Day 13 - Debounce + Bottom Half (Workqueue / Timer)

Overview

In this chapter, we improve the GPIO interrupt driver by introducing:

• Software debounce using delayed work
• Proper top half / bottom half design
• Event-driven architecture with stable input

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Why Debounce is Needed

Mechanical buttons do not produce clean transitions.

Instead of:

0 → 1

We often get:

0 → 1 → 0 → 1 → 1 → 0 → 1

This is called bounce.

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Design Approach

We implement debounce using:

IRQ (top half)
    ↓
schedule delayed work
    ↓
bottom half (process context)
    ↓
read stable GPIO value
    ↓
generate event (if changed)

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Top Half vs Bottom Half

Top Half (ISR)

Responsibilities:

• Triggered by hardware interrupt
• Must be fast and non-blocking
• Cannot sleep

Implementation:

mod_delayed_work(system_wq, &mydev->btn_dwork, delay);

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Bottom Half (Delayed Work)

Responsibilities:

• Runs in process context
• Can sleep
• Performs debounce and event generation

Key API:

gpiod_get_value_cansleep()

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Debounce Logic

Core idea:

if (new_value != last_reported_state)
    generate event

This ensures:

• No duplicate events
• Only stable transitions are reported

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Event Flow

Complete pipeline:

GPIO IRQ
 → ISR (top half)
 → delayed work (bottom half)
 → update event state
 → wake_up_interruptible()
 → read() / poll()
 → user space

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Logical vs Raw GPIO

With Device Tree:

GPIO_ACTIVE_LOW

Kernel automatically converts:

Physical Level	Logical Value
LOW	1 (pressed)
HIGH	0 (released)

Driver should always expose:

pressed / released (logical)

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Key Kernel APIs

Workqueue

INIT_DELAYED_WORK()
mod_delayed_work()
cancel_delayed_work_sync()

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Wait Queue

wait_event_interruptible()
wake_up_interruptible()

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Poll

poll_wait()
POLLIN | POLLRDNORM

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Design Guidelines

• Keep ISR minimal
• Use bottom half for heavy work
• Always debounce GPIO input
• Expose logical state to user space
• Avoid duplicate events

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Summary

Day13 transforms the driver into a more realistic Linux design:

• Interrupt-safe
• Debounced
• Event-driven
• User-space friendly

This is a foundation for real input drivers.