Overview

下图来自文章 Linux kernel GPIO user space interface ，可概括这篇文章的内容。本文介绍 Linux GPIO driver的初始化，用户态访问 GPIO 和内核态访问 GPIO。

GPIO init

DTS

        gpio0: gpio@GPIO0_FULL_ADDR {
            compatible = "snps,dw-apb-gpio";
            reg = <GPIO0_REGS GPIO_REGS_SIZE>;
            #address-cells = <1>;
            #size-cells = <0>;
            gpio0p0: gpio-controller@0 {
                compatible = "snps,dw-apb-gpio-port";
                gpio-controller;
                #gpio-cells = <2>;
                snps,nr-gpios = <8>;
                snps,gpio-base = <0>;
                reg = <0>;
                interrupt-controller;
                #interrupt-cells = <2>;
                interrupt-parent = <&gic>;
                interrupts = <GIC_SPI GPIO_0_SPI IRQ_TYPE_LEVEL_HIGH>;
            };
        };

core_initcall(gpiolib_dev_init)

gpiolib_dev_init
    bus_register(&gpio_bus_type)
    alloc_chrdev_region(&gpio_devt, 0, GPIO_DEV_MAX, "gpiochip")
    gpiolib_initialized = true
    gpiochip_setup_devs() //traverse gpio_devices 
        gpiochip_setup_dev(gdev)
            cdev_add(&gdev->chrdev, gdev->dev.devt, 1)
            device_add(&gdev->dev)
            gpiochip_sysfs_register(gdev)
                device_create_with_groups(&gpio_class, parent

postcore_initcall(gpiolib_sysfs_init)

class_register(&gpio_class)
gpiochip_sysfs_register

导出 sysfs 文件 /sys/class/gpio/export 和 /sys/class/gpio/unexport

subsys_initcall(gpiolib_debugfs_init)

debugfs_create_file("gpio",  // create "/sys/kernel/debug/gpio"

module_platform_driver(bgpio_driver)

Generic driver for memory-mapped GPIO controllers.

module_platform_driver(dwapb_gpio_driver)

dwapb_gpio_probe
    dwapb_gpio_add_port
        bgpio_init
        dwapb_configure_irqs(gpio, port, pp)
            irq_domain_create_linear
            irq_alloc_domain_generic_chips
            irq_get_domain_generic_chip
            irq_set_chained_handler_and_data or
            devm_request_irq or irq_set_chained_handler_and_data
            irq_create_mapping(gpio->domain, hwirq)
            port->gc.to_irq = dwapb_gpio_to_irq
        gpiochip_add_data(&port->gc, port)
            gpiodev_add_to_list //add to gpio_devices
            gpiochip_set_desc_names
            gpiochip_irqchip_init_valid_mask
            of_gpiochip_add
            gpiochip_setup_dev

user drivers

UIO platform driver

uio_irq_platform_probe
    uio_irq_parse
    uio_register_device

builtin_platform_driver(gpio_clk_driver)

Gpio controlled clock implementation

module_platform_driver(mdio_gpio_driver)

GPIO based MDIO bitbang driver.

module_platform_driver(gpio_led_driver)

LEDs driver for GPIOs

user interface

Char device

~ # ls /dev/gpiochip* -l
crw-rw----    1 root     root      254,   0 Jan  1  1970 /dev/gpiochip0
crw-rw----    1 root     root      254,   1 Jan  1  1970 /dev/gpiochip1
crw-rw----    1 root     root      254,   2 Jan  1  1970 /dev/gpiochip2

sysfs

/sys/class/gpio # ls -l
--w-------    1 root     root          4096 Dec  3 19:07 export
lrwxrwxrwx    1 root     root             0 Dec  3 19:07 gpiochip0 -> ../../devices/platform/axi/4f200000.gpio/gpio/gpiochip0
lrwxrwxrwx    1 root     root             0 Dec  3 19:07 gpiochip16 -> ../../devices/platform/axi/4f600000.gpio/gpio/gpiochip16
lrwxrwxrwx    1 root     root             0 Dec  3 19:07 gpiochip8 -> ../../devices/platform/axi/4f400000.gpio/gpio/gpiochip8
--w-------    1 root     root          4096 Dec  3 19:07 unexport

内核态访问 GPIO

用户态访问 GPIO

libgpiod

buildroot 选择 package libgpiod 和 gpio tools：

libgpiod - C library and tools for interacting with the linux GPIO character device (gpiod stands for GPIO device)

https://git.kernel.org/pub/scm/libs/libgpiod/libgpiod.git/

Since linux 4.8 the GPIO sysfs interface is deprecated. User space should use the character device instead. This library encapsulates the ioctl calls and data structures behind a straightforward API.

gpio tools

There are currently six command-line tools available:

gpiodetect - list all gpiochips present on the system, their names, labels and number of GPIO lines
gpioinfo - list all lines of specified gpiochips, their names, consumers, direction, active state and additional flags
gpioget - read values of specified GPIO lines
gpioset - set values of specified GPIO lines, potentially keep the lines exported and wait until timeout, user input or signal
gpiofind - find the gpiochip name and line offset given the line name
gpiomon - wait for events on GPIO lines, specify which events to watch, how many events to process before exiting or if the events should be reported to the console

Linux GPIO driver