For a thumb press on the T440s I get an ABS_MT_PRESSURE minimum of 100 to 110 (range [0, 255]), with the first one or sometimes two events being in the 80-100 range. Finger data is in the 60-90 range. On an X220t, the thumb pressure is all over the place. This touchpad is quite small, so the only way to use a thumb here is to only partially touch the touchpad, so most thumb presses are actually in the 30-60 range. Finger data is in the 50-90 range. Based on those two touchpads: what may work is having a zone on the touchpad, similar to touchpad buttons to label a thumb (or at least increase confidence that it is a thumb). We may need to limit thumb detection to certain touchpads over a specific size.
On an Elan touchpad (10x7 cm): regular finger 25-50, thumb 75-100. On an Atmel touchpad (10x7 cm): regular finger -> initial reported value ~30 then roughly 60-80, for the thumb -> intial reported value of 70-90, then roughly 110-130. Good luck with that :)
commit 3dcf28b919ae598506852b408829a6627168e803 Author: Peter Hutterer <peter.hutterer@who-t.net> Date: Wed May 27 18:25:49 2015 +1000 touchpad: add pressure-based thumb-detection commit 4a463ca70228eaf0d3b676b6d7392b3cd5eb8f00 Author: Peter Hutterer <peter.hutterer@who-t.net> Date: Mon Jul 6 15:22:45 2015 +1000 touchpad: work thumb detection into the tap state machine
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