Interfacing with GPIOs from User Space in Linux

Introduction

General-Purpose Input/Output (GPIO) pins are versatile and fundamental features of computers, particularly with single-board computers like the Raspberry Pi and BeagleBone. They enable the computer to interact with the physical world, serving as a bridge between software and hardware. Linux, being one of the most used operating systems for such devices, provides interfaces to interact with these GPIOs. This article sheds light on how to interface with GPIOs from user space in Linux, using both older and newer mechanisms provided by the kernel.

Sysfs Interface: The Legacy Method

In earlier Linux kernel versions, GPIOs were commonly accessed via the sysfs interface. This interface is now deprecated in favor of the character device interface. However, for those working with legacy systems, understanding sysfs remains important.

Here's a basic outline of how one would interact with a GPIO pin using the sysfs interface:

1. Export the GPIO to make it available for user space interaction.
2. Define the GPIO direction (input or output).
3. Write (or read) values to the GPIO.
4. Unexport the GPIO when done.

For older Linux kernels (before the introduction of the GPIO character device interface), GPIOs were commonly accessed via the sysfs interface. Here's how you'd do the same GPIO toggle using sysfs, written in C:

Explanation:

1. Export GPIO: To start controlling a GPIO, it first needs to be exported to /sys/class/gpio. This makes the necessary files and folders for controlling the GPIO available in sysfs.
2. Set Direction: Once exported, the direction of the GPIO (input or output) must be set. In this example, we're setting the direction to "out" because we want to send output signals to toggle the GPIO value.
3. Toggle GPIO: To toggle the GPIO, we write to the "value" file. Writing "1" sets the GPIO high, and "0" sets it low.
4. Unexport GPIO: After using the GPIO, it's a good practice to unexport the GPIO. This releases the GPIO and cleans up the files and folders in sysfs.

GPIO Character Device Interface: The Modern Method

In more recent Linux kernel versions, the GPIO character device interface has been introduced, replacing the older sysfs interface. It offers a cleaner and more intuitive interface for GPIO operations. Most users will interact with this interface through the libgpiod library, which provides both command-line utilities and a C library.

Here's a step-by-step breakdown using the GPIO character device interface:

1. Open the desired GPIO chip, a collection of GPIO lines controlled by the same kernel driver.
2. Get a reference to the specific GPIO line you wish to control.
3. Request control of the GPIO line and set its direction (input or output).
4. Set or get the GPIO value using appropriate functions.
5. Release the GPIO line after use.

Below is a simple example in C to toggle a GPIO pin on and off. The example assumes you're using a modern version of the Linux kernel that supports the GPIO character device interface and that you've installed the libgpiod library:

Explanation:

1. We begin by including the necessary headers.
2. We then open the desired GPIO chip (in this case, "gpiochip0"). In Linux, a chip is a set of GPIO lines (pins) controlled by the same kernel driver.
3. After that, we get a reference to our specific GPIO line using gpiod_chip_get_line.
4. We request control of the GPIO line as an output using gpiod_line_request_output. The initial state is set to low (0).
5. We toggle the GPIO value a few times gpiod_line_set_value and use sleep to introduce a delay between the toggles.
6. Finally, we release the GPIO line and close our reference to the GPIO chip.

Conclusion

Interfacing with GPIO pins in Linux offers the capability to build a bridge between the digital world of software and the tangible world of hardware. While the sysfs method provided the foundation for GPIO interaction in Linux, the newer character device interface simplifies and streamlines the process, making it more intuitive for developers. Regardless of the method, understanding how to communicate with GPIO pins is essential for anyone looking to dive deeper into hardware-software integration on Linux platforms.