GPIO interface uses deprecated GPIO sysfs interface and should use the GPIO character device

[Siggi Skulason]

The snapd builtin gpio interface provides access to the GPIO sysfs Interface for userspace at sys/class/gpio/gpio

https://github.com/snapcore/snapd/blob/45d65822c95b7f7f4120ec4a9723f2af0bb5b4b8/interfaces/builtin/gpio.go#L44

The GPIO sysfs interface has however been deprecated since version 4.8 of the kernel and the snapd GPIO interface should provide access to /sys/bus/gpiochipN (or /dev/gpiochipN) as well.

Reference:

1) https://www.kernel.org/doc/html/latest/admin-guide/gpio/sysfs.html

"Warning
THIS ABI IS DEPRECATED, THE ABI DOCUMENTATION HAS BEEN MOVED TO Documentation/ABI/obsolete/sysfs-gpio AND NEW USERSPACE CONSUMERS ARE SUPPOSED TO USE THE CHARACTER DEVICE ABI. THIS OLD SYSFS ABI WILL NOT BE DEVELOPED (NO NEW FEATURES), IT WILL JUST BE MAINTAINED."

2)https://www.kernel.org/doc/Documentation/ABI/obsolete/sysfs-gpio

"This ABI is deprecated and will be removed after 2020. It is
replaced with the GPIO character device."

3) https://www.kernel.org/doc/html/latest/driver-api/gpio/intro.html

The descriptor-based interface is the preferred way to manipulate GPIOs, and is described by all the files in this directory excepted gpio-legacy.txt.
The legacy integer-based interface which is considered deprecated (but still usable for compatibility reasons) is documented in gpio-legacy.txt.

4) https://www.beyondlogic.org/an-introduction-to-chardev-gpio-and-libgpiod-on-the-raspberry-pi/

sysfs GPIO
User-mode GPIO (General Purpose Input/Output) has historically been performed via the legacy “integer-based”sysfs pseudo file system. For example, to set GPIO25, one would:

# echo out > /sys/class/gpio/gpio25/direction
# echo 1 > /sys/class/gpio/gpio25/value
GPIO access via this legacy sysfs interface has been deprecated since version 4.8 of the Linux kernel.

chardev GPIO
The new way of doing GPIO is via the “descriptor-based” character device ABI (Application Binary Interface). The interface is exposed at /dev/gpiochipN or /sys/bus/gpiochipN where N is the chip number.