libinput: Touchpad jitter

Touchpad jitter

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 `src/evdev.c`.

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 with the `--fuzz=<value>` argument.

    $ 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.

Check with `udevadm info /sys/class/input/eventX` (replace your device node number) whether an existing hwdb override exists. If the `EVDEV_ABS_` properties are present, the hwdb overried exists. Find the file that contains that entry, most likely in `/etc/udev/hwdb.d` or `/usr/lib/udev/hwdb.d`.

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 `fuzz` part, leaving the remainder of the property as-is. Values may be empty, e.g. a property that only sets resolution and fuzz reads as `EVDEV_ABS_00=::32:8`.

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

Substitute the `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 should show the new fuzz value for the axes.

Restart the host and libinput should pick up the revised fuzz values.

# Kernel fuzz

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.