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CHROMIUM: 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>

Change-Id: I92d92b0235b47000afba0f6f7c7b1ba11550720f
BUG=chromium-os:19094
TEST=read the doc
Reviewed-on: http://gerrit.chromium.org/gerrit/6217
Reviewed-by: Andrew de los Reyes <adlr@chromium.org>
Tested-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
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  1. +14 −0 Documentation/input/multi-touch-protocol.txt
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14 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.
+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
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