linux-kernel - Re: [PATCH v3 1/3] input: mouse: trackpoint: Add doubletap enable/disable support

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Message-ID: <fgizjuhcnl6rfawrqnabkhijf5uzbczen6k5zp6z44hycjqylx@6qzi54baxivx>
Date: Thu, 18 Sep 2025 18:12:27 -0700
From: Dmitry Torokhov <dmitry.torokhov@...il.com>
To: Hans de Goede <hansg@...nel.org>
Cc: Vishnu Sankar <vishnuocv@...il.com>, hmh@....eng.br, 
	ilpo.jarvinen@...ux.intel.com, derekjohn.clark@...il.com, mpearson-lenovo@...ebb.ca, 
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org, 
	ibm-acpi-devel@...ts.sourceforge.net, platform-driver-x86@...r.kernel.org, vsankar@...ovo.com
Subject: Re: [PATCH v3 1/3] input: mouse: trackpoint: Add doubletap
 enable/disable support

On Thu, Sep 18, 2025 at 09:31:47AM +0200, Hans de Goede wrote:
> Hi Vishnu,
> 
> On 18-Sep-25 4:37 AM, Vishnu Sankar wrote:
> > Hello all,
> > 
> > Do we have any questions or concerns?
> > Thanks in advance!
> > 
> > On Mon, Sep 1, 2025 at 10:53 PM Vishnu Sankar <vishnuocv@...il.com> wrote:
> >>
> >> Add support for enabling and disabling doubletap on TrackPoint devices
> >> that support this functionality. The feature is detected using firmware
> >> ID and exposed via sysfs as `doubletap_enabled`.
> 
> Hmm, you seem to be using a firmware ID prefix match, combined with
> a deny list of some firmware IDs with that prefix which do not support
> this. How do we know this deny list is complete?
> 
> Also as Dmitry says you really should use the is_visible() callback
> to not show the attribute at all on unsupported systems.
> 
> >> The feature is only available on newer ThinkPads (2023 and later).The driver
> >> exposes this capability via a new sysfs attribute:
> >> "/sys/bus/serio/devices/seriox/doubletap_enabled".
> >>
> >> The attribute is only created if the device is detected to be capable of
> >> doubletap via firmware and variant ID checks. This functionality will be
> >> used by platform drivers such as thinkpad_acpi to expose and control doubletap
> >> via user interfaces.
> 
> Hmm, you refer to thinkpad_acpi as a possible consumer of this
> functionality. But you only add a sysfs interface.
> 
> thinkpad_acpi will need some in kernel interface to use this.
> 
> Which brings me to my main question: thinkpad_acpi is the driver
> receiving the doubletap events since these are send out-of-bound
> and not through the ps/2 trackpoint protocol.
> 
> thinkpad_acpi already has the capability to filter out these doubletap
> events and report nothing. Why is it necessary / better to disable
> the doubletap at the trackpoint fw-level, rather then just filtering
> it at the thinkpad_acpi level ?
> 
> I don't really see a big advantage in filtering these events at
> the fw-level rather then in the kernel and we already have the
> in kernel filtering.

That is an excellent observation, thank you Hans. The frequency of these
events should be extremely low, so cost of simply ignoring events should
be miniscule...

Thanks.

-- 
Dmitry