linux/Documentation/ABI/obsolete/sysfs-gpio
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   1What:           /sys/class/gpio/
   2Date:           July 2008
   3KernelVersion:  2.6.27
   4Contact:        Linus Walleij <linusw@kernel.org>
   5Description:
   6
   7  As a Kconfig option, individual GPIO signals may be accessed from
   8  userspace.  GPIOs are only made available to userspace by an explicit
   9  "export" operation.  If a given GPIO is not claimed for use by
  10  kernel code, it may be exported by userspace (and unexported later).
  11  Kernel code may export it for complete or partial access.
  12
  13  GPIOs are identified as they are inside the kernel, using integers in
  14  the range 0..INT_MAX.  See Documentation/admin-guide/gpio for more information.
  15
  16  ::
  17
  18    /sys/class/gpio
  19        /export ... asks the kernel to export a GPIO to userspace
  20        /unexport ... to return a GPIO to the kernel
  21        /gpioN ... for each exported GPIO #N OR
  22        /<LINE-NAME> ... for a properly named GPIO line
  23            /value ... always readable, writes fail for input GPIOs
  24            /direction ... r/w as: in, out (default low); write: high, low
  25            /edge ... r/w as: none, falling, rising, both
  26        /gpiochipN ... for each gpiochip; #N is its first GPIO
  27            /base ... (r/o) same as N
  28            /label ... (r/o) descriptive, not necessarily unique
  29            /ngpio ... (r/o) number of GPIOs; numbered N to N + (ngpio - 1)
  30
  31  This ABI is deprecated and will be removed after 2020. It is
  32  replaced with the GPIO character device.
  33