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>

1 files changed, 57 insertions, 0 deletions

diff --git a/Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst b/Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst
new file mode 100644
index 000000000000..126c2626ba6b
--- /dev/null
+++ b/Documentation/userspace-api/gpio/gpio-v2-line-set-config-ioctl.rst
@@ -0,0 +1,57 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+.. _GPIO_V2_LINE_SET_CONFIG_IOCTL:
+
+*****************************
+GPIO_V2_LINE_SET_CONFIG_IOCTL
+*****************************
+
+Name
+====
+
+GPIO_V2_LINE_SET_CONFIG_IOCTL - Update the configuration of previously requested lines.
+
+Synopsis
+========
+
+.. c:macro:: GPIO_V2_LINE_SET_CONFIG_IOCTL
+
+``int ioctl(int req_fd, GPIO_V2_LINE_SET_CONFIG_IOCTL, struct gpio_v2_line_config *config)``
+
+Arguments
+=========
+
+``req_fd``
+    The file descriptor of the GPIO character device, as returned in the
+    :c:type:`request.fd<gpio_v2_line_request>` by gpio-v2-get-line-ioctl.rst.
+
+``config``
+    The new :c:type:`configuration<gpio_v2_line_config>` to apply to the
+    requested lines.
+
+Description
+===========
+
+Update the configuration of previously requested lines, without releasing the
+line or introducing potential glitches.
+
+The new configuration must specify the configuration of all requested lines.
+
+The same :ref:`gpio-v2-get-line-config-rules` that apply when requesting the lines
+also apply when updating the line configuration.
+
+The motivating use case for this command is changing direction of
+bi-directional lines between input and output, but it may also be used to
+dynamically control edge detection, or more generally move lines seamlessly
+from one configuration state to another.
+
+To only change the value of output lines, use
+gpio-v2-line-set-values-ioctl.rst.
+
+Return Value
+============
+
+On success 0.
+
+On error -1 and the ``errno`` variable is set appropriately.
+Common error codes are described in error-codes.rst.