The Input Stack on Linux: An End-to-End Architecture Overview

Summary

Intro Let’s explore and deobfuscate the input stack on Linux. Our aim is to understand its components and what each does. Input handling can be divided into two parts, separated by a common layer: Kernel-level handling: It deals with what happens in the kernel and how events are exposed to user-space The actual hardware connected to the machine, along with the different buses and I/O/transport subsystems The input core subsystem, and the specific device drivers that register on it Exposed layer (middle) The event abstraction subsystem (evdev) devtmpfs for device nodes sysfs for kernel objects and device attributes procfs for an introspection interface of the input core User-space handling: The user-space device manager (udev) and hardware database (hwdb) for device management and setup The libinput library for general input, and other libraries such as XKB for keyboards, to interpret the events and make them manageable The Widgets, X Server, X11 window managers, and Wayland compositors, which rely on everything else We’ll try to make sense of all this, one thing at a time, with a logical and coherent approach. NB: This article compiles my understand, for any correction please contact me. The Kernel’s Input Core How are input devices and their events handled in the kernel? You might think it is useless to know, but understanding some of the kernel logic is what makes things click. The input core is the central piece of the kernel responsible for handling input devices and their events. Most input devices go through it, although some bypass it entirely but these are special use-cases. It provides common abstract components that sit between the low-level hardware, and the more useful features for user-space, along with a sort of publish-subscribe system. To follow along you can either download the kernel source, or view it in any browser explorer (such as this, this, or this). Practically, the input core is found in the kernel under drivers/input/input.c, it defines th...