Day44 - Input Userspace Application

Objective

• Read input events from /dev/input/eventX
• Decode key events (short / long / double click)
• Implement a userspace application
• Control LED based on input events

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System Flow

GPIO Button
    ↓
Kernel Input Driver
    ↓
input_report_key()
    ↓
/dev/input/eventX
    ↓
Python Application
    ↓
LED Control (sysfs)

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Input Event Mapping

Action	Key Code
Short Click	KEY_ENTER
Long Click	KEY_ESC
Double Click	KEY_SPACE

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Userspace Reader

File

d44-input-app/input_led_app.py

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Implementation Overview

1. Open input device

open("/dev/input/eventX", "rb")

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2. Parse input_event

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

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3. Filter key press events

if ev_type == EV_KEY and value == 1:
    # key press detected

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4. Map key to action

KEY_ENTER → SHORT
KEY_ESC   → LONG
KEY_SPACE → DOUBLE

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LED Control

Check LED device

ls /sys/class/leds/

Example:

ACT

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Switch to manual mode

echo none | sudo tee /sys/class/leds/ACT/trigger

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Control LED

echo 1 | sudo tee /sys/class/leds/ACT/brightness
echo 0 | sudo tee /sys/class/leds/ACT/brightness

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Application Logic

SHORT  → Toggle LED
LONG   → LED ON
DOUBLE → LED OFF

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Build & Run

chmod +x input_led_app.py
sudo ./input_led_app.py

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Expected Output

[EVENT] SHORT
[ACTION] Toggle LED

[EVENT] LONG
[ACTION] LED ON

[EVENT] DOUBLE
[ACTION] LED OFF

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Validation Steps

1. Verify input events

sudo evtest /dev/input/eventX

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2. Run application

sudo ./input_led_app.py

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3. Perform actions

• Press button shortly → LED toggles
• Hold button (>2s) → LED ON
• Double click → LED OFF

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Observations

• Input driver does not define behavior
• Userspace decides how to interpret events
• Same key event can map to different actions

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Key Learning

Kernel:
    generates input events

Userspace:
    interprets and applies logic

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Extension Ideas

• Support multiple input devices
• Use select/poll for event loop
• Add debounce in userspace (compare behavior)
• Build CLI or GUI application
• Integrate with other subsystems (e.g. IIO trigger)

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Summary

Day43:
    kernel generates input events

Day44:
    userspace consumes and uses events