GPIO

owned by linux kernel driver

1. Openfirmware GPIO

#include <linux/of.h>
#include <linux/of_gpio.h>
#include <linux/gpio.h>

zed_oled {
	compatible = "pmodoled-gpio";
	vbat-gpio = <&gpio_oled 0 0>;
	vdd-gpio = <&gpio_oled 1 0>;
	...
}

1. Registering GPIO through OF

of_get_named_gpio(struct device_node *np, const char *propname, int index))

1. Checking and Requesting GPIO pin

gpio_is_valid() // ensure GPIO exists
gpio_request()
gpio_request_array(led_gpios, ARRAY_SIZE(led_gpios))

struct gpio {
	unsigned gpio;
	unsigned long flags;
	const char *label;
}

1. Configuring and Accessing GPIO

int gpio_direction_input(unsigned gpio)
int gpio_direction_output(unsigned gpio)
int gpio_get_value(unsigned gpio)
int gpio_set_value(unsigned gpio, int value)

// 0 for low, nonzero for high

GPIOs defined on external chips require sleeping due to being slow from processor’s perspective

int gpio_get_value_cansleep(unsigned gpio)
int gpio_set_value_cansleep(unsigned gpio, int value)

Bit banging

using GPIO lines of a processor to emulate a communications protocol when a dedicated hardware controller isn’t available

Linux includes drivers over GPIO: I2C, SPI, HID over I2C, MDIO

Not a real time garunteed

Documentation/devicetree/bindings/<bus>/<bus>-gpio.txt

GPIO Keys

Keyboard

autorepeat: hold to repeat