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Linux-GPIO-API.md

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libgpiod

Zielgruppe: Applikations-Entwickler (auch HW) die in Linux GPIOs bedienen wollen.

Was macht das OS eigentlich?

Userpace <-> Kernelspace am Beispiel GPIO-Driver

sysfs = bad

  • State not tied to process
  • Concurrent access to sysfs attributes -> keine Zugriffskontrolle, kein locking
  • If process crashes, the GPIOs remain exported (zwingt zu echo unexport usw.)
  • Cumbersome API -> echo export beispiel
  • Single sequence of GPIO numbers representing a two-level hierarchy - necessary to calculate the number of the GPIO

GPIO Nummer ausrechnen:

GPIO-Nummer = ( GPIO-Bank-Nr * 16|32 ) + GPIO-Offset

z.b. GPIO #31 auf GPIO-Bank #4: 3*32+31=127

Achtung: GPIO-Bank und GPIO-Offset zählen ab 0 (HWer und Manual zählen meist ab 1)

echo 127 > /sys/class/gpio/export

echo out > /sys/class/gpio/gpio127/direction
echo 1 > /sys/class/gpio/gpio127/value

echo in > /sys/class/gpio/gpio127/direction
cat /sys/class/gpio/gpio127/value

echo 127 > /sys/class/gpio/unexport

GPIO character device - new user API

  • v1.x Seit Linux v4.8 (2016)
  • v2 seit Linux v5.10
  • One device file per gpiochip (Bank)
    • /dev/gpiochip0, /dev/gpiochip1, /dev/gpiochipX…
  • Similar to other kernel interfaces: open() + ioctl() + poll() + read() + close()
  • Possible to request multiple lines at once (for reading/setting values)
  • Possible to find GPIO lines and chips by name
  • Open-source and open-drain flags, user/consumer strings, uevents
  • Reliable polling

Racy GPIO event polling

  • Z.B. Schalter an gpio und schnell drücken -> Wettlaufsituation 😉 Wer ist schneller: Der Read-Code oder der GPIO-Impuls?

sysfs: Events can get lost! | gpiod: Events never get lost!

Event polling mit sysfs

#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <poll.h>

#define GPIO_PIN "17"  // Change this to the GPIO pin you want to use

int main() {
    int gpio_fd;
    char gpio_path[256];
    struct pollfd pfd;

    // Export the GPIO pin
    gpio_fd = open("/sys/class/gpio/export", O_WRONLY);
    if (gpio_fd == -1) {
        perror("Unable to export GPIO");
        exit(1);
    }
    write(gpio_fd, GPIO_PIN, sizeof(GPIO_PIN));
    close(gpio_fd);

    // Set the direction of the GPIO pin to input
    sprintf(gpio_path, "/sys/class/gpio/gpio%s/direction", GPIO_PIN);
    gpio_fd = open(gpio_path, O_WRONLY);
    if (gpio_fd == -1) {
        perror("Unable to set direction");
        exit(1);
    }
    write(gpio_fd, "in", 2);
    close(gpio_fd);

    // Open the value file for reading
    sprintf(gpio_path, "/sys/class/gpio/gpio%s/value", GPIO_PIN);
    gpio_fd = open(gpio_path, O_RDONLY);
    if (gpio_fd == -1) {
        perror("Unable to open value file");
        exit(1);
    }

    pfd.fd = gpio_fd;
    pfd.events = POLLPRI | POLLERR;

    while (1) {
        int ret = poll(&pfd, 1, -1); // Wait for events indefinitely
        if (ret < 0) {
            perror("Poll error");
            exit(1);
        }
        if (pfd.revents & POLLPRI) {
            char buf[1];
            lseek(gpio_fd, 0, SEEK_SET); // Move to the beginning of the file
            read(gpio_fd, buf, 1);
            printf("GPIO value: %c\n", buf[0]);
        }
    }

    // Unexport the GPIO pin when done
    gpio_fd = open("/sys/class/gpio/unexport", O_WRONLY);
    if (gpio_fd == -1) {
        perror("Unable to unexport GPIO");
        exit(1);
    }
    write(gpio_fd, GPIO_PIN, sizeof(GPIO_PIN));
    close(gpio_fd);

    close(gpio_fd);

    return 0;
}

GPIO event polling - now reliable using libgpiod:

#include <stdio.h>
#include <gpiod.h>

#define GPIO_CHIPNAME "gpiochip0"  // Modify this to match your GPIO chip
#define GPIO_LINE_OFFSET 17       // Modify this to match your GPIO pin number

int main() {
    struct gpiod_chip *chip;
    struct gpiod_line *line;

    // Initialize the GPIO chip
    chip = gpiod_chip_open_by_name(GPIO_CHIPNAME);
    if (!chip) {
        perror("Failed to open GPIO chip");
        return 1;
    }

    // Get the GPIO line
    line = gpiod_chip_get_line(chip, GPIO_LINE_OFFSET);
    if (!line) {
        perror("Failed to get GPIO line");
        gpiod_chip_close(chip);
        return 1;
    }

    // Request the line for input
    if (gpiod_line_request_input(line, "example-gpio") < 0) {
        perror("Failed to request GPIO line");
        gpiod_chip_close(chip);
        return 1;
    }

    while (1) {
        int value = gpiod_line_get_value(line);
        printf("GPIO value: %d\n", value);
    }

    // Release resources
    gpiod_line_release(line);
    gpiod_chip_close(chip);

    return 0;
}

libgpiod

Userspace Library zur Interaktion mit der

Demo

  • locking sysfs vs. gpiod