Input Subsystem

The Linux input subsystem is used for handling input devices such as:

• Keyboard
• Mouse
• Touchscreen
• GPIO buttons

It provides a standard interface between kernel drivers and userspace.

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Architecture

Input Driver
    ↓
Input Core
    ↓
Event Handler (evdev)
    ↓
/dev/input/eventX
    ↓
Userspace (evtest / application)

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Key Components

1. input_dev

Represents an input device.

struct input_dev *input;
input = devm_input_allocate_device(dev);

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2. Capability

Defines what events the device can generate.

input_set_capability(input, EV_KEY, KEY_ENTER);

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3. Event Reporting

input_report_key(input, KEY_ENTER, 1); // press
input_sync(input);

input_report_key(input, KEY_ENTER, 0); // release
input_sync(input);

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4. input_event (userspace)

struct input_event {
    struct timeval time;
    __u16 type;
    __u16 code;
    __s32 value;
};

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Event Model

Value	Meaning
1	Key press
0	Key release
2	Auto-repeat

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SYN_REPORT

input_sync() generates:

EV_SYN / SYN_REPORT

It indicates the end of one event frame.

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Design Philosophy

Kernel:
    provides events (mechanism)

Userspace:
    decides behavior (policy)

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Input vs IIO

Feature	Input	IIO
Data type	Event	Streaming data
Example	Button	ADC / sensor
Model	Event-driven	Buffered
Userspace	evdev	iio buffer

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Summary

Input = "something happened"
IIO   = "data is flowing"