Touchpad jitter describes random movement by a few pixels even when the user’s finger is unmoving.
libinput has a mechanism called a hysteresis to avoid that jitter. When
active, movement with in the hysteresis margin is discarded. If the
movement delta is larger than the margin, the movement is passed on as
pointer movement. This is a simplified summary, developers should
read the implementation of the hysteresis in
libinput uses the kernel
fuzz value to determine the size of the
hysteresis. Users should override this with a udev hwdb entry where the
device itself does not provide the correct value.
Overriding the hysteresis margins
libinput provides the debugging tool
libinput measure fuzz to help edit or
test a fuzz value. This tool is interactive and provides a udev hwdb entry
that matches the device. To check if a fuzz is currently present, simply run
without arguments or with the touchpad’s device node:
$ sudo libinput measure fuzz Using Synaptics TM2668-002: /dev/input/event17 Checking udev property... not set Checking axes... x=16 y=16
In the above output, the axis fuzz is set to 16. To set a specific fuzz, run
$ sudo libinput measure fuzz --fuzz=8
The tool will attempt to construct a hwdb file that matches your touchpad device. Follow the printed prompts.
In the ideal case, the tool will provide you with a file that can be submitted to the systemd repo for inclusion.
However, hwdb entry creation is difficult to automate and it’s likely that the tools fails in doing so, especially if an existing entry is already present.
Below is the outline of what a user needs to do to override a device’s fuzz
value in case the
libinput measure fuzz tool fails.
udevadm info /sys/class/input/eventX (replace your device node
number) whether an existing hwdb override exists. If the
properties are present, the hwdb override exists. Find the file that
contains that entry, most likely in
The content of the property is a set of values in the format
EVDEV_ABS_00=min:max:resolution:fuzz. You need to set the
leaving the remainder of the property as-is. Values may be empty, e.g. a
property that only sets resolution and fuzz reads as
If no properties exist, your hwdb.entry should look approximately like this:
evdev:name:Synaptics TM2668-002:dmi:*:svnLENOVO*:pvrThinkPadT440s*: EVDEV_ABS_00=:::8 EVDEV_ABS_01=:::8 EVDEV_ABS_35=:::8 EVDEV_ABS_36=:::8
name field with the device name (see the output of
libinput measure fuzz and the DMI match content with your hardware. See
Modifying the hwdb for details.
Once the hwdb entry has been modified, added, or created, reload the hwdb. Once reloaded, libinput record and libinput replay “libinput record” should show the new fuzz value for the axes.
Restart the host and libinput should pick up the revised fuzz values.
A fuzz set on an absolute axis in the kernel causes the kernel to apply
hysteresis-like behavior to the axis. Unfortunately, this behavior leads to
inconsistent deltas. To avoid this, libinput sets the kernel fuzz on the
device to 0 to disable this kernel behavior but remembers what the fuzz was
on startup. The fuzz is stored in the
LIBINPUT_FUZZ_XX udev property, on
startup libinput will check that property as well as the axis itself.