linux-kernel - Re: [PATCH 1/2] HID: multitouch: report MT_TOOL

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Date:   Mon, 4 Jun 2018 19:55:57 +0200
From:   Henrik Rydberg <rydberg@...math.org>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>
Cc:     Jiri Kosina <jikos@...nel.org>,
        Jason Gerecke <killertofu@...il.com>,
        Dennis Kempin <denniskempin@...gle.com>,
        Andrew de los Reyes <adlr@...gle.com>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] HID: multitouch: report MT_TOOL_PALM for
 non-confident touches

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