linux 11.24-3

[*]   /sys/class/gpio/... (sysfs interface)

There are three kinds of entry in /sys/class/gpio:

   -    Control interfaces used to get userspace control over GPIOs;

   -    GPIOs themselves; and

   -    GPIO controllers ("gpio_chip" instances).

That's in addition to standard files including the "device" symlink.

The control interfaces are write-only:

    /sys/class/gpio/

        "export" ... Userspace may ask the kernel to export control of
        a GPIO to userspace by writing its number to this file.

        Example:  "echo 19 > export" will create a "gpio19" node
        for GPIO #19, if that's not requested by kernel code.

        "unexport" ... Reverses the effect of exporting to userspace.

        Example:  "echo 19 > unexport" will remove a "gpio19"
        node exported using the "export" file.

GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
and have the following read/write attributes:

    /sys/class/gpio/gpioN/

    "direction" ... reads as either "in" or "out".  This value may
        normally be written.  Writing as "out" defaults to
        initializing the value as low.  To ensure glitch free
        operation, values "low" and "high" may be written to
        configure the GPIO as an output with that initial value.

        Note that this attribute *will not exist* if the kernel
        doesn't support changing the direction of a GPIO, or
        it was exported by kernel code that didn't explicitly
        allow userspace to reconfigure this GPIO's direction.

    "value" ... reads as either 0 (low) or 1 (high).  If the GPIO
        is configured as an output, this value may be written;
        any nonzero value is treated as high.

    "edge" ... reads as either "none", "rising", "falling", or
        "both". Write these strings to select the signal edge(s)
        that will make poll(2) on the "value" file return.

        This file exists only if the pin can be configured as an
        interrupt generating input pin.

    "active_low" ... reads as either 0 (false) or 1 (true).  Write
        any nonzero value to invert the value attribute both
        for reading and writing.  Existing and subsequent
        poll(2) support configuration via the edge attribute
        for "rising" and "falling" edges will follow this
        setting.

GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
controller implementing GPIOs starting at #42) and have the following
read-only attributes:

    /sys/class/gpio/gpiochipN/

        "base" ... same as N, the first GPIO managed by this chip

        "label" ... provided for diagnostics (not always unique)

        "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)