Re: [PATCH] Documentation: input: define ABS_PRESSURE/ABS_MT_PRESSURE resolution as grams

[Jonathan Corbet <corbet@lwn.net>]

On Wed, 13 Jan 2021 09:03:10 +1000
Peter Hutterer <peter.hutterer@who-t.net> wrote:

> ABS_PRESSURE and ABS_MT_PRESSURE on touch devices usually represent
> contact size (as a finger flattens with higher pressure the contact size
> increases) and userspace translates the kernel pressure value back into
> contact size. For example, libinput has pressure thresholds when a touch is
> considered a palm (palm == large contact area -> high pressure). The values
> themselves are on an arbitrary scale and device-specific.
> 
> On pressurepads however, the pressure axis may represent the real physical
> pressure. Pressurepads are touchpads without a hinge but an actual pressure
> sensor underneath the device instead, for example the Lenovo Yoga 9i.
> 
> A high-enough pressure is converted to a button click by the firmware.
> Microsoft does not require a pressure axis to be present, see [1], so as seen
> from userspace most pressurepads are identical to clickpads - one button and
> INPUT_PROP_BUTTONPAD set.
> 
> However, pressurepads that export the pressure axis break userspace because
> that axis no longer represents contact size, resulting in inconsistent touch
> tracking, e.g. [2]. Userspace needs to know when a pressure axis represents
> real pressure and the best way to do so is to define what the resolution
> field means. Userspace can then treat data with a pressure resolution as
> true pressure.
> 
> This patch documents that the pressure resolution is in units/gram. This
> allows for fine-grained detail and tops out at roughly ~2000t, enough for the
> devices we're dealing with. Grams is not a scientific pressure unit but the
> alternative is:
> - Pascal: defined as force per area and area is unreliable on many devices and
>   seems like the wrong option here anyway, especially for devices with a
>   single pressure sensor only.
> - Newton: defined as mass * distance/acceleration and for the purposes of a
>   pressure axis, the distance is tricky to interpret and we get the data to
>   calculate acceleration from event timestamps anyway.
> 
> For the purposes of touch devices and digitizers, grams seems the best choice
> and the easiest to interpret.
> 
> Bonus side effect: we can use the existing hwdb infrastructure in userspace to
> fix devices that advertise false pressure.
> 
> [1] https://docs.microsoft.com/en-us/windows-hardware/design/component-guidelines/windows-precision-touchpad-required-hid-top-level-collections#windows-precision-touchpad-input-reports
> [2] https://gitlab.freedesktop.org/libinput/libinput/-/issues/562
> 
> Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
> ---
>  Documentation/input/event-codes.rst          | 15 +++++++++++++++
>  Documentation/input/multi-touch-protocol.rst |  4 ++++
>  2 files changed, 19 insertions(+)

It looks like nobody has picked this up, so I went ahead and applied it.

Thanks,

jon