What is libinput?
This page describes what libinput is, but more importantly it also describes what libinput is not.
What libinput is
libinput is an input stack for processes that need to provide events from commonly used input devices. That includes mice, keyboards, touchpads, touchscreens and graphics tablets. libinput handles device-specific quirks and provides an easy-to-use API to receive events from devices.
libinput is designed to handle all input devices available on a system but it is possible to limit which devices libinput has access to. For example, the use of xf86-input-libinput depends on xorg.conf snippets for specific devices. But libinput works best if it handles all input devices as this allows for smarter handling of features that affect multiple devices.
libinput restricts device-specific features to those devices that require those features. One example for this are the top software buttons on the touchpad in the Lenovo T440. While there may be use-cases for providing top software buttons on other devices, libinput does not do so.
This introductory blog post from 2015 describes some of the motivations.
What libinput is not
libinput is not a project to support experimental devices. Unless a device is commonly available off-the-shelf, libinput will not support this device. libinput can serve as a useful base for getting experimental devices enabled and reduce the amount of boilerplate required. But such support will not land in libinput master until the devices are commonly available.
libinput is not a box of legos. It does not provide the pieces to assemble a selection of features. Many features can be disabled through configuration options, but some features are hardcoded and/or only available on some devices. There are plenty of use-cases to provide niche features, but libinput is not the place to support these.
libinput is not a showcase for features. There are a lot of potential features that could be provided on input devices. But unless they have common usage, libinput is not the place to implement them. Every feature multiplies the maintenance effort, any feature that is provided but unused is a net drain on the already sparse developer resources libinput has available.
libinput is boring. It does not intend to break new grounds on how devices are handled. Instead, it takes best practice and the common use-cases and provides it in an easy-to-consume package for compositors or other processes that need those interactions typically expected by users.
libinput and Wayland
libinput is not used directly by Wayland applications, it is an input stack used by the compositor. The typical software stack for a system running Wayland is:
The Wayland compositor may be Weston, mutter, KWin, etc. Note that Wayland encourages the use of toolkits, so the Wayland client (your application) does not usually talk directly to the compositor but rather employs a toolkit (e.g. GTK) to do so. The Wayland client does not know whether libinput is in use.
libinput is not a requirement for Wayland or even a Wayland compositor. There are some specialized compositors that do not need or want libinput.
libinput and X.Org
libinput is not used directly by X applications but rather through the custom xf86-input-libinput driver. The simplified software stack for a system running X.Org is:
libinput is not employed directly by the X server but by the xf86-input-libinput driver instead. That driver is loaded by the server on demand, depending on the xorg.conf.d configuration snippets. The X client does not know whether libinput is in use.
libinput and xf86-input-libinput are not a requirement, the driver will only handle those devices explicitly assigned through an xorg.conf.d snippets. It is possible to mix xf86-input-libinput with other X.Org drivers.
libinput handles all common devices used to interact with a desktop system. This includes mice, keyboards, touchscreens, touchpads and graphics tablets. libinput does not expose the device type to the caller, it solely provides capabilities and the attached features (see this blog post).
For example, a touchpad in libinput is a device that provides pointer events, gestures and has a number of Configuration options such as Tap-to-click behaviour. A caller may present the device as touchpad to the user, or simply as device with a config knob to enable or disable tapping.
Handled device types
Virtual absolute pointing devices such as those used by QEMU or VirtualBox
Switches (Lid Switch and Tablet Mode switch)
If a device falls into one of the above categories but does not work as expected, please file a bug.
Unhandled device types
libinput does not handle some devices. The primary reason is that these device have no clear interaction with a desktop environment.
Joysticks have one or more axes and one or more buttons. Beyond that it is difficult to find common ground between joysticks and much of the interaction is application-specific, not system-specific. libinput does not provide support for joysticks for that reason, any abstraction libinput would provide for joysticks would be so generic that libinput would merely introduce complexity and processing delays for no real benefit.