Input: mt - document devices reporting more touches than slots

Some devices are capable of identifying and/or tracking more contacts than
they can report to the driver.  Document how a driver should handle this,
and what userspace should expect.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

diff --git a/Documentation/input/multi-touch-protocol.txt b/Documentation/input/multi-touch-protocol.txt
index 71536e7..543101c 100644 (file)
--- a/Documentation/input/multi-touch-protocol.txt
+++ b/Documentation/input/multi-touch-protocol.txt
@@ -65,6 +65,20 @@ the full state of each initiated contact has to reside in the receiving
 end.  Upon receiving an MT event, one simply updates the appropriate
 attribute of the current slot.
 
 end.  Upon receiving an MT event, one simply updates the appropriate
 attribute of the current slot.
 

+Some devices identify and/or track more contacts than they can report to the
+driver.  A driver for such a device should associate one type B slot with each
+contact that is reported by the hardware.  Whenever the identity of the
+contact associated with a slot changes, the driver should invalidate that
+slot by changing its ABS_MT_TRACKING_ID.  If the hardware signals that it is
+tracking more contacts than it is currently reporting, the driver should use
+a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
+being tracked by the hardware at that moment.  The driver should do this by
+explicitly sending the corresponding BTN_TOOL_*TAP event and setting
+use_count to false when calling input_mt_report_pointer_emulation().
+The driver should only advertise as many slots as the hardware can report.
+Userspace can detect that a driver can report more total contacts than slots
+by noting that the largest supported BTN_TOOL_*TAP event is larger than the
+total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.

 
 Protocol Example A
 ------------------
 
 Protocol Example A
 ------------------