[PATCH 23/23] Documentation: gpio: add documentation for gpio-mockup

[Bartosz Golaszewski <brgl@bgdev.pl>]

From: Bartosz Golaszewski <bgolaszewski@baylibre.com>

There's some documentation for gpio-mockup's debugfs interface in the
driver's source but it's not much. Add proper documentation for this
testing module.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
---
 .../admin-guide/gpio/gpio-mockup.rst          | 87 +++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 Documentation/admin-guide/gpio/gpio-mockup.rst

diff --git a/Documentation/admin-guide/gpio/gpio-mockup.rst b/Documentation/admin-guide/gpio/gpio-mockup.rst
new file mode 100644
index 000000000000..1d452ee55f8d
--- /dev/null
+++ b/Documentation/admin-guide/gpio/gpio-mockup.rst
@@ -0,0 +1,87 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+GPIO Testing Driver
+===================
+
+The GPIO Testing Driver (gpio-mockup) provides a way to create simulated GPIO
+chips for testing purposes. There are two ways of configuring the chips exposed
+by the module. The lines can be accessed using the standard GPIO character
+device interface as well as manipulated using the dedicated debugfs directory
+structure.
+
+Creating simulated chips using debugfs
+--------------------------------------
+
+When the gpio-mockup module is loaded (or builtin) it creates its own directory
+in debugfs. Assuming debugfs is mounted at /sys/kernel/debug/, the directory
+will be located at /sys/kernel/debug/gpio-mockup/. Inside this directory there
+are two attributes: new_device and delete_device.
+
+New chips can be created by writing a single line containing a number of
+options to "new_device". For example:
+
+.. code-block:: sh
+
+    $ echo "label=my-mockup num_lines=4 named_lines" > /sys/kernel/debug/gpio-mockup/new_device
+
+Supported options:
+
+    num_lines=<num_lines> - number of GPIO lines to expose
+
+    label=<label> - label of the dummy chip
+
+    named_lines - defines whether dummy lines should be named, the names are
+                  of the form X-Y where X is the chip's label and Y is the
+                  line's offset
+
+Note: only num_lines is mandatory.
+
+Chips can be dynamically removed by writing the chip's label to
+"delete_device". For example:
+
+.. code-block:: sh
+
+    echo "gpio-mockup.0" > /sys/kernel/debug/gpio-mockup/delete_device
+
+Creating simulated chips using module params
+--------------------------------------------
+
+Note: this is an older, now deprecated method kept for backward compatibility
+for user-space tools.
+
+When loading the gpio-mockup driver a number of parameters can be passed to the
+module.
+
+    gpio_mockup_ranges
+
+        This parameter takes an argument in the form of an array of integer
+        pairs. Each pair defines the base GPIO number (if any) and the number
+        of lines exposed by the chip. If the base GPIO is -1, the gpiolib
+        will assign it automatically.
+
+        Example: gpio_mockup_ranges=-1,8,-1,16,405,4
+
+        The line above creates three chips. The first one will expose 8 lines,
+        the second 16 and the third 4. The base GPIO for the third chip is set
+        to 405 while for two first chips it will be assigned automatically.
+
+    gpio_named_lines
+
+        This parameter doesn't take any arguments. It lets the driver know that
+        GPIO lines exposed by it should be named.
+
+        The name format is: gpio-mockup-X-Y where X is the letter associated
+        with the mockup chip and Y is the line offset.
+
+Manipulating simulated lines
+----------------------------
+
+Each mockup chip creates its own subdirectory in /sys/kernel/debug/gpio-mockup/.
+The directory is named after the chip's label. A symlink is also created, named
+after the chip's name, which points to the label directory.
+
+Inside each subdirectory, there's a separate attribute for each GPIO line. The
+name of the attribute represents the line's offset in the chip.
+
+Reading from a line attribute returns the current value. Writing to it (0 or 1)
+changes its pull.
-- 
2.26.1