linux-kernel - Re: ALPS Trackpoint & pressure

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Message-ID: <20180208232141.2jxjd2xzkfua42mr@pali>
Date:   Fri, 9 Feb 2018 00:21:41 +0100
From:   Pali Rohár <pali.rohar@...il.com>
To:     Peter Hutterer <peter.hutterer@...-t.net>
Cc:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Masaki Ota <masaki.ota@...alps.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: ALPS Trackpoint & pressure

On Tuesday 06 February 2018 10:29:47 Peter Hutterer wrote:
> On Mon, Feb 05, 2018 at 02:49:55PM -0800, Dmitry Torokhov wrote:
> > On Sun, Feb 04, 2018 at 04:08:39PM +0100, Pali Rohár wrote:
> > > Hi! Now playing again with trackpoint connected to ALPS rushmore
> > > touchpad and I'm seeking a nice feature. Via ALPS PS/2 protocol it
> > > reports pressure of trackpoint. Parser for it is already implemented in
> > > alps.c and value is assigned to variable "z". When I just move
> > > trackpoint z is zero, when I push trackpoint while moving, then z is
> > > number higher, maximally 32. Variable "z" is set, but unused.
> > > 
> > > Do we have some input interface which can be used to report this
> > > pressure of trackpoint to userspace? I can use this feature e.g. as
> > > additional button...
> > 
> > We could either do the conversion in kernel and emit BTN_LEFT, or
> > report ABS_PRESSURE and see if userspace will be able to handle
> > REL_X/REL_Y/ABS_PRESSURE device.
> > 
> > Adding Peter.
> 
> judging by trackpoint history, I'd leave the pressure->click conversion to
> userspace because every trackpoint may need a different threshold setting.
> "easier" to have this in userspace with dmi matches etc. plus, converting to
> BTN_LEFT in the kernel means we cannot use it as a separate interaction
> anymore.

Also BTN_LEFT is already reported when left button under trackpoint is
pressed. Therefore it would not be possible to distinguish between
trackpoint "press" and real left button press.

> That aside, I think exporting ABS_PRESSURE is fine, that's what it's there
> for. Nothing will use it for now though, tbh not sure yet how that would be
> exported from libinput. but worth filing a bug for, please assign it to me.

Ok, so ABS_PRESSURE? Also then question is, how to report minimal and
maximal (possible) value? If we are going to "standardize" API for it,
we should also define min/max values, so userspace would be able to
normalize this pressure event. I can imagine that some devices can
report 8bit value, but ALPS rushmore reports only 5bit value.

-- 
Pali Rohár
pali.rohar@...il.com

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