<style> .reveal { font-size: 24px; } </style> # 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 ---- <!-- .slide: style="text-align: left" --> 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) ```bash= 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 --- - Neue Kernel API: GPIO character device - One device file per gpiochip (Bank) * `/dev/gpiochip0, /dev/gpiochip1, /dev/gpiochipX`… - Similar to other kernel interfaces: open() + ioctl() + poll() + read() + close() - v1.x seit Linux v4.8 (2016), v2 seit Linux v5.10 - Userspace Bibliothek - libgpiod mit API calls z.b. `gpiod_chip_open_by_name(), gpiod_line_request_input(), gpiod_line_get_value(), gpiod_line_set_values(), ...` - Tools: `gpioinfo, gpioset, gpioget, gpiomon` - Features: - 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 - Tools: Eine Zeile statt >=4 ---- #### Racy GPIO event polling --- - Z.B. Schalter an gpio und schnell drücken -> Wettlaufsituation :wink: Wer ist schneller: Der Read-Code oder der GPIO-Impuls?  sysfs: Events can get lost! | gpiod: Events never get lost! ---- #### Event polling mit sysfs --- ```c #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: --- ```c #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