Input: update input_mt_report_pointer_emulation to support hovering

For devices that are incapable of reporting per-contact distance and can
only report ABS_DISTANCE, just distance is not enough for upper layers of
OS to determine whether contact is leaving the area or if it is continuing
hovering, we need BTN_TOOL_FINGER for that:

Contact State
Inactive Hovering Active
(far away) (inside hover area) (touching surface)
BTN_TOUCH 0 0 1
BTN_TOOL_FINGER 0 1 1
ABS_DISTANCE N/A 1 0

Signed-off-by: KT Liao <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

drivers/input/input-mt.c

···
 	}
 
 	input_event(dev, EV_KEY, BTN_TOUCH, count > 0);
-	if (use_count)
+
+	if (use_count) {
+		if (count == 0 &&
+		    !test_bit(ABS_MT_DISTANCE, dev->absbit) &&
+		    test_bit(ABS_DISTANCE, dev->absbit) &&
+		    input_abs_get_val(dev, ABS_DISTANCE) != 0) {
+			/*
+			 * Force reporting BTN_TOOL_FINGER for devices that
+			 * only report general hover (and not per-contact
+			 * distance) when contact is in proximity but not
+			 * on the surface.
+			 */
+			count = 1;
+		}
+
 		input_mt_report_finger_count(dev, count);
+	}
 
 	if (oldest) {
 		int x = input_mt_get_value(oldest, ABS_MT_POSITION_X);