Manual Pages  — GPIO

On some embedded architectures (like MIPS), discovery of the bus and configuration of the pins is done via device.hints(5) in the platform's kernel config(5) file.

On some others (like ARM), where FDT(4) is used to describe the device tree, the bus discovery is done via the DTS passed to the kernel, being either statically compiled in, or by a variety of ways where the boot loader (or Open Firmware enabled system) passes the DTS blob to the kernel at boot.

The following device.hints(5) are only provided by the ar71xx_gpio driver:

hint.gpio.%d.pinmask
	This is a bitmask of pins on the GPIO board that we would like to expose for use to the host operating system. To expose pin 0, 4 and 7, use the bitmask of 10010001 converted to the hexadecimal value 0x0091.
hint.gpio.%d.pinon
	This is a bitmask of pins on the GPIO board that will be set to ON at host start. To set pin 2, 5 and 13 to be set ON at boot, use the bitmask of 10000000010010 converted to the hexadecimal value 0x2012.
hint.gpio.function_set hint.gpio.function_clear
	These are bitmasks of pins that will remap a pin to handle a specific function (USB, UART TX/RX, etc) in the Atheros function registers. This is mainly used to set/clear functions that we need when they are set up or not set up by uBoot.

Simply put, each pin of the GPIO interface is connected to an input/output of some device in a system.