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Well, I say "measure" but really at this point it just reads the properties/axes and then does it's best to auto-generate a hwdb entry that matches the user's hardware and sets a fuzz value on the device. Ideally this reduces the number of hand-holding required in bugzillas. There are plenty of things that can go wrong, so our fallback is still to throw up our hands and point to the documentation. https://bugs.freedesktop.org/show_bug.cgi?id=98839 Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
90 lines
3.6 KiB
Text
90 lines
3.6 KiB
Text
/**
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@page touchpad_jitter Touchpad jitter
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Touchpad jitter describes random movement by a few pixels even when the
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user's finger is unmoving.
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libinput has a mechanism called a **hysteresis** to avoid that jitter. When
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active, movement with in the **hysteresis margin** is discarded. If the
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movement delta is larger than the margin, the movement is passed on as
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pointer movement. This is a simplified summary, developers should
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read the implementation of the hysteresis in `src/evdev.c`.
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libinput uses the kernel `fuzz` value to determine the size of the
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hysteresis. Users should override this with a udev hwdb entry where the
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device itself does not provide the correct value.
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@section touchpad_jitter_fuzz_override Overriding the hysteresis margins
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libinput provides the debugging tool `libinput measure fuzz` to help edit or
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test a fuzz value. This tool is interactive and provides a udev hwdb entry
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that matches the device. To check if a fuzz is currently present, simply run
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without arguments or with the touchpad's device node:
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@verbatim
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$ sudo libinput measure fuzz
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Using Synaptics TM2668-002: /dev/input/event17
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Checking udev property... not set
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Checking axes... x=16 y=16
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@endverbatim
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In the above output, the axis fuzz is set to 16. To set a specific fuzz, run
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with the `--fuzz=<value>` argument.
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@verbatim
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$ sudo libinput measure fuzz --fuzz=8
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@endverbatim
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The tool will attempt to construct a hwdb file that matches your touchpad
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device. Follow the printed prompts.
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In the ideal case, the tool will provide you with a file that can be
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submitted to the systemd repo for inclusion.
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However, hwdb entry creation is difficult to automate and it's likely
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that the tools fails in doing so, especially if an existing entry is already
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present.
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Below is the outline of what a user needs to do to override a device's fuzz
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value in case the `libinput measure fuzz` tool fails.
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Check with `udevadm info /sys/class/input/eventX` (replace your device node
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number) whether an existing hwdb override exists. If the `EVDEV_ABS_`
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properties are present, the hwdb overried exists. Find the file that
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contains that entry, most likely in `/etc/udev/hwdb.d` or
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`/usr/lib/udev/hwdb.d`.
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The content of the property is a set of values in the format
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`EVDEV_ABS_00=min:max:resolution:fuzz`. You need to set the `fuzz` part,
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leaving the remainder of the property as-is. Values may be empty, e.g. a
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property that only sets resolution and fuzz reads as `EVDEV_ABS_00=::32:8`.
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If no properties exist, your hwdb.entry should look approximately like this:
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@verbatim
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evdev:name:Synaptics TM2668-002:dmi:*:svnLENOVO*:pvrThinkPadT440s*:
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EVDEV_ABS_00=:::8
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EVDEV_ABS_01=:::8
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EVDEV_ABS_35=:::8
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EVDEV_ABS_36=:::8
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@endverbatim
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Substitute the `name` field with the device name (see the output of
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`libinput measure fuzz` and the DMI match content with your hardware. See
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@ref hwdb_modifying for details.
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Once the hwdb entry has been modified, added, or created, @ref
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hwdb_reloading "reload the hwdb". Once reloaded, @ref libinput-record
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"libinput record" should show the new fuzz value for the axes.
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Restart the host and libinput should pick up the revised fuzz values.
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@section kernel_fuzz Kernel fuzz
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A fuzz set on an absolute axis in the kernel causes the kernel to apply
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hysteresis-like behavior to the axis. Unfortunately, this behavior leads to
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inconsistent deltas. To avoid this, libinput sets the kernel fuzz on the
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device to 0 to disable this kernel behavior but remembers what the fuzz was
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on startup. The fuzz is stored in the `LIBINPUT_FUZZ_XX` udev property, on
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startup libinput will check that property as well as the axis itself.
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*/
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