Mastering Linux GPIO: Driver APIs, Sysfs, and PinCtrl Integration

Introduction

This article targets developers working with Linux kernel GPIO on versions 3.13 and later. It covers basic GPIO concepts, sysfs and debug interfaces, and introduces the pinctrl subsystem for more advanced pin configuration.

GPIO Subsystem API Overview

Key Functions

int gpio_request(unsigned gpio, const char *label)

– request a GPIO line; returns 0 on success.

int gpio_request_one(unsigned gpio, unsigned long flags, const char *label)

– request a GPIO with direction flags. void gpio_free(unsigned gpio) – release a previously requested GPIO. int gpio_direction_input(unsigned gpio) – set GPIO as input. int gpio_direction_output(unsigned gpio, int value) – set GPIO as output with an initial value. void __gpio_set_value(unsigned gpio, int value) (macro gpio_set_value) – write a value. int __gpio_get_value(unsigned gpio) (macro gpio_get_value) – read a value. int gpio_to_irq(unsigned gpio) – map a GPIO to an IRQ number.

Parameter Details

Each function’s parameters are documented, e.g., gpio is the numeric identifier, label is a descriptive name, and return values are 0 for success or a negative error code.

Example: Controlling a Single GPIO

Device‑tree snippet to request gpio4 as an output set high:

gpio_test{
  status="ok";
  gpio_req=<&tlmn 4 0>;
}

Driver code:

struct device dev;
int gpio4 = of_get_named_gpio(dev.of_node, "gpio_req", 0);
int err = gpio_request(gpio4, "qti-can-reset");
if (err < 0) return err;
gpio_direction_output(gpio4, 0);
gpio_set_value(gpio4, 1);

For a GPIO array, the device‑tree can list multiple pins and associated labels, e.g.,

gpios = <&tlmm 36 0>, <&tlmm 42 0>, <&tlmm 132 0>;

with corresponding qcom,gpio-reset, qcom,gpio-standby, etc.

PinCtrl Subsystem Integration

Relevant APIs

struct pinctrl *devm_pinctrl_get(struct device *dev)

– obtain pinctrl handle from device tree.

struct pinctrl_state *pinctrl_lookup_state(struct pinctrl *p, const char *name)

– retrieve a named pin configuration state.

int pinctrl_select_state(struct pinctrl *p, struct pinctrl_state *state)

– apply the selected state to hardware.

Example: Qualcomm Pin Configuration

&tlmn{
  pin_test_default:pin_test_default{
    mux {
      pins = "gpio0", "gpio1";
      function = "qup00";
    };
    config {
      pins = "gpio0", "gpio1";
      drive-strength = <2>;
      bias-disable;
    };
  };
}

Device‑tree entry to use the state:

pinctrl_test{
  status="ok";
  pinctrl-name="default";
  pinctrl-0=<&pin_test_default>;
}

Driver code to bind and select the state:

dev->pins->p = devm_pinctrl_get(dev);
dev->pins->default_state = pinctrl_lookup_state(dev->pins->p, PINCTRL_STATE_DEFAULT);
pinctrl_select_state(dev->pins->p, dev->pins->default_state);

During the probe phase, the kernel automatically binds pinctrl resources, so the driver can rely on the pre‑initialized states without extra manual configuration.