On Wed, Jan 13, 2021 at 12:03 AM 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 that axis no longer represents contact size, resulting in inconsistent touch 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> FWIW, and because I was involved in the initial discussion: Acked-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx> Cheers, Benjamin > --- > Documentation/input/event-codes.rst | 15 +++++++++++++++ > Documentation/input/multi-touch-protocol.rst | 4 ++++ > 2 files changed, 19 insertions(+) > > diff --git a/Documentation/input/event-codes.rst b/Documentation/input/event-codes.rst > index b24b5343f5eb..3118fc1c1e26 100644 > --- a/Documentation/input/event-codes.rst > +++ b/Documentation/input/event-codes.rst > @@ -236,6 +236,21 @@ A few EV_ABS codes have special meanings: > - Used to describe multitouch input events. Please see > multi-touch-protocol.txt for details. > > +* ABS_PRESSURE/ABS_MT_PRESSURE: > + > + - For touch devices, many devices converted contact size into pressure. > + A finger flattens with pressure, causing a larger contact area and thus > + pressure and contact size are directly related. This is not the case > + for other devices, for example digitizers and touchpads with a true > + pressure sensor ("pressure pads"). > + > + A device should set the resolution of the axis to indicate whether the > + pressure is in measurable units. If the resolution is zero, the > + pressure data is in arbitrary units. If the resolution is nonzero, the > + pressure data is in units/gram. For example, a value of 10 with a > + resolution of 1 represents 10 gram, a value of 10 with a resolution on > + 1000 represents 10 microgram. > + > EV_SW > ----- > > diff --git a/Documentation/input/multi-touch-protocol.rst b/Documentation/input/multi-touch-protocol.rst > index 307fe22d9668..21c1e6a22888 100644 > --- a/Documentation/input/multi-touch-protocol.rst > +++ b/Documentation/input/multi-touch-protocol.rst > @@ -260,6 +260,10 @@ ABS_MT_PRESSURE > of TOUCH and WIDTH for pressure-based devices or any device with a spatial > signal intensity distribution. > > + If the resolution is zero, the pressure data is in arbitrary units. > + If the resolution is nonzero, the pressure data is in units/gram. See > + :ref:`input-event-codes` for details. > + > ABS_MT_DISTANCE > The distance, in surface units, between the contact and the surface. Zero > distance means the contact is touching the surface. A positive number means > -- > 2.29.2 >
