DESK-MX8M-L/Peripherals/GPIOs

History
Version	Issue Date	Notes
1.0.0	Jan 2022	First DESK-MX8M release

Peripheral GPIOs

i.MX8 can handle external pins in many different ways and most of them can be configured as GPIOs. When a pin is set as a GPIO, it is possible to read its value, change its direction or change output value directly from the shell.

Device tree configuration

Here below a simple example of device tree configuration for using a GPIO on DAVE's kit for the ORCA SOM with gpio-leds sysfs interface:

From imx8mp-mito8mplus-cb1001.dts:

       leds {
       compatible = "gpio-leds";

       gpio-test {
               gpios = <&gpio4 24 GPIO_ACTIVE_HIGH>;
               default-state = "off";
           };
       };
...
...
&iomuxc {
       pinctrl-names = "default";
       pinctrl-0 = <&pinctrl_hog>;

       pinctrl_hog: hoggrp {
        fsl,pins = <
                       MX8MP_IOMUXC_SAI2_TXFS__GPIO4_IO24 0x19
        >;
    };
};

Accessing the peripheral

Linux messages at boot time

With the previous leds configuration for the GPIOs, it is possible to find them on sysfs under /sys/class/leds sub-directory:

root@desk-mx8mp:~# ls -la /sys/class/leds/
total 0
drwxr-xr-x  2 root root 0 Jan 14 12:08 .
drwxr-xr-x 82 root root 0 Jan 14 12:08 ..
lrwxrwxrwx  1 root root 0 Jan 14 12:08 gpio-test -> ../../devices/platform/leds/leds/gpio-test
lrwxrwxrwx  1 root root 0 Jan 14 12:08 mmc0:: -> ../../devices/platform/soc@0/30800000.bus/30b40000.mmc/leds/mmc0::
lrwxrwxrwx  1 root root 0 Jan 14 12:08 mmc1:: -> ../../devices/platform/soc@0/30800000.bus/30b50000.mmc/leds/mmc1::
lrwxrwxrwx  1 root root 0 Jan 14 12:08 mmc2:: -> ../../devices/platform/soc@0/30800000.bus/30b60000.mmc/leds/mmc2::
root@desk-mx8mp:~#

Usage as led function

Setting the brightness node, it is possible to change the GPIO status:

• read actual led/GPIO status

root@desk-mx8mp:~# cat /sys/class/leds/gpio-test/brightness
0
root@desk-mx8mp:~#

• change led/GPIO status to High (1)

root@desk-mx8mp:~# echo 1 > /sys/class/leds/gpio-test/brightness

Usage with sysfs

It is possible to configure the GPIO pin directly at Linux userspace using sysfs even if it is discouraged: see below .

See here below, an example of configuring a GPIO pin using J8 connector:

• set J8.11 SAI2_RXD0 (MX8MP_IOMUXC_SAI2_RXD0__GPIO4_IO23) as output GPIO
  • GPIO4_IO23 => (n-1)*32 + IO = (4-1)*32+23 = 119

• export the GPIO

root@desk-mx8mp:~# echo 119 > /sys/class/gpio/export

Using the GPIO as output

root@desk-mx8mp:~# echo out > /sys/class/gpio/gpio119/direction

• set the GPIO level

root@desk-mx8mp:~# echo 1 > /sys/class/gpio/gpio119/value

For using the GPIO as input:

root@desk-mx8mp:~# echo in > /sys/class/gpio/gpio119/direction

• read the GPIO level

root@desk-mx8mp:~# cat /sys/class/gpio/gpio119/value
1
root@desk-mx8mp:~#

Note that on i.MX SoC the GPIO value can be read back only if the SION bit is set the pin muxing.

Additional information

Information about GPIOs usage under sysfs directory https://www.kernel.org/doc/Documentation/gpio/sysfs.txt

GPIO sysfs deprecated

sysfs GPIO ABI has been deprecated. See more information here about it. A character device access has to be used, more information here

Information about GPIOs library libgpiod - C library and tools - can be found on git.kernel.org

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