What:		/sys/class/gpio/
Date:		July 2008
KernelVersion:	2.6.27
Contact:	Linus Walleij <linusw@kernel.org>
Description:

  As a Kconfig option, individual GPIO signals may be accessed from
  userspace.  GPIOs are only made available to userspace by an explicit
  "export" operation.  If a given GPIO is not claimed for use by
  kernel code, it may be exported by userspace (and unexported later).
  Kernel code may export it for complete or partial access.

  GPIOs are identified as they are inside the kernel, using integers in
  the range 0..INT_MAX.  See Documentation/admin-guide/gpio for more information.

    /sys/class/gpio
	/export ... asks the kernel to export a GPIO to userspace
	/unexport ... to return a GPIO to the kernel
	/gpioN ... for each exported GPIO #N OR
	/<LINE-NAME> ... for a properly named GPIO line
	    /value ... always readable, writes fail for input GPIOs
	    /direction ... r/w as: in, out (default low); write: high, low
	    /edge ... r/w as: none, falling, rising, both
	/gpiochipN ... for each gpiochip; #N is its first GPIO
	    /base ... (r/o) same as N
	    /label ... (r/o) descriptive, not necessarily unique
	    /ngpio ... (r/o) number of GPIOs; numbered N to N + (ngpio - 1)

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