linux-kernel - Re: [PATCH] x86/Mouse: thinkpad_acpi/Trackpoint: Trackpoint Doubletap handling

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Message-ID: <a85c0207-880c-53a4-3c52-6c1e13cb1232@linux.intel.com>
Date: Wed, 2 Jul 2025 11:57:39 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
cc: Vishnu Sankar <vishnuocv@...il.com>, 
    Mark Pearson <mpearson-lenovo@...ebb.ca>, pali@...nel.org, 
    Henrique de Moraes Holschuh <hmh@....eng.br>, hansg@...nel.org, 
    tglx@...utronix.de, mingo@...nel.org, jon_xie@...art.com, 
    jay_lee@...art.com, zhoubinbin@...ngson.cn, linux-input@...r.kernel.org, 
    LKML <linux-kernel@...r.kernel.org>, ibm-acpi-devel@...ts.sourceforge.net, 
    "platform-driver-x86@...r.kernel.org" <platform-driver-x86@...r.kernel.org>, 
    Vishnu Sankar <vsankar@...ovo.com>
Subject: Re: [PATCH] x86/Mouse: thinkpad_acpi/Trackpoint: Trackpoint Doubletap
 handling

On Mon, 30 Jun 2025, Dmitry Torokhov wrote:
> On Mon, Jun 30, 2025 at 08:50:27PM +0900, Vishnu Sankar wrote:
> > On Mon, Jun 30, 2025 at 2:20 PM Dmitry Torokhov
> > <dmitry.torokhov@...il.com> wrote:
> > > On Mon, Jun 30, 2025 at 05:42:45AM +0900, Mark Pearson wrote:
> > > > On Fri, Jun 27, 2025, at 2:14 PM, Dmitry Torokhov wrote:
> > > > > On Fri, Jun 20, 2025 at 09:42:08AM +0900, Vishnu Sankar wrote:
> > > > >> Newer ThinkPads have a doubletap feature that needs to be turned
> > > > >> ON/OFF via the trackpoint registers.
> > > > >> Systems released from 2023 have doubletap disabled by default and
> > > > >> need the feature enabling to be useful.
> > > > >>
> > > > >> This patch introduces support for exposing and controlling the
> > > > >> trackpoint doubletap feature via a sysfs attribute.
> > > > >> /sys/devices/platform/thinkpad_acpi/tp_doubletap
> > > > >> This can be toggled by an "enable" or a "disable".
> > > > >>
> > > > >> With this implemented we can remove the masking of events, and rely on
> > > > >> HW control instead, when the feature is disabled.
> > > > >>
> > > > >> Note - Early Thinkpads (pre 2015) used the same register for hysteris
> > > > >> control, Check the FW IDs to make sure these are not affected.
> > > > >>
> > > > >> trackpoint.h is moved to linux/input/.
> > > > >
> > > > > No, please keep everything private to trackpoint.c and do not involve
> > > > > thinkpad_acpi driver. By doing so you are introducing unwanted
> > > > > dependencies (for both module loading, driver initialization, and
> > > > > operation) and unsafe use of non-owned pointers/dangling pointers, etc.
> > > > >
> > > >
> > > > Do you have recommendations on how to handle this case then?
> > > >
> > > > This is a Thinkpad specific feature and hence the logic for involving
> > > > thinkpad_acpi. There are Thinkpad hotkeys that will enable/disable the
> > > > trackpoint doubletap feature - so there is some linkage. I'm not sure
> > > > how to avoid that.
> > > >
> > > > Is there a cleaner way to do this that you'd recommend we look at
> > > > using? It's a feature (albeit a minor one) on the laptops that we'd
> > > > like to make available to Linux users.
> > >
> > > I believe if you define the doubletap as an attribute (see
> > > TRACKPOINT_INT_ATTR or TRACKPOINT_BIT_ATTR in
> > > drivers/input/mouse/trackpoint.c) then whatever process is handling the
> > > hot keys switching this function on or off should be able to toggle the
> > > behavior. The difference is that it will have to locate trackpoint node
> > > in /sys/bus/serio/devices/* (or maybe scan
> > > /sys/devices/platform/i8042/serio*) instead of expecting the attributes
> > > be atached to thinkpad_acpi instance.
> > >
> > > You just don't want to have one driver directly peeking into another,
> > > because then it starts breaking if you unbind or force use of a
> > > different protocol, etc.
> > >
> > > Thanks.
> > >
> > > --
> > > Dmitry
> > 
> > Thanks for the suggestion. I understand the concern about avoiding
> > direct driver-to-driver calls and unwanted dependencies.
> > 
> > Just to clarify: if we move the sysfs attribute to the trackpoint
> > driver itself (under /sys/bus/serio/devices/...), then thinkpad_acpi
> > would no longer be able to directly enable/disable the doubletap
> > feature in response to the Fn+G hotkey press. Don't we need userspace
> > to listen for the hotkey event, find the trackpoint sysfs node, and
> > toggle the attribute there?
> 
> Yes.
> 
> > That's possible, of course, but it means the feature won't work
> > out-of-the-box without extra userspace integration. For example, there
> > would be no automatic linkage between pressing Fn+G and toggling the
> > feature unless a udev rule or userspace daemon is configured to do it.
> > Or is there an approach you'd recommend to preserve the automatic
> > hotkey integration while avoiding the direct dependency between
> > thinkpad_acpi and trackpoint?
> > Sorry, I missed something.
> 
> Well, I guess you can look into interacting with sysfs file from
> thinkpad_acpi.c... There is kernel_read_file_from_path() and others, you
> will need to implement write counterpart of it. Pretty ugly but safer
> than following pointers that may go away.

Could device links be used here to ensure the correct shutdown order?

-- 
 i.