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+What: /sys/class/gpio/
+Date: July 2008
+KernelVersion: 2.6.27
+Contact: David Brownell <dbrownell@users.sourceforge.net>
+ 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/gpio.txt 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
+ /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)