libinput: Button debouncing

Button debouncing

Physical buttons experience wear-and-tear with usage.

On some devices this can result in an effect called "contact bouncing" or "chatter". This effect can cause the button to send multiple events within a short time frame, even though the user only pressed or clicked the button once. This effect can be counteracted by "debouncing" the buttons, usually by ignoring erroneous events.

libinput provides two methods of debouncing buttons, referred to as the "bounce" and "spurious" methods:

- In the "bounce" method, libinput monitors hardware bouncing on button state changes, i.e. when a user clicks or releases a button. For example, if a user presses a button but the hardware generates a press-release-press sequence in quick succession, libinput ignores the release and second press event. This method is always enabled.
- in the "spurious" method, libinput detects spurious releases of a button while the button is physically held down by the user. These releases are immediately followed by a press event. libinput monitors for these events and ignores the release and press event. This method is disabled by default and enables once libinput detects the first faulty event sequence.

The "bounce" method guarantees that all press events are delivered immediately and most release events are delivered immediately. The "spurious" method requires that release events are delayed, libinput thus does not enable this method unless a faulty event sequence is detected. A message is printed to the log when spurious deboucing was detected.

Note that libinput's debouncing intended to correct hardware damage or substandard hardware. Debouncing is also used as an accessibility feature but the requirements are different. In the accessibility feature, multiple physical key presses, usually caused by involuntary muscle movement, must be filtered to only one key press. This feature must be implemented higher in the stack, libinput is limited to hardware debouncing.

Below is an illustration of the button debouncing modes to show the relation of the physical button state and the application state. Where applicable, an extra line is added to show the timeouts used by libinput that affect the button state handling. The waveform's high and low states correspond to the buttons 'pressed' and 'released' states, respectively.

Diagram illustrating button debouncing