input: mt: Add hovering distance axis

Touch devices capable of hovering, i.e., fingers detected a
distance from the surface, are not supported by the current
input MT protocol. This patch adds ABS_MT_DISTANCE, which may
be used to indicate the distance between the contact and the
surface.
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>

Documentation/input/multi-touch-protocol.txt

@@ -161,7 +161,8 @@ against the glass. The inner region will increase, and in general, the
 ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
 unity, is related to the contact pressure. For pressure-based devices,
 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
-instead.
+instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
+indicate the distance between the contact and the surface.
 
 In addition to the MAJOR parameters, the oval shape of the contact can be
 described by adding the MINOR parameters, such that MAJOR and MINOR are the
@@ -213,6 +214,12 @@ The pressure, in arbitrary units, on the contact area. May be used instead
 of TOUCH and WIDTH for pressure-based devices or any device with a spatial
 signal intensity distribution.
 
+ABS_MT_DISTANCE
+
+The distance, in surface units, between the contact and the surface. Zero
+distance means the contact is touching the surface. A positive number means
+the contact is hovering above the surface.
+
 ABS_MT_ORIENTATION
 
 The orientation of the ellipse. The value should describe a signed quarter

include/linux/input.h

@@ -733,11 +733,12 @@ struct input_keymap_entry {
 #define ABS_MT_BLOB_ID		0x38	/* Group a set of packets as a blob */
 #define ABS_MT_TRACKING_ID	0x39	/* Unique ID of initiated contact */
 #define ABS_MT_PRESSURE		0x3a	/* Pressure on contact area */
+#define ABS_MT_DISTANCE		0x3b	/* Contact hover distance */
 
 #ifdef __KERNEL__
 /* Implementation details, userspace should not care about these */
 #define ABS_MT_FIRST		ABS_MT_TOUCH_MAJOR
-#define ABS_MT_LAST		ABS_MT_PRESSURE
+#define ABS_MT_LAST		ABS_MT_DISTANCE
 #endif
 
 #define ABS_MAX			0x3f