Hi Dmitry,
Logically, the confidence state is a property of a contact, not a new type
of contact. Trying to use it in any other way is bound to lead to confusion.
Problem is that MT_TOOL_PALM has been introduced in the kernel since
v4.0 (late 2015 by a736775db683 "Input: add MT_TOOL_PALM").
It's been used in the Synaptics RMI4 driver since and by hid-asus in late 2016.
I can't find any other users in the current upstream tree, but those
two are already making a precedent and changing the semantic is a
little bit late :/
I am sorry I did not respond and lost track of this issue back then, but
I disagree with Henrik here. While confidence is a property of contact,
so is the type of contact and it can and will change throughout life of
a contact, especially if we will continue adding new types, such as, for
example, thumb. In this case the firmware can transition through
finger->thumb or finger->thumb->palm or finger->palm as the nature of
contact becomes better understood. Still it is the same contact and we
should not attempt to signal userspace differently.
We agree that the contact should stay the same, but the fear, and I
think somewhere along the blurry history of this thread, the problem was
that userspace interpreted the property change as a new contact (lift
up/double click/etc). Finger/thumb/palm is one set of hand properties,
but what about Pen? It would be hard for an application to consider a
switch from finger to pen as the same contact, which is where the
natural implementation starts to diverge from the intention.
We could introduce the ABS_MT_CONFIDENCE (0/1 or even 0..n range), to
complement ABS_MT_TOOL, but that would not really solve the issue with
Wacom firmware (declaring contact non-confident and releasing it right
away) and given MS explanation of the confidence as "contact is too big"
MT_TOOL_PALM fits it perfectly.
Indeed, the Wacom firmware seems to need some special handling, which
should be fine by everyone. I do think it would make sense to add
ABS_MT_TOOL_TOO_BIG, or something, and use it if it exists. This would
apply also to a pen lying down on a touchpad, for instance.
Henrik
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