Input: add MT_TOOL_PALM

Currently there are only two "tools" that can be specified by a multi-touch
driver: MT_TOOL_FINGER and MT_TOOL_PEN. In working with Elan (The touch
vendor) and discussing their next-gen devices it seems that it will be
useful to have more tools so that their devices can give the upper layers
of the stack hints as to what is touching the sensor.

In particular they have new experimental firmware that can better
differentiate between palms vs fingertips and would like to plumb a patch
so that we can use their hints in higher-level gesture soft- ware. The
firmware on the device can reasonably do a better job of palm detection
because it has access to all of the raw sensor readings as opposed to just
the width/pressure/etc that are exposed by the driver. As such, the
firmware can characterize what a palm looks like in much finer-grained
detail and this change would allow such a device to share its findings with
the kernel.

Signed-off-by: Charlie Mooney <[e-mail]>
Acked-by: Peter Hutterer <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/input/multi-touch-protocol.txt

···
 
 The type of approaching tool. A lot of kernel drivers cannot distinguish
 between different tool types, such as a finger or a pen. In such cases, the
-event should be omitted. The protocol currently supports MT_TOOL_FINGER and
-MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
-drivers should instead use input_mt_report_slot_state().
+event should be omitted. The protocol currently supports MT_TOOL_FINGER,
+MT_TOOL_PEN, and MT_TOOL_PALM [2]. For type B devices, this event is handled
+by input core; drivers should instead use input_mt_report_slot_state().
+A contact's ABS_MT_TOOL_TYPE may change over time while still touching the
+device, because the firmware may not be able to determine which tool is being
+used when it first appears.
 
 ABS_MT_BLOB_ID
 

include/uapi/linux/input.h

···
  */
 #define MT_TOOL_FINGER		0
 #define MT_TOOL_PEN		1
-#define MT_TOOL_MAX		1
+#define MT_TOOL_PALM		2
+#define MT_TOOL_MAX		2
 
 /*
  * Values describing the status of a force-feedback effect