dev_err(&client->dev,
+			"failed to re-enable touchpad after wake: %d\n",
+			error);
+		return error;
+	}
+
+	return 0;
 }
 
 static int elan_smbus_power_control(struct i2c_client *client, bool enable)
-- 
2.55.0