From Variscite Wiki

1 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

2 Manipulating a single GPIO via /sys/class/gpio

2.1 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

2.2 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));


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); 


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); 


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 */


Once finished, free (unexport) the GPIO:

fd = open("/sys/class/gpio/unexport", O_WRONLY);

sprintf(buf, "%d", gpio);

write(fd, buf, strlen(buf));


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.

3 Kernel Device Tree GPIO configuration

3.1 Device Tree GPIO files

3.1.1 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.

3.2 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 */ 

3.2.1 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.