Documentation: gpio: document configfs interface for gpio-aggregator

BugLink: https://bugs.launchpad.net/bugs/2103496

Add documentation for the newly added configfs-based interface for GPIO
aggregator.

Signed-off-by: Koichiro Den<koichiro.den@canonical.com>
Link:https://lore.kernel.org/r/20250407043019.4105613-9-koichiro.den@canonical.com
Signed-off-by: Bartosz Golaszewski<bartosz.golaszewski@linaro.org>

(cherry picked from commit 10f94d092bba9ab08004c624c604e976e6e3cd22 linux-next)
Signed-off-by: Koichiro Den<koichiro.den@canonical.com>
Acked-by: Jacob Martin <jacob.martin@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Documentation/admin-guide/gpio/gpio-aggregator.rst

@@ -69,6 +69,113 @@ write-only attribute files in sysfs.
 		    $ echo gpio-aggregator.0 > delete_device
 
 
+Aggregating GPIOs using Configfs
+--------------------------------
+
+**Group:** ``/config/gpio-aggregator``
+
+    This is the root directory of the gpio-aggregator configfs tree.
+
+**Group:** ``/config/gpio-aggregator/<example-name>``
+
+    This directory represents a GPIO aggregator device. You can assign any
+    name to ``<example-name>`` (e.g. ``agg0``), except names starting with
+    ``_sysfs`` prefix, which are reserved for auto-generated configfs
+    entries corresponding to devices created via Sysfs.
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/live``
+
+    The ``live`` attribute allows to trigger the actual creation of the device
+    once it's fully configured. Accepted values are:
+
+    * ``1``, ``yes``, ``true`` : enable the virtual device
+    * ``0``, ``no``, ``false`` : disable the virtual device
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/dev_name``
+
+    The read-only ``dev_name`` attribute exposes the name of the device as it
+    will appear in the system on the platform bus (e.g. ``gpio-aggregator.0``).
+    This is useful for identifying a character device for the newly created
+    aggregator. If it's ``gpio-aggregator.0``,
+    ``/sys/devices/platform/gpio-aggregator.0/gpiochipX`` path tells you that the
+    GPIO device id is ``X``.
+
+You must create subdirectories for each virtual line you want to
+instantiate, named exactly as ``line0``, ``line1``, ..., ``lineY``, when
+you want to instantiate ``Y+1`` (Y >= 0) lines.  Configure all lines before
+activating the device by setting ``live`` to 1.
+
+**Group:** ``/config/gpio-aggregator/<example-name>/<lineY>/``
+
+    This directory represents a GPIO line to include in the aggregator.
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/<lineY>/key``
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/<lineY>/offset``
+
+    The default values after creating the ``<lineY>`` directory are:
+
+    * ``key`` : <empty>
+    * ``offset`` : -1
+
+    ``key`` must always be explicitly configured, while ``offset`` depends.
+    Two configuration patterns exist for each ``<lineY>``:
+
+    (a). For lookup by GPIO line name:
+
+         * Set ``key`` to the line name.
+         * Ensure ``offset`` remains -1 (the default).
+
+    (b). For lookup by GPIO chip name and the line offset within the chip:
+
+         * Set ``key`` to the chip name.
+         * Set ``offset`` to the line offset (0 <= ``offset`` < 65535).
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/<lineY>/name``
+
+    The ``name`` attribute sets a custom name for lineY. If left unset, the
+    line will remain unnamed.
+
+Once the configuration is done, the ``'live'`` attribute must be set to 1
+in order to instantiate the aggregator device. It can be set back to 0 to
+destroy the virtual device. The module will synchronously wait for the new
+aggregator device to be successfully probed and if this doesn't happen, writing
+to ``'live'`` will result in an error. This is a different behaviour from the
+case when you create it using sysfs ``new_device`` interface.
+
+.. note::
+
+   For aggregators created via Sysfs, the configfs entries are
+   auto-generated and appear as ``/config/gpio-aggregator/_sysfs.<N>/``. You
+   cannot add or remove line directories with mkdir(2)/rmdir(2). To modify
+   lines, you must use the "delete_device" interface to tear down the
+   existing device and reconfigure it from scratch. However, you can still
+   toggle the aggregator with the ``live`` attribute and adjust the
+   ``key``, ``offset``, and ``name`` attributes for each line when ``live``
+   is set to 0 by hand (i.e. it's not waiting for deferred probe).
+
+Sample configuration commands
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: sh
+
+    # Create a directory for an aggregator device
+    $ mkdir /sys/kernel/config/gpio-aggregator/agg0
+
+    # Configure each line
+    $ mkdir /sys/kernel/config/gpio-aggregator/agg0/line0
+    $ echo gpiochip0 > /sys/kernel/config/gpio-aggregator/agg0/line0/key
+    $ echo 6         > /sys/kernel/config/gpio-aggregator/agg0/line0/offset
+    $ echo test0     > /sys/kernel/config/gpio-aggregator/agg0/line0/name
+    $ mkdir /sys/kernel/config/gpio-aggregator/agg0/line1
+    $ echo gpiochip0 > /sys/kernel/config/gpio-aggregator/agg0/line1/key
+    $ echo 7         > /sys/kernel/config/gpio-aggregator/agg0/line1/offset
+    $ echo test1     > /sys/kernel/config/gpio-aggregator/agg0/line1/name
+
+    # Activate the aggregator device
+    $ echo 1         > /sys/kernel/config/gpio-aggregator/agg0/live
+
+
 Generic GPIO Driver
 -------------------