Device Tree, Driver, and /dev Node Mapping

Core Concept

The Device Tree describes hardware and binds it to a kernel driver through the compatible property.
The driver then creates /dev/mygpio as the user-space interface.

---

Three-Layer Architecture Overview

[Device Tree / Hardware Description]
    mygpio-led
    compatible = "myvendor,mygpio-led"
    led-gpios = <...>

            ↓ (match)

[Kernel Driver]
    mygpio driver
    mygpio_probe()

            ↓ (create device node)

[User Space]
    /dev/mygpio

---

Naming Reference Table

Name	Location	Purpose	Used By
/dev/mygpio	User space	Device operation interface	User space
mygpio	Driver / macro	Driver name and device node name	Kernel
mygpio-led	DT node name	Device Tree node name	DTS
mygpio_led	DT label	Internal DTS reference label	DTS
myvendor,mygpio-led	compatible	Driver matching identifier	Kernel
"led"	gpiod API	GPIO function name	Driver
led-gpios	DT property	GPIO hardware mapping	Device Tree

---

Detailed Naming Explanation

---

1️⃣ /dev/mygpio (User-Space Interface)

#define DEVICE_NAME "mygpio"
device_create(..., DEVICE_NAME);

👉 This creates:

/dev/mygpio

Purpose

Provides a user-space interface for device operation:

• echo 1 > /dev/mygpio
• cat /dev/mygpio

---

2️⃣ compatible (Device Tree ↔ Driver Binding)

Device Tree:

compatible = "myvendor,mygpio-led";

Driver:

static const struct of_device_id mygpio_of_match[] = {
    { .compatible = "myvendor,mygpio-led" },
};

Purpose

Determines:

Which kernel driver should handle this Device Tree node.

---

3️⃣ mygpio-led (DT Node Name)

mygpio-led {
    ...
};

Purpose

Used only as a readable node name in the Device Tree source.

Important

• Does NOT affect driver matching
• Does NOT become the /dev node name

---

4️⃣ mygpio_led: (DT Label)

mygpio_led: mygpio-led {
};

Purpose

Used as an internal DTS reference label.

Can be referenced by other nodes through phandles.

---

5️⃣ gpiod "led" vs led-gpios

Driver:

devm_gpiod_get(dev, "led", GPIOD_OUT_LOW);

Device Tree:

led-gpios = <&gpio 17 GPIO_ACTIVE_HIGH>;

---

Core Naming Rule

"xxx" → "xxx-gpios"

Examples:

Driver	Device Tree
"led"	led-gpios
"reset"	reset-gpios
"irq"	irq-gpios

---

Correct Interpretation

"led" represents a GPIO function name, NOT a GPIO number.

---

gpiod Design Philosophy

---

Legacy GPIO API

gpio_request(17);

Problems

• Driver is tightly coupled to GPIO numbers
• Poor portability
• Hardware configuration hardcoded inside driver

---

Modern GPIO Design (gpiod + Device Tree)

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

Driver perspective:

"I want the GPIO used for the LED function."

Device Tree perspective:

"The LED is connected to GPIO17."

Kernel responsibility:

Map the function to the actual GPIO hardware.

---

GPIO Data Flow

devm_gpiod_get(dev, "led", ...)
        ↓
DT: led-gpios = <&gpio 17 ...>
        ↓
GPIO subsystem
        ↓
gpio_desc
        ↓
gpiod_set_value()

---

Common Mistakes

---

Inconsistent Naming

Driver:

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

Device Tree:

my-led-gpios = <...>;

👉 ❌ Driver will fail to find the GPIO property.

---

❌ Invalid Property Name

led_gpio = <...>;

👉 ❌ The kernel GPIO framework will not recognize it.

---

Quick Reference

---

Device Tree

• compatible → selects driver
• xxx-gpios → GPIO hardware mapping

---

Driver

• "xxx" → maps to xxx-gpios
• probe() → initializes device

---

User Space

• /dev/xxx → device operation interface

---

Summary

Device Tree (describes hardware)
        ↓ compatible
Kernel Driver (controls hardware)
        ↓ device_create
/dev/mygpio (user-space interface)