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
	    /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
	    /active_low ... r/w as: 0, 1
	/gpiochipN ... for each gpiochip; #N is its first GPIO
	    /base ... (r/o) same as N
	    /label ... (r/o) descriptive chip name
	    /ngpio ... (r/o) number of GPIOs; numbered N to N + (ngpio - 1)
	    /gpio<OFFSET>
	        /value ... always readable, writes fail for input GPIOs
	        /direction ... r/w as: in, out (default low); write: high, low
	/chipX ... for each gpiochip; #X is the gpio device ID
	    /export ... asks the kernel to export a GPIO at HW offset X to userspace
	    /unexport ... to return a GPIO at HW offset X to the kernel
	    /label ... (r/o) descriptive chip name
	    /ngpio ... (r/o) number of GPIOs exposed by the chip

  This ABI is obsoleted by Documentation/ABI/testing/gpio-cdev and will be
  removed after 2020.