HID: wacom: Avoid sending empty sync events

Empty sync events clutter up logs and are a waste of CPU cycles. We can
avoid sending mt_sync events if touch is disabled or a specific slot is
unused. We can avoid sending full sync events if no events were generated.

Signed-off-by: Ping Cheng <ping.cheng@wacom.com>
Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/wacom_wac.c

@@ -2551,8 +2551,17 @@ static void wacom_wac_finger_slot(struct wacom_wac *wacom_wac,
 		int slot;
 
 		slot = input_mt_get_slot_by_key(input, hid_data->id);
-		if (slot < 0)
+		if (slot < 0) {
 			return;
+		} else {
+			struct input_mt_slot *ps = &input->mt->slots[slot];
+			int mt_id = input_mt_get_value(ps, ABS_MT_TRACKING_ID);
+
+			if (!prox && mt_id < 0) {
+				// No data to send for this slot; short-circuit
+				return;
+			}
+		}
 
 		input_mt_slot(input, slot);
 		input_mt_report_slot_state(input, MT_TOOL_FINGER, prox);
@@ -2696,6 +2705,10 @@ static void wacom_wac_finger_report(struct hid_device *hdev,
 	struct input_dev *input = wacom_wac->touch_input;
 	unsigned touch_max = wacom_wac->features.touch_max;
 
+	/* if there was nothing to process, don't send an empty sync */
+	if (wacom_wac->hid_data.num_expected == 0)
+		return;
+
 	/* If more packets of data are expected, give us a chance to
 	 * process them rather than immediately syncing a partial
 	 * update.