Touchpad pressure-based touch detection


libinput uses the touchpad pressure values and/or touch size values to detect wether a finger has been placed on the touchpad.

This is Information provided by the kernel and combines with a libinput-specific hardware database to adjust the thresholds on a per-device basis. libinput uses these thresholds primarily to filter out accidental light touches but the information is also used for some Palm detection.

Pressure and touch size thresholds are not directly configurable by the user. Instead, libinput provides these thresholds for each device where necessary. See Debugging touchpad pressure ranges for instructions on how to adjust the pressure ranges and Debugging touch size ranges for instructions on how to adjust the touch size ranges.

Information provided by the kernel

The kernel sends multiple values to inform userspace about a finger touching the touchpad. The most basic is the EV_KEY/BTN_TOUCH boolean event that simply announces physical contact with the touchpad. The decision when this event is sent is usually made by the kernel driver and may depend on device-specific thresholds. These thresholds are transparent to userspace and cannot be modified. On touchpads where pressure or touch size is not available, libinput uses BTN_TOUCH to determine when a finger is logically down.

Many contemporary touchpad devices provide an absolute pressure axis in addition to BTN_TOUCH. This pressure generally increases as the pressure increases, however few touchpads are capable of detecting true pressure. The pressure value is usually related to the covered area - as the pressure increases a finger flattens and thus covers a larger area. The range provided by the kernel is not mapped to a specific physical range and often requires adjustment. Pressure is sent by the ABS_PRESSURE axis for single-touch touchpads or ABS_MT_PRESSURE on multi-touch capable touchpads. Some devices can detect multiple fingers but only provide ABS_PRESSURE.

Some devices provide additional touch size information through the ABS_MT_TOUCH_MAJOR/ABS_MT_TOUCH_MINOR axes and/or the ABS_MT_WIDTH_MAJOR/ABS_MT_WIDTH_MINOR axes. These axes specifcy the size of the touch ellipse. While the kernel documentation specifies how these axes are supposed to be mapped, few devices forward reliable information. libinput uses these values together with a device-specific Initialization and manipulation of input devices quirks entry. In other words, touch size detection does not work unless a device quirk is present for the device.

Debugging touchpad pressure ranges

This section describes how to determine the touchpad pressure ranges required for a touchpad device and how to add the required Device quirks locally. Note that the quirk is not public API and may change at any time. Users are advised to report a bug with the updated pressure ranges when testing has completed.

Use the libinput measure touchpad-pressure tool provided by libinput. This tool will search for your touchpad device and print some pressure statistics, including whether a touch is/was considered logically down.

Note
This tool will only work on touchpads with pressure.

Example output of the tool is below:

$ sudo libinput measure touchpad-pressure
Ready for recording data.
Pressure range used: 8:10
Palm pressure range used: 65535
Place a single finger on the touchpad to measure pressure values.
Ctrl+C to exit
 
Sequence 1190 pressure: min:  39 max:  48 avg:  43 median:  44 tags: down
Sequence 1191 pressure: min:  49 max:  65 avg:  62 median:  64 tags: down
Sequence 1192 pressure: min:  40 max:  78 avg:  64 median:  66 tags: down
Sequence 1193 pressure: min:  36 max:  83 avg:  70 median:  73 tags: down
Sequence 1194 pressure: min:  43 max:  76 avg:  72 median:  74 tags: down
Touchpad pressure:  47 min:  47 max:  86 tags: down

The example output shows five completed touch sequences and one ongoing one. For each, the respective minimum and maximum pressure values are printed as well as some statistics. The tags show that sequence was considered logically down at some point. This is an interactive tool and its output may change frequently. Refer to the libinput-measure-touchpad-pressure(1) man page for more details.

By default, this tool uses the Device quirks for the pressure range. To narrow down on the best values for your device, specify the 'logically down' and 'logically up' pressure thresholds with the --touch-thresholds argument:

$ sudo libinput measure touchpad-pressure --touch-thresholds=10:8 --palm-threshold=20

Interact with the touchpad and check if the output of this tool matches your expectations.

Note
This is an interactive process. You will need to re-run the tool with varying thresholds until you find the right range for your touchpad. Attaching output logs to a bug will not help, only you with access to the hardware can figure out the correct ranges.

Once the thresholds are decided on (e.g. 10 and 8), they can be enabled with Device quirks entry similar to this:

$> cat /etc/libinput/local-overrides.quirks
[Touchpad pressure override]
MatchUdevType=touchpad
MatchName=*SynPS/2 Synaptics TouchPad
MatchDMIModalias=dmi:*svnLENOVO:*:pvrThinkPadX230*
AttrPressureRange=10:8

The file name must be /etc/libinput/local-overrides.quirks. The The first line is the section name and can be free-form. The Match directives limit the quirk to your touchpad, make sure the device name matches your device's name (see libinput record's output). The dmi modalias match should be based on the information in /sys/class/dmi/id/modalias. This modalias should be shortened to the specific system's information, usually system vendor (svn) and product name (pn).

Once in place, run the following command to verify the quirk is valid and works for your device:

$ sudo libinput list-quirks /dev/input/event10
AttrPressureRange=10:8

Replace the event node with the one from your device. If the AttrPressureRange quirk does not show up, re-run with --verbose and check the output for any error messages.

If the pressure range quirk shows up correctly, restart X or the Wayland compositor and libinput should now use the correct pressure thresholds. The Helper tools can be used to verify the correct functionality first without the need for a restart.

Once the pressure ranges are deemed correct, report a bug to get the pressure ranges into the repository.

Debugging touch size ranges

This section describes how to determine the touchpad size ranges required for a touchpad device and how to add the required Device quirks locally. Note that the quirk is not public API and may change at any time. Users are advised to report a bug with the updated pressure ranges when testing has completed.

Use the libinput measure touch-size tool provided by libinput. This tool will search for your touchpad device and print some touch size statistics, including whether a touch is/was considered logically down.

Note
This tool will only work on touchpads with the ABS_MT_MAJOR axis.

Example output of the tool is below:

$ sudo libinput measure touch-size --touch-thresholds 10:8 --palm-threshold 14
Using ELAN Touchscreen: /dev/input/event5
 
Ready for recording data.
Touch sizes used: 10:8
Palm size used: 14
Place a single finger on the device to measure touch size.
Ctrl+C to exit
 
Sequence: major: [  9.. 11] minor: [  7..  9]
Sequence: major: [  9.. 10] minor: [  7..  7]
Sequence: major: [  9.. 14] minor: [  6..  9]  down
Sequence: major: [ 11.. 11] minor: [  9..  9]  down
Sequence: major: [  4.. 33] minor: [  1..  5]  down palm

The example output shows five completed touch sequences. For each, the respective minimum and maximum pressure values are printed as well as some statistics. The down and palm tags show that sequence was considered logically down or a palm at some point. This is an interactive tool and its output may change frequently. Refer to the libinput-measure-touch-size(1) man page for more details.

By default, this tool uses the Device quirks for the touch size range. To narrow down on the best values for your device, specify the 'logically down' and 'logically up' pressure thresholds with the --touch-thresholds arguments as in the example above.

Interact with the touchpad and check if the output of this tool matches your expectations.

Note
This is an interactive process. You will need to re-run the tool with varying thresholds until you find the right range for your touchpad. Attaching output logs to a bug will not help, only you with access to the hardware can figure out the correct ranges.

Once the thresholds are decided on (e.g. 10 and 8), they can be enabled with Device quirks entry similar to this:

$> cat /etc/libinput/local-overrides.quirks
[Touchpad touch size override]
MatchUdevType=touchpad
MatchName=*SynPS/2 Synaptics TouchPad
MatchDMIModalias=dmi:*svnLENOVO:*:pvrThinkPadX230*
AttrTouchSizeRange=10:8

The first line is the match line and should be adjusted for the device name (see evemu-record's output) and for the local system, based on the information in /sys/class/dmi/id/modalias. The modalias should be shortened to the specific system's information, usually system vendor (svn) and product name (pn).

Once in place, run the following command to verify the quirk is valid and works for your device:

$ sudo libinput list-quirks /dev/input/event10
AttrTouchSizeRange=10:8

Replace the event node with the one from your device. If the AttrTouchSizeRange quirk does not show up, re-run with --verbose and check the output for any error messages.

If the touch size range property shows up correctly, restart X or the Wayland compositor and libinput should now use the correct thresholds. The Helper tools can be used to verify the correct functionality first without the need for a restart.

Once the touch size ranges are deemed correct, report abug" to get the thresholds into the repository.