GPIO Driver with IRQ (Platform Driver)¶

This note summarizes how to implement a GPIO driver with interrupt support using the Linux platform driver model and Device Tree.

1. Architecture Overview¶

User Space ↓ /dev/mygpio (char device) ↓ Platform Driver ↓ GPIO subsystem (gpiod) ↓ Hardware (LED / Button)

2. Device Tree Mapping¶

Example:

led1-gpios = <&gpio 17 0>;
led2-gpios = <&gpio 27 0>;
btn-gpios  = <&gpio 22 1>;

0 = active high
1 = active low

Driver mapping:

devm_gpiod_get(dev, "led1", GPIOD_OUT_LOW);
devm_gpiod_get(dev, "led2", GPIOD_OUT_LOW);
devm_gpiod_get(dev, "btn",  GPIOD_IN);

3. devm_ Managed Resource¶

Manual (traditional)¶

gpio_request()
gpio_free()

devm_ version¶

devm_gpiod_get()

Automatically released when driver is removed
Cleaner and safer

4. IRQ Flow¶

Step 1: Convert GPIO → IRQ¶

irq = gpiod_to_irq(btn_gpio);

Step 2: Register IRQ¶

devm_request_irq(dev,
                 irq,
                 handler,
                 IRQF_TRIGGER_RISING | IRQF_TRIGGER_FALLING,
                 "mygpio_btn",
                 data);

Step 3: ISR (Interrupt Service Routine)¶

static irqreturn_t mygpio_btn_isr(int irq, void *data)
{
    int value = gpiod_get_value(...);

    // Update state
    mydev->btn_state = value;

    return IRQ_HANDLED;
}

5. IRQ Design Principles¶

DO:¶

Keep ISR short
Update state
Trigger event

DON'T:¶

sleep
use mutex
heavy computation
excessive logging

6. Logging in IRQ Context¶

Allowed:

dev_info()

But NOT recommended for production: - increases latency - may flood logs (bounce) - may cause contention

Better:

dev_info_ratelimited()
dev_dbg()

7. Character Device Interface¶

Write¶

echo led1=1 > /dev/mygpio
echo led2=0 > /dev/mygpio
echo all=1 > /dev/mygpio

Read¶

cat /dev/mygpio
→ led1=1,led2=0,btn=1

8. Permission Issue¶

Problem:

Permission denied

Solution:

sudo chmod 666 /dev/mygpio

or:

echo led1=1 | sudo tee /dev/mygpio

9. Common Pitfalls¶

Missing header¶

#include <linux/interrupt.h>

Wrong format specifier¶

%d  (for int)
%ld (for long)

Shell redirect issue¶

sudo echo ... > /dev/mygpio   ❌
echo ... | sudo tee /dev/mygpio   ✅

10. Button Bounce¶

Observed: - Multiple IRQ triggers per press

Reason: - Mechanical switch bounce

Solutions (later): - jiffies filter - timer - workqueue

11. Summary¶

This driver demonstrates:

Platform driver + Device Tree
Multiple GPIO handling
Interrupt-based input
Character device interface
Basic driver design principles

This is a complete minimal Linux driver architecture.