Controlling GPIO from Linux User Space

This application note shows how to control the i.MX RT1170 GPIOs from the user level using the standard Linux GPIOLIB interface.

1. Changes to the Kernel Configuration

The generic GPIO interface is controlled by the CONFIG_GPIOLIB kernel option enabled by default in the rootfs project. Most of the i.MX RT1170 GPIO pins can be used in different multiplexed I/O roles (for instance, some GPIO pins can be also configured as an SPI interface, etc). Depending on the requirements of your application, you need to configure the pins that you want to use as GPIO for the GPIO role and other pins for an alternative I/O function.

The device driver for i.MX family GPIO controller is enabled in the kernel config. The gpio-leds and gpio-keys drivers will be enabled and configured in the DTS file, in order to support the USER_LED and USER_BUTTON of the IMXRT1170 EVK board.

  1. Verify that the USER LED and USER BUTTON interfaces are functional via the standard gpio-leds and gpio-keys interfaces:

    1. Use the following command to turn on the USER LED1:

      # echo 1 > /sys/class/leds/user-led1/brightness
    2. Use the following command to turn off the USER LED1:

      # echo 0 > /sys/class/leds/user-led1/brightness
    3. Use the following command to turn on the USER LED2:

      # echo 1 > /sys/class/leds/user-led2/brightness
    4. Use the following command to turn off the USER LED2:

  2. From the Linux shell, start the evtest utility. Then press the USER BUTTON and make sure the evtest has reported the events correctly:

  3. In order to test raw GPIO functions, disable definition of the USER BUTTON in the kernel DTS file, then rebuild and reinstall the project:

  4. Each GPIO is assigned a unique integer GPIO number within the GPIO chip range of 0 to 416 by Linux. The i.MX RT1170 supports 13 GPIO blocks of 32 pieces. To calculate that number for a specific GPIO, use the following formula
    gpio_idX(GPIOX_IOY) = (X - 1) * 32 + Y
    where an X value is a GPIO number and Y is a GPIO pin number.
    For example: gpio number for GPIO13.0 will be (13 -- 1) * 32 + 0 = 384.

  5. Export GPIO5_00 and configure it as an input:

  6. Make sure the value of GPIO13_00 is 1 when the USER BUTTON is untouched (due to the internal PULL-UP being enabled):

  7. Press and hold the USER BUTTON and make sure the GPIO13_00 value has changed to 0:

2. Alternative Ways to Access GPIO

In Linux, you may access GPIOs using different approaches, not only the ones described in this application note above. Here are some external links that might be useful if you decide to try an alternative approach.

The following article describes accessing GPIOs from the kernel context:
https://lwn.net/Articles/532714/

To work with GPIOs from the user space, there are the following possibilities:

  1. Using the GPIOLIB interface (described in this application note):
    https://www.kernel.org/doc/Documentation/gpio/sysfs.txt

  2. Using the drivers of the Linux LED/Input subsystems:
    https://www.kernel.org/doc/Documentation/leds/leds-class.txt
    https://www.kernel.org/doc/Documentation/input/input.txt
    These drivers allow to use different GPIO-related mechanisms already implemented in Linux. For example, you may simply force a LED connected to GPIO output to blink with the specified frequency, or simply force input subsystem to generate a some-button-pressed event on changing GPIO input.