xf86-input-wacom 0.34 workaround for pressure range bugs

Back in 2003, the pressure range value for Wacom pen tablets was set to 2048. That's a #define in the driver, but it shouldn't matter because we also advertise this range as part of the device description in the X Input protocol. Clients should be using that advertised min/max range and scale appropriately, in the same way as they should be doing this for the x/y axis ranges.

Fast-forward to 2017 and we changed the pressure range. New Wacom devices now use ~8000 levels, but we opted to just #define it in the driver to 65536 and be done with it. We now scale all models into that range, with varying granularity based on the physical hardware. It shouldn't matter because it's not tied to a reliable physical property anyway and the only thing that matters is the percentage of the max value (which is why libinput just gives you a [0, 1] range. Hindsight is a bliss.).

Slow-forward to 2017-and-a-bit and we received complaints that pressure handling is now broken. Turns out that some applications hardcoded the old 2048 range and now get confused, because virtually any pressure will now hit that maximum. Since those applications are largely proprietary ones and cannot be updated easily, we needed a workaround to this. Jason Gerecke from Wacom got busy and we now have a "Pressure2K" option available in the driver. If set, this option will scale everything into the 2048 range to make sure those applications still work. To get this to work, the following xorg.conf.d snippet is recommended:

Section "InputClass"
    Identifier "Wacom pressure compatibility"
    MatchDriver "wacom"
    Option "Pressure2K" "true"
EndSection

Put it in a file that sorts higher than the wacom driver itself (e.g. /etc/X11/xorg.conf.d/99-wacom-pressure2k.conf) and restart X. Check the Xorg.log/journal for a "Using 2K pressure levels" message, then verify it works by running xinput list "device name". xinput should show a range of 0 to 2048 on the third valuator.

$>  xinput list "Wacom Intuos4 6x9 Pen stylus"
Wacom Intuos4 6x9 Pen stylus                    id=25   [slave  pointer  (2)]
        Reporting 8 classes:
                Class originated from: 25. Type: XIButtonClass
                Buttons supported: 9
                Button labels: None None None None None None None None None
                Button state:
                Class originated from: 25. Type: XIKeyClass
                Keycodes supported: 248
                Class originated from: 25. Type: XIValuatorClass
                Detail for Valuator 0:
                  Label: Abs X
                  Range: 0.000000 - 44704.000000
                  Resolution: 200000 units/m
                  Mode: absolute
                  Current value: 22340.000000
                Class originated from: 25. Type: XIValuatorClass
                Detail for Valuator 1:
                  Label: Abs Y
                  Range: 0.000000 - 27940.000000
                  Resolution: 200000 units/m
                  Mode: absolute
                  Current value: 13970.000000
                Class originated from: 25. Type: XIValuatorClass
                Detail for Valuator 2:
                  Label: Abs Pressure
                  Range: 0.000000 - 2048.000000
                  Resolution: 1 units/m
                  Mode: absolute
                  Current value: 0.000000
                Class originated from: 25. Type: XIValuatorClass
                Detail for Valuator 3:
                  Label: Abs Tilt X
                  Range: -64.000000 - 63.000000
                  Resolution: 57 units/m
                  Mode: absolute
                  Current value: 0.000000
                Class originated from: 25. Type: XIValuatorClass
                Detail for Valuator 4:
                  Label: Abs Tilt Y
                  Range: -64.000000 - 63.000000
                  Resolution: 57 units/m
                  Mode: absolute
                  Current value: 0.000000
                Class originated from: 25. Type: XIValuatorClass
                Detail for Valuator 5:
                  Label: Abs Wheel
                  Range: -900.000000 - 899.000000
                  Resolution: 1 units/m
                  Mode: absolute
                  Current value: 0.000000

This is an application bug, but this workaround will make sure new versions of the driver can be used until those applications have been fixed. The option will be available in the soon-to-be-released xf86-input-wacom 0.35. The upstream commit is d958ab79d21b57141415650daac88f9369a1c861.

Edit 02/06/2017: wacom devices have 8k pressure levels now.