linux-kernel - Re: [PATCH v2] Documentation: input: expand INPUT_PROP_HAPTIC

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Message-ID: <ueksimssfw7eqxa5mfmxruj2ghtuncznhbgdxvoorjf6jhjfhk@hsuapbn3kie3>
Date: Tue, 4 Nov 2025 21:09:09 -0800
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Peter Hutterer <peter.hutterer@...-t.net>
Cc: Jonathan Denose <jdenose@...gle.com>, Jiri Kosina <jikos@...nel.org>, 
	Benjamin Tissoires <bentiss@...nel.org>, Jonathan Corbet <corbet@....net>, 
	Henrik Rydberg <rydberg@...math.org>, linux-input@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, Angela Czubak <aczubak@...gle.com>, 
	Sean O'Brien <seobrien@...gle.com>, Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v2] Documentation: input: expand
 INPUT_PROP_HAPTIC_TOUCHPAD to all pressure pads

Hi Peter,

On Fri, Oct 31, 2025 at 02:12:45PM +1000, Peter Hutterer wrote:
> Definition: "pressure pad" used here as all touchpads that use physical
> pressure to convert to click without physical hinges. Also called haptic
> touchpads in general parlance, Synaptics calls them ForcePads.
> 
> Most (all?) pressure pads are currently advertised as
> INPUT_PROP_BUTTONPAD. The suggestion to identify them as pressure pads
> by defining the resolution on ABS_MT_PRESSURE has been in the docs since
> commit 20ccc8dd38a3 ("Documentation: input: define
> ABS_PRESSURE/ABS_MT_PRESSURE resolution as grams") but few devices
> provide this information.
> 
> In userspace it's thus impossible to determine whether a device is a
> true pressure pad (pressure equals pressure) or a normal clickpad with
> (pressure equals finger size).
> 
> Commit 7075ae4ac9db ("Input: add INPUT_PROP_HAPTIC_TOUCHPAD") introduces
> INPUT_PROP_HAPTIC_TOUCHPAD but restricted it to those touchpads that
> have support for userspace-controlled effects. Let's expand that
> definition to include all haptic touchpads (pressure pads) since those
> that do support FF effects can be identified by the presence of the
> FF_HAPTIC bit.
> 
> This means:
> - clickpad: INPUT_PROP_BUTTONPAD
> - pressurepad: INPUT_PROP_BUTTONPAD + INPUT_PROP_HAPTIC_TOUCHPAD
> - pressurepad with haptics:
>   INPUT_PROP_BUTTONPAD + INPUT_PROP_HAPTIC_TOUCHPAD + FF_HAPTIC

Should we maybe rename it to INPUT_PROP_PRESSURE_TOUCHPAD? We are within
a release of introducing it, so we should be able to rename it without
much fallout.

Thanks.

-- 
Dmitry