On Wed, 13 Jan 2021 09:03:10 +1000 Peter Hutterer <peter.hutterer@xxxxxxxxx> 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 that axis no longer represents contact size, resulting in inconsistent touch 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@xxxxxxxxx> > --- > 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
