[PATCH v4 4/5] docs: gpio: Add GPIO Aggregator documentation

[Geert Uytterhoeven <geert+renesas@glider.be>]

Document the GPIO Aggregator, and the two typical use-cases.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>
---
v4:
  - Add Reviewed-by,
  - Drop controversial GPIO repeater,
  - Clarify industrial control use case,
  - Fix typo s/communicated/communicate/,
  - Replace abstract frobnicator example by concrete door example with
    gpio-line-names,

v3:
  - New.
---
 .../admin-guide/gpio/gpio-aggregator.rst      | 102 ++++++++++++++++++
 Documentation/admin-guide/gpio/index.rst      |   1 +
 2 files changed, 103 insertions(+)
 create mode 100644 Documentation/admin-guide/gpio/gpio-aggregator.rst

diff --git a/Documentation/admin-guide/gpio/gpio-aggregator.rst b/Documentation/admin-guide/gpio/gpio-aggregator.rst
new file mode 100644
index 0000000000000000..954ed568b0b833b3
--- /dev/null
+++ b/Documentation/admin-guide/gpio/gpio-aggregator.rst
@@ -0,0 +1,102 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+GPIO Aggregator
+===============
+
+The GPIO Aggregator allows to aggregate GPIOs, and expose them as a new
+gpio_chip.  This supports the following use cases.
+
+
+Aggregating GPIOs using Sysfs
+-----------------------------
+
+GPIO controllers are exported to userspace using /dev/gpiochip* character
+devices.  Access control to these devices is provided by standard UNIX file
+system permissions, on an all-or-nothing basis: either a GPIO controller is
+accessible for a user, or it is not.
+
+The GPIO Aggregator allows access control for individual GPIOs, by aggregating
+them into a new gpio_chip, which can be assigned to a group or user using
+standard UNIX file ownership and permissions.  Furthermore, this simplifies and
+hardens exporting GPIOs to a virtual machine, as the VM can just grab the full
+GPIO controller, and no longer needs to care about which GPIOs to grab and
+which not, reducing the attack surface.
+
+Aggregated GPIO controllers are instantiated and destroyed by writing to
+write-only attribute files in sysfs.
+
+    /sys/bus/platform/drivers/gpio-aggregator/
+
+	"new_device" ...
+		Userspace may ask the kernel to instantiate an aggregated GPIO
+		controller by writing a string describing the GPIOs to
+		aggregate to the "new_device" file, using the format
+
+		.. code-block:: none
+
+		    [<gpioA>] [<gpiochipB> <offsets>] ...
+
+		Where:
+
+		    "<gpioA>" ...
+			    is a GPIO line name,
+
+		    "<gpiochipB>" ...
+			    is a GPIO chip label or name, and
+
+		    "<offsets>" ...
+			    is a comma-separated list of GPIO offsets and/or
+			    GPIO offset ranges denoted by dashes.
+
+		Example: Instantiate a new GPIO aggregator by aggregating GPIO
+		19 of "e6052000.gpio" and GPIOs 20-21 of "gpiochip2" into a new
+		gpio_chip:
+
+		.. code-block:: bash
+
+		    echo 'e6052000.gpio 19 gpiochip2 20-21' > new_device
+
+	"delete_device" ...
+		Userspace may ask the kernel to destroy an aggregated GPIO
+		controller after use by writing its device name to the
+		"delete_device" file.
+
+		Example: Destroy the previously-created aggregated GPIO
+		controller "gpio-aggregator.0":
+
+		.. code-block:: bash
+
+		    echo gpio-aggregator.0 > delete_device
+
+
+Generic GPIO Driver
+-------------------
+
+The GPIO Aggregator can also be used as a generic driver for a simple
+GPIO-operated device described in DT, without a dedicated in-kernel driver.
+This is useful in industrial control, and is not unlike e.g. spidev, which
+allows to communicate with an SPI device from userspace.
+
+Binding a device to the GPIO Aggregator is performed either by modifying the
+gpio-aggregator driver, or by writing to the "driver_override" file in Sysfs.
+
+Example: If "door" is a GPIO-operated device described in DT, using its own
+compatible value::
+
+        door {
+                compatible = "myvendor,mydoor";
+
+                gpios = <&gpio2 19 GPIO_ACTIVE_HIGH>,
+                        <&gpio2 20 GPIO_ACTIVE_LOW>;
+		gpio-line-names = "open", "lock";
+        };
+
+it can be bound to the GPIO Aggregator by either:
+
+1. Adding its compatible value to ``gpio_aggregator_dt_ids[]``,
+2. Binding manually using "driver_override":
+
+.. code-block:: bash
+
+    echo gpio-aggregator > /sys/bus/platform/devices/door/driver_override
+    echo door > /sys/bus/platform/drivers/gpio-aggregator/bind
diff --git a/Documentation/admin-guide/gpio/index.rst b/Documentation/admin-guide/gpio/index.rst
index a244ba4e87d5398a..ef2838638e967777 100644
--- a/Documentation/admin-guide/gpio/index.rst
+++ b/Documentation/admin-guide/gpio/index.rst
@@ -7,6 +7,7 @@ gpio
 .. toctree::
     :maxdepth: 1
 
+    gpio-aggregator
     sysfs
 
 .. only::  subproject and html
-- 
2.17.1