Template:GPIO LIBGPIOD DRAFT

Manipulating GPIO using libgpiod

libgpiod via command line

libgpiod provides command line utilities for GPIO:

gpiodetect	List all gpiochips present on the system, their names, labels, and the number of GPIO lines.
gpioinfo	List all lines of specified gpiochips, their names, consumers, direction, active state, and additional flags.
gpioget	Read the values of the specified GPIO lines (not valid if the line is already requested). The line will be then configured as an input.
gpioset	Set the values of the specified GPIO lines, potentially keeping the lines exported, and wait until timeout, user input, or signal (not valid if the line is already requested). The line will be then configured as output.
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.

i.MX GPIOs are organized in banks of 32 pins. Each bank corresponds to a character device /dev/gpiochip<bank index>. The gpiodetect utility can be used to inspect the available gpiochip character devices:

# gpiodetect
gpiochip0 [30200000.gpio] (32 lines)
gpiochip1 [30210000.gpio] (32 lines)
...

The gpioinfo utility can be used to inspect the lines for a given gpiochip:

# gpioinfo gpiochip0
gpiochip0 - 32 lines:
        line   0:      unnamed    "spi_imx"  output  active-high [used]
        line   1:      unnamed       unused   input  active-high
        line   2:      unnamed       unused   input  active-high
        ...

The gpioset and gpioget utilities can be used to manipulate GPIO from the command line.

For example, assuming GPIO4_21 is configured as a GPIO in your device tree:

Set GPIO4_21 high:

gpioset gpiochip3 21=1

Set GPIO4_21 low:

gpioset gpiochip3 21=0

Read GPIO4_21:

gpioget gpiochip3 21GPIO_SYSFS_DRAFT

The gpiomon utility is useful for polling the lines to expect incoming input events.
For example, wait for three rising edge events on a given GPIO line:

gpiomon --num-events=3 --rising-edge gpiochip3 21
event:  RISING EDGE offset: 3 timestamp: [    1151.814356387]
event:  RISING EDGE offset: 3 timestamp: [    1151.815449803]
event:  RISING EDGE offset: 3 timestamp: [    1152.091556803]

Porting scripts using sysfs GPIO interface to libgpiod

Migrating from sysfs GPIO interface to libgpiod can be accomplished using the libgpiod package's command-line tools. For bash scripts, instead of writing or reading from /sys/class/gpio, you'll use these tools.
Here are a few examples of how you'd read and write in a GPIO using sysfs interface and the equivalent operations through libgpiod:

NOTE:

GPIOs in i.MX SoCs are grouped in groups of 32 pins. That is relevant when converting the GPIO absolute number used by sysfs GPIO interface.
For example:
GPIO 17 belongs to gpiochip0, pin 17: 32 * 0 + 17 equals 17.
GPIO 83 belongs to gpiochip2, pin 19: 32 * 2 + 19 equals 83.
GPIO 149 belongs to gpiochip4, pin 21: 32 * 4 + 21 equals 149.

# writing in a GPIO using sysfs GPIO interface
echo 149 > /sys/class/gpio/export
echo out > /sys/class/gpio/gpio149/direction
echo 1 > /sys/class/gpio/gpio149/value

# writing in a GPIO using libgpiod
gpioset gpiochip4 21=1

# reading a GPIO using sysfs GPIO interface
echo 149 > /sys/class/gpio/export
echo in > /sys/class/gpio/gpio149/direction
cat /sys/class/gpio/gpio149/value

# reading a GPIO using libgpiod
gpioget gpiochip4 21

Remember, it is also important to check the return status of these commands in your scripts to ensure they are working correctly. If a command fails, it will return a non-zero status which can be checked in the $? variable, this variable holds the exit status of the last command that was executed. For example:

gpioget gpiochip4 21
if [ $? -ne 0 ]; then
  echo "Failed to get GPIO value"
  exit 1
fi

libgpiod C Application

libgpiod provides bindings for C/C++ applications. C++ examples are available in the libgpiod /tree/bindings/cxx/examples directory.

Below is a simple C application demonstrating how to use the bindings with GPIO4_IO21:

Makefile:

all: main.cpp
	$(CC) $(CCFLAGS) -Og -lgpiod main.c -g -o hello.bin
clean:
	rm -f hello.bin

main.c

#include <gpiod.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

#define    CONSUMER    "Variscite Demo"

int main(int argc, char **argv)
{
    unsigned int i, ret, val;
    struct gpiod_chip *chip;
    struct gpiod_line *line;
    const char * chipname = "gpiochip3";
    const unsigned int line_num = 21;

    chip = gpiod_chip_open_by_name(chipname);
    if (!chip) {
        perror("Open chip failed\n");
        goto end;
    }

    line = gpiod_chip_get_line(chip, line_num);
    if (!line) {
        perror("Get line failed\n");
        goto close_chip;
    }

    ret = gpiod_line_request_output(line, CONSUMER, 0);
    if (ret < 0) {
        perror("Request line as output failed\n");
        goto release_line;
    }

    /* Blink 5 times */
    val = 0;
    for (i = 0; i < 5; i++) {
        ret = gpiod_line_set_value(line, val);
        if (ret < 0) {
            perror("Set line output failed\n");
            goto release_line;
        }
        printf("Output %u on line #%u\n", val, line_num);
        sleep(1);
        val = !val;
    }

release_line:
    gpiod_line_release(line);
close_chip:
    gpiod_chip_close(chip);
end:
    return 0;
}

libgpiod Python Application

libgpiod provides bindings for python applications:

# pip3 install gpiod

Python examples are available in the libgpiod /tree/bindings/python/examples directory.