Documentation: gpio: add chardev userspace API documentation

Add documentation for the GPIO character device userspace API.

Added to the userspace-api book, but also provide a link from the
admin-guide book, as historically the GPIO documentation has been
there.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Documentation/userspace-api/gpio/error-codes.rst

@@ -0,0 +1,78 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _gpio_errors:
+
+*******************
+GPIO Error Codes
+*******************
+
+.. _gpio-errors:
+
+.. tabularcolumns:: |p{2.5cm}|p{15.0cm}|
+
+.. flat-table:: Common GPIO error codes
+    :header-rows:  0
+    :stub-columns: 0
+    :widths: 1 16
+
+    -  -  ``EAGAIN`` (aka ``EWOULDBLOCK``)
+
+       -  The device was opened in non-blocking mode and a read can't
+          be performed as there is no data available.
+
+    -  -  ``EBADF``
+
+       -  The file descriptor is not valid.
+
+    -  -  ``EBUSY``
+
+       -  The ioctl can't be handled because the device is busy. Typically
+          returned when an ioctl attempts something that would require the
+          usage of a resource that was already allocated. The ioctl must not
+          be retried without performing another action to fix the problem
+          first.
+
+    -  -  ``EFAULT``
+
+       -  There was a failure while copying data from/to userspace, probably
+	  caused by an invalid pointer reference.
+
+    -  -  ``EINVAL``
+
+       -  One or more of the ioctl parameters are invalid or out of the
+          allowed range. This is a widely used error code.
+
+    -  -  ``ENODEV``
+
+       -  Device not found or was removed.
+
+    -  -  ``ENOMEM``
+
+       -  There's not enough memory to handle the desired operation.
+
+    -  -  ``EPERM``
+
+       -  Permission denied. Typically returned in response to an attempt
+          to perform an action incompatible with the current line
+          configuration.
+
+    -  -  ``EIO``
+
+       -  I/O error. Typically returned when there are problems communicating
+          with a hardware device or requesting features that hardware does not
+          support. This could indicate broken or flaky hardware.
+          It's a 'Something is wrong, I give up!' type of error.
+
+    -  - ``ENXIO``
+
+       -  No device corresponding to this device special file exists.
+
+.. note::
+
+  #. This list is not exhaustive; ioctls may return other error codes.
+     Since errors may have side effects such as a driver reset,
+     applications should abort on unexpected errors, or otherwise
+     assume that the device is in a bad state.
+
+  #. Request-specific error codes are listed in the individual
+     requests descriptions.