libinput and the Lenovo x220 touchpad - after a firmware update to version 8.1

This post only applies to users of the Lenovo x220 laptop experiencing issues when using the touchpad. Specifically, the touchpad is imprecise and "jumpy" after a firmware update, as outlined in Fedora bug 1264453. The cause is buggy touchpad firmware, identifiable by the string "fw: 8.1" in the dmesg output for the touchpad:

[  +0.005261] psmouse serio1: synaptics: Touchpad model: 1, fw: 8.1, 
id: 0x1e2b1, caps: 0xd002a3/0x940300/0x123800, board id: 1611, fw id: 1099905

If you are experiencing these touchpad issues and your dmesg shows the 8.1 firmware version, please read on for a solution. By default, the x220 shipped with version 8.0 so unless you updated the firmware as part of a Lenovo update, you are not affected by this bug.

The touchpad issues seem identical as the ones seen on the Lenovo x230 model which has the same physical hardware and also ships with a firmware version 8.1. The root cause as seen by libinput is that the touchpad only sends events once the finger moves approximately 50 device units in either direction. The touchpad advertises a resolution of 65 units/mm horizontally and 136 units/mm vertically, but the effective resolution is reduced by roughly 75% and 30% This bugzilla attachment 1082925 shows the recording, you can easily see that while the pressure is upgraded with high granularity, the motion coordinates jump from one position to the next. From what we know this was introduced by the touchpad firmware v8.1, presumably as part of a filter to reduce the jitter some x230 users saw.

libinput automatically detects the x230 and enables a custom acceleration function for just that model. That same acceleration function works for the x220 v8.1, but unfortunately we cannot automatically detect it. As of libinput 1.1.3, libinput recognises a special udev tag, LIBINPUT_MODEL_LENOVO_X220_TOUCHPAD_FW81, to mark such an updated x220 and enable a better pointer behaviour. To apply this tag, please do the following:

1. Create a new file /etc/udev/hwdb.d/90-libinput-x220-fw8.1.hwdb
2. Look for X220 in the 90-libinput-model-quirks.hwdb file, copy the match and the property assignment into the file. As of the time of writing, the two lines are as below, but make sure you take the latest from your locally installed libinput version or the link above.

   libinput:name:SynPS/2 Synaptics TouchPad:dmi:*svnLENOVO:*:pvrThinkPadX220*
    LIBINPUT_MODEL_LENOVO_X220_TOUCHPAD_FW81=1
   

3. Update the udev hwdb with sudo udevadm hwdb --update
4. Verify the tag shows up with sudo udevadm test /sys/class/input/event4 (adjust the event node if necessary)
5. Reboot

The touchpad is now marked as requiring special treatment and libinput will apply a different pointer acceleration for this touchpad.

Note that any udev property starting with LIBINPUT_MODEL_ is private API and subject to change at any time. We will never break the meaning of the LIBINPUT_MODEL_LENOVO_X220_TOUCHPAD_FW81 property, but the exact behaviour of the property is implementation-dependent and may change at any time. Do not use it for any other purpose than marking the touchpad on a Lenovo x220 with an updated touchpad firmware version v8.1.