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Input Subsystem

The Linux input subsystem is used for devices that report user input events, such as keys, buttons, touch sensors, rotary encoders, and gesture-like controls.

What Problem It Solves

Without the input subsystem, each button or touch device could require a custom character device and a custom user-space protocol. The input subsystem provides a standard event ABI through /dev/input/eventX.

Core Concepts

Concept Description
input_dev Input device object registered by the driver.
Event type Class of event, such as EV_KEY, EV_ABS, or EV_REL.
Event code Specific key, axis, or event identifier.
input_report_*() Reports a value change.
input_sync() Terminates one event frame.
evdev User-space event interface exposed as /dev/input/eventX.

Typical Driver Flow

probe()
    ↓
allocate input_dev
    ↓
set supported event bits
    ↓
register input device
    ↓
IRQ / workqueue reports input events

A GPIO button event usually looks like this:

GPIO IRQ
    ↓
debounce / gesture logic
    ↓
input_report_key()
    ↓
input_sync()
    ↓
/dev/input/eventX

Common Event Types

Event Type Typical Use
EV_KEY Buttons, keys, simple touch detection
EV_ABS Absolute values, touch coordinates, pressure
EV_REL Relative movement, mouse-like deltas
EV_SW Switch state, lid state, docking state

Design Notes

  • Use EV_KEY for button-like events and simple touch actions.
  • Use EV_ABS when the device reports position, pressure, or absolute sensor values.
  • Call input_sync() after reporting one logical event frame.
  • Keep gesture recognition policy clear: driver-side gesture recognition is useful for simple hardware-specific events, but complex UX logic may belong in user space.