Linux GPIO Driver (gpiod)

1. GPIO Architecture

User space ↓ /dev/mygpio (char driver) ↓ Kernel driver ↓ gpiod API ↓ GPIO controller ↓ Hardware

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2. Legacy vs gpiod

Legacy API (deprecated)

gpio_request()
gpio_direction_output()
gpio_set_value()

Problems: - Not DT-friendly - Platform dependent - Hard to maintain

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gpiod API (modern)

devm_gpiod_get()
gpiod_set_value()
gpiod_get_value()

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3. Device Tree Binding

led-gpios = <&gpio 17 0>;

Driver:

devm_gpiod_get(dev, "led", ...)

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4. Platform Driver Binding

DT: compatible = "myvendor,mygpio-led"

Driver: .of_match_table

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5. Descriptor-based Design

• No global GPIO number
• Use logical names ("led")

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6. devm_ Resource Management

devm_gpiod_get()

• Auto cleanup on driver remove
• No manual free needed

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7. Common Pitfalls

• GPIO number ≠ BCM number (Raspberry Pi)
• dtoverlay reload may leave stale state
• dmesg | tail can mislead debugging
• need EOF handling in read()

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