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+GPIO Sysfs Interface for Userspace
+==================================
+
+Platforms which use the "gpiolib" implementors framework may choose to
+configure a sysfs user interface to GPIOs. This is different from the
+debugfs interface, since it provides control over GPIO direction and
+value instead of just showing a gpio state summary. Plus, it could be
+present on production systems without debugging support.
+
+Given appropriate hardware documentation for the system, userspace could
+know for example that GPIO #23 controls the write protect line used to
+protect boot loader segments in flash memory. System upgrade procedures
+may need to temporarily remove that protection, first importing a GPIO,
+then changing its output state, then updating the code before re-enabling
+the write protection. In normal use, GPIO #23 would never be touched,
+and the kernel would have no need to know about it.
+
+Again depending on appropriate hardware documentation, on some systems
+userspace GPIO can be used to determine system configuration data that
+standard kernels won't know about. And for some tasks, simple userspace
+GPIO drivers could be all that the system really needs.
+
+Note that standard kernel drivers exist for common "LEDs and Buttons"
+GPIO tasks:  "leds-gpio" and "gpio_keys", respectively. Use those
+instead of talking directly to the GPIOs; they integrate with kernel
+frameworks better than your userspace code could.
+
+
+Paths in Sysfs
+--------------
+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.
+
+                If the pin can be configured as interrupt-generating interrupt
+                and if it has been configured to generate interrupts (see the
+                description of "edge"), you can poll(2) on that file and
+                poll(2) will return whenever the interrupt was triggered. If
+                you use poll(2), set the events POLLPRI and POLLERR. If you
+                use select(2), set the file descriptor in exceptfds. After
+                poll(2) returns, either lseek(2) to the beginning of the sysfs
+                file and read the new value or close the file and re-open it
+                to read the value.
+
+        "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)
+
+Board documentation should in most cases cover what GPIOs are used for
+what purposes. However, those numbers are not always stable; GPIOs on
+a daughtercard might be different depending on the base board being used,
+or other cards in the stack. In such cases, you may need to use the
+gpiochip nodes (possibly in conjunction with schematics) to determine
+the correct GPIO number to use for a given signal.
+
+
+Exporting from Kernel code
+--------------------------
+Kernel code can explicitly manage exports of GPIOs which have already been
+requested using gpio_request():
+
+        /* export the GPIO to userspace */
+        int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
+
+        /* reverse gpio_export() */
+        void gpiod_unexport(struct gpio_desc *desc);
+
+        /* create a sysfs link to an exported GPIO node */
+        int gpiod_export_link(struct device *dev, const char *name,
+                      struct gpio_desc *desc);
+
+        /* change the polarity of a GPIO node in sysfs */
+        int gpiod_sysfs_set_active_low(struct gpio_desc *desc, int value);
+
+After a kernel driver requests a GPIO, it may only be made available in
+the sysfs interface by gpiod_export(). The driver can control whether the
+signal direction may change. This helps drivers prevent userspace code
+from accidentally clobbering important system state.
+
+This explicit exporting can help with debugging (by making some kinds
+of experiments easier), or can provide an always-there interface that's
+suitable for documenting as part of a board support package.
+
+After the GPIO has been exported, gpiod_export_link() allows creating
+symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
+use this to provide the interface under their own device in sysfs with
+a descriptive name.
+
+Drivers can use gpiod_sysfs_set_active_low() to hide GPIO line polarity
+differences between boards from user space. Polarity change can be done both
+before and after gpiod_export(), and previously enabled poll(2) support for
+either rising or falling edge will be reconfigured to follow this setting.