Document the GPIO Aggregator/Repeater, and the three typical use-cases.
Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
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v3:
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.../admin-guide/gpio/gpio-aggregator.rst | 111 ++++++++++++++++++
Documentation/admin-guide/gpio/index.rst | 1 +
2 files changed, 112 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..826146e260253299
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@@ -0,0 +1,111 @@
+.. SPDX-License-Identifier: GPL-2.0-only
+
+GPIO Aggregator/Repeater
+========================
+
+The GPIO Aggregator/Repeater 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
+
+
+GPIO Repeater in Device Tree
+----------------------------
+
+A GPIO Repeater is a node in a Device Tree representing a repeater for one or
+more GPIOs, possibly including physical signal property translation (e.g.
+polarity inversion). This allows to model e.g. inverters in DT.
+
+See Documentation/devicetree/bindings/gpio/gpio-repeater.yaml
+
+
+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 not unlike e.g. spidev, which allows to communicated 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 "frobnicator" is a GPIO-operated device described in DT, using its
+own compatible value::
+
+ frobnicator {
+ compatible = "myvendor,frobnicator";
+
+ gpios = <&gpio2 19 GPIO_ACTIVE_HIGH>,
+ <&gpio2 20 GPIO_ACTIVE_LOW>;
+ };
+
+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/frobnicator/driver_override
+ echo frobnicator > /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
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2.17.1
