[RFC PATCH 2/2] Documentation: gpio: document configfs interface for gpio-aggregator

[Koichiro Den <koichiro.den@xxxxxxxxxxxxx>]

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

Signed-off-by: Koichiro Den <koichiro.den@xxxxxxxxxxxxx>
---
 .../admin-guide/gpio/gpio-aggregator.rst      | 86 +++++++++++++++++++
 1 file changed, 86 insertions(+)

diff --git a/Documentation/admin-guide/gpio/gpio-aggregator.rst b/Documentation/admin-guide/gpio/gpio-aggregator.rst
index 5cd1e7221756..219ef2bb70ab 100644
--- a/Documentation/admin-guide/gpio/gpio-aggregator.rst
+++ b/Documentation/admin-guide/gpio/gpio-aggregator.rst
@@ -69,6 +69,92 @@ 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``).
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/live``
+
+    The ``'live'`` attribute allows to trigger the actual creation of the device
+    once it's fully configured. The accepted values are: ``'1'`` to enable the
+    virtual device and ``'0'`` to disable and tear it down.
+
+**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``.
+
+**Attribute:** ``/config/gpio-aggregator/<example-name>/num_lines``
+
+    The ``num_lines`` attribute sets the number of lines to aggregate. It
+    must be set to a value greater than 0 before proceeding with line
+    configuration. For a value of Y+1 (where Y>=0), you must create exactly
+    ``line0``, ``line1``, ..., ``lineY`` subdirectories and 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``
+
+    If ``offset`` is >= 0:
+        * ``key`` specifies the name of the chip this GPIO belongs to
+        * ``offset`` specifies the line offset within that chip.
+    If ``offset`` is <0:
+        * ``key`` needs to specify the GPIO line name.
+
+**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.
+
+Sample configuration commands
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: sh
+
+    # Create a directory for an aggregator device
+    $ mkdir /sys/kernel/config/gpio-aggregator/agg0
+
+    # Set how many lines to aggregate beforehand
+    $ echo 2         > /sys/kernel/config/gpio-aggregator/agg0/num_lines
+
+    # 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
 -------------------
 
-- 
2.45.2