Linux GPIO Consumer Driver¶

Overview¶

Linux GPIO subsystem separates GPIO providers and GPIO consumers.

GPIO Consumer Driver
        ↓
    gpio_desc
        ↓
      gpiolib
        ↓
    gpio_chip
        ↓
GPIO Controller Driver
        ↓
 Hardware Registers

The GPIO controller driver exposes GPIO lines through gpio_chip.

Consumer drivers acquire GPIO resources from Device Tree and access them through GPIO descriptors.

GPIO Controller vs GPIO Consumer¶

GPIO Controller Driver¶

Responsible for:

Registering GPIO lines
Managing GPIO hardware
Implementing GPIO operations

Typical structure:

struct gpio_chip {
    int (*direction_input)(...);
    int (*direction_output)(...);
    int (*get)(...);
    void (*set)(...);
};

Examples:

SoC GPIO controller driver
I2C GPIO expander driver
SPI GPIO expander driver

GPIO Consumer Driver¶

Responsible for:

Requesting GPIO resources
Configuring GPIO direction
Reading GPIO inputs
Controlling GPIO outputs

Consumer drivers never access GPIO controller hardware directly.

Instead they use GPIO descriptor APIs.

Device Tree GPIO Binding¶

GPIO resources are described in Device Tree.

Example:

my-device {
    reset-gpios = <&gpio0 7 GPIO_ACTIVE_LOW>;
    enable-gpios = <&gpio0 5 GPIO_ACTIVE_HIGH>;
};

A GPIO specifier contains:

<&gpio_controller offset flags>

Meaning:

Field	Description
GPIO controller	GPIO provider
offset	GPIO line number within controller
flags	Active-high or active-low configuration

GPIO Descriptor Model¶

Consumer drivers do not operate GPIO numbers directly.

Instead:

Device Tree
        ↓
gpio_desc
        ↓
gpiod API

Each GPIO line is represented by a descriptor.

Conceptually:

struct gpio_desc {
    struct gpio_chip *chip;
    unsigned int offset;
    bool active_low;
};

The descriptor layer provides:

Ownership tracking
Active-low translation
GPIO abstraction

GPIO Consumer APIs¶

Request GPIO¶

Mandatory GPIO:

reset_gpio =
    devm_gpiod_get(dev, "reset");

Looks up:

reset-gpios

from Device Tree.

Optional GPIO¶

Optional GPIO:

enable_gpio =
    devm_gpiod_get_optional(dev, "enable");

Looks up:

enable-gpios

If the property does not exist:

NULL

is returned.

Configure Direction¶

Output:

gpiod_direction_output(desc, value);

Input:

gpiod_direction_input(desc);

Set Output Value¶

gpiod_set_value(desc, value);

Logical values are used.

For active-low GPIOs:

logical 1
        ↓
physical 0

logical 0
        ↓
physical 1

Read Input Value¶

value = gpiod_get_value(desc);

Returned value is a logical value.

GPIO Consumer Driver Flow¶

Typical probe flow:

probe()
    ↓
devm_gpiod_get()
    ↓
gpiod_direction_output()
    ↓
gpiod_set_value()
    ↓
device ready

Example:

reset_gpio =
    devm_gpiod_get(dev, "reset");

enable_gpio =
    devm_gpiod_get_optional(dev, "enable");

gpiod_direction_output(reset_gpio, 1);
gpiod_direction_output(enable_gpio, 1);

gpiod_set_value(reset_gpio, 0);

Remove Flow¶

Typical remove flow:

remove()
    ↓
disable device
    ↓
assert reset
    ↓
release resources

Example:

gpiod_set_value(enable_gpio, 0);
gpiod_set_value(reset_gpio, 1);

Relationship with Platform Drivers¶

GPIO consumer APIs are commonly used inside platform drivers.

Device Tree
        ↓
Platform Driver
        ↓
probe()
        ↓
devm_gpiod_get()
        ↓
gpio_desc
        ↓
gpiod_set_value()

This is the standard GPIO access model used throughout the Linux kernel.