MX8 GPIO
GPIO state
The current state of the system's GPIOs can be obtained in user-mode, as shown in the following example:
# cat /sys/kernel/debug/gpio gpiochip0: GPIOs 0-31, parent: platform/5d080000.gpio, 5d080000.gpio: gpio-12 ( |ov5640_mipi_reset ) out hi gpio-16 ( |fsl_lpspi ) in hi gpio-26 ( |ov5640_mipi_pwdn ) out lo gpiochip1: GPIOs 32-63, parent: platform/5d090000.gpio, 5d090000.gpio: gpio-32 ( |fsl_lpspi ) in hi gpio-34 ( |phy-reset ) out hi gpio-39 ( |reg_sd_pwr ) out hi gpio-60 ( |reg_ethphy0 ) out hi gpiochip2: GPIOs 64-95, parent: platform/5d0a0000.gpio, 5d0a0000.gpio: gpiochip3: GPIOs 96-127, parent: platform/5d0b0000.gpio, 5d0b0000.gpio: gpio-120 ( |cam_buf_en ) out hi gpiochip4: GPIOs 128-159, parent: platform/5d0c0000.gpio, 5d0c0000.gpio: gpio-149 ( |usb_otg1_vbus ) out hi gpio-150 ( |cd ) in lo IRQ gpiochip5: GPIOs 160-191, parent: platform/5d0d0000.gpio, 5d0d0000.gpio: gpio-169 ( |connect ) in hi IRQ gpiochip6: GPIOs 192-223, parent: platform/5d0e0000.gpio, 5d0e0000.gpio: gpiochip7: GPIOs 224-255, parent: platform/5d0f0000.gpio, 5d0f0000.gpio: gpiochip8: GPIOs 504-511, parent: i2c/1-0020, pca9534, can sleep: gpio-504 ( |heartbeat ) out lo gpio-505 ( |Back ) in hi IRQ gpio-506 ( |Home ) in hi IRQ gpio-507 ( |Menu ) in hi IRQ gpio-508 ( |usb3_sel ) out lo gpio-509 ( |phy-reset ) out hi gpio-510 ( |reg_vselect ) out hi gpio-511 ( |reg_ethphy1 ) out hi
Each GPIO is defined as in or out and the state is shown as lo or hi.
For example pin 150 is the SD card card-detect.
When an SD card is plugged in, the state will be:
gpio-150 ( |cd ) in lo IRQ
When the SD card is removed, the state will be:
gpio-150 ( |cd ) in hi IRQ
Manipulating GPIO using libgpiod
The Linux GPIO sysfs interface is being deprecated. Moving forward, user space should use the character device /dev/gpiochip* instead. libgpiod provides bindings and utilities for for manipulating GPIO via user space.
libgpiod via command line
libgpiod provides command line utilities for GPIO:
| 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 |
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 21
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.
Manipulating a single GPIO via /sys/class/gpio
Using a command line or a script
GPIOs in i.MX are grouped in groups of 32 pins.
For example, GPIO0_3 belongs to the first group, pin 3. Its absolute number will be 3.
GPIO4_21 will be 4*32+21=149.
Assuming this GPIO is defined in your device tree, the following is an example of how to use it from userspace.
To export the GPIO for userspace use:
# echo 149 > /sys/class/gpio/export
To configure as output:
# echo out > /sys/class/gpio/gpio149/direction
Set GPIO high:
# echo 1 > /sys/class/gpio/gpio149/value
Set GPIO low:
# echo 0 > /sys/class/gpio/gpio149/value
To configure as input:
# echo in > /sys/class/gpio/gpio149/direction
Read the current value:
# cat /sys/class/gpio/gpio149/value
To free the GPIO after you're done using it:
# echo 149 > /sys/class/gpio/unexport
Using a C application
All of the command line operations above can be translated to C code:
Reserve (export) the GPIO:
#define IMX_GPIO_NR(port, index) ((port)*32)+((index)&31))
int fd;
char buf[MAX_BUF];
int gpio = IMX_GPIO_NR(4, 21); /* Just an example */
fd = open("/sys/class/gpio/export", O_WRONLY);
sprintf(buf, "%d", gpio);
write(fd, buf, strlen(buf));
close(fd);
Set the GPIO direction:
sprintf(buf, "/sys/class/gpio/gpio%d/direction", gpio); fd = open(buf, O_WRONLY); /* Set out direction */ write(fd, "out", 3); /* Set in direction */ write(fd, "in", 2); close(fd);
In case of out direction set the GPIO value:
sprintf(buf, "/sys/class/gpio/gpio%d/value", gpio); fd = open(buf, O_WRONLY); /* Set GPIO high status */ write(fd, "1", 1); /* Set GPIO low status */ write(fd, "0", 1); close(fd);
In case of in direction get the current GPIO value:
char value;
sprintf(buf, "/sys/class/gpio/gpio%d/value", gpio);
fd = open(buf, O_RDONLY);
read(fd, &value, 1);
if (value == '0') {
/* Current GPIO status low */
} else {
/* Current GPIO status high */
}
close(fd);
Once finished, free (unexport) the GPIO:
fd = open("/sys/class/gpio/unexport", O_WRONLY);
sprintf(buf, "%d", gpio);
write(fd, buf, strlen(buf));
close(fd);
Important notes:
- Remember that after the first read operation the file pointer will move to the next position in the file, so to get a correct value for each read operation you simply have to set the file pointer at the beginning of the file before read by using the following command:
lseek(fd, 0, SEEK_SET);
- This is only a short example. If you want to use it in your code remember add error handling to it.
Kernel Device Tree GPIO configuration
Device Tree GPIO files
Pin Func files
In the directory include/dt-bindings/pinctrl/ of the Linux kernel source you will find the pin functions definition files.
The relevant file is pads-imx8qxp.h.
If you search it for GPIO0_IO20, for example, you will see a goup of definitions with the same prefix (pad name) "SC_P_MCLK_OUT0",
#define SC_P_MCLK_OUT0_ADMA_ACM_MCLK_OUT0 SC_P_MCLK_OUT0 0 #define SC_P_MCLK_OUT0_ADMA_ESAI0_TX_HF_CLK SC_P_MCLK_OUT0 1 #define SC_P_MCLK_OUT0_ADMA_LCDIF_CLK SC_P_MCLK_OUT0 2 #define SC_P_MCLK_OUT0_ADMA_SPI2_SDO SC_P_MCLK_OUT0 3 #define SC_P_MCLK_OUT0_LSIO_GPIO0_IO20 SC_P_MCLK_OUT0 4
Adding only the one with the GPIO0_IO20 suffix (function) to your DTS file will let you use the pin as GPIO.
Define a pin as GPIO in the kernel Device Tree
You need to add the relevant definitions to your device tree, as explained in the Pin Func files section above.
Edit arch/arm64/boot/dts/freescale/fsl-imx8qxp-var-som-common.dtsi and add the definition for the GPIO you need in the iomuxc node.
&iomuxc {
pinctrl-names = "default";
pinctrl-0 = <&pinctrl_hog>;
imx8qxp-var-som {
pinctrl_hog: hoggrp {
fsl,pins = <
/* Add your GPIO definitions here */
>;
};
};
...
};
Device Tree GPIO attribute
If you look at the pin control definitions in arch/arm64/boot/dts/freescale/imx8qxp-var-som-common.dtsi in the Linux kernel source tree, the number to the right of the pin mux macro can be used for additional attributes like pull-up, slew rate, open drain, drive strength, etc.
This value is written to the IOMUXD register of the relevant pad.
Please consult the SOC reference manual for details about the relevant register.