> On November 27, 2019 at 9:42 AM Geert Uytterhoeven <geert+renesas@xxxxxxxxx> wrote:
>
>
> Document the GPIO Aggregator/Repeater, and the three typical use-cases.
>
> Signed-off-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> ---
> v3:
> - New.
> ---
> .../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
> --- /dev/null
> +++ b/Documentation/admin-guide/gpio/gpio-aggregator.rst
> @@ -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
> --
> 2.17.1
>
Reviewed-by: Ulrich Hecht <uli+renesas@xxxxxxxx>
CU
Uli
