From ce7a2f77f9766378b51422f8f45fd06cdc44ef0b Mon Sep 17 00:00:00 2001 From: Geert Uytterhoeven Date: Mon, 11 May 2020 16:52:56 +0200 Subject: [PATCH] docs: gpio: Add GPIO Aggregator documentation Document the GPIO Aggregator, and the two typical use-cases. Signed-off-by: Geert Uytterhoeven Tested-by: Eugeniu Rosca Reviewed-by: Ulrich Hecht Reviewed-by: Eugeniu Rosca Link: https://lore.kernel.org/r/20200511145257.22970-6-geert+renesas@glider.be Signed-off-by: Linus Walleij --- Documentation/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 0000000..5cd1e72 --- /dev/null +++ b/Documentation/admin-guide/gpio/gpio-aggregator.rst @@ -0,0 +1,111 @@ +.. SPDX-License-Identifier: GPL-2.0-only + +GPIO Aggregator +=============== + +The GPIO Aggregator provides a mechanism 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 provides access control for a set of one or more 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 + + [] [ ] ... + + Where: + + "" ... + is a GPIO line name, + + "" ... + is a GPIO chip label, and + + "" ... + 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 + line 19 of "e6052000.gpio" and GPIO lines 20-21 of + "e6050000.gpio" into a new gpio_chip: + + .. code-block:: sh + + $ echo 'e6052000.gpio 19 e6050000.gpio 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, assumed to be "gpio-aggregator.0": + + .. code-block:: sh + + $ 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 the user 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:: sh + + $ echo gpio-aggregator > /sys/bus/platform/devices/door/driver_override + $ echo door > /sys/bus/platform/drivers/gpio-aggregator/bind + +After that, a new gpiochip "door" has been created: + +.. code-block:: sh + + $ gpioinfo door + gpiochip12 - 2 lines: + line 0: "open" unused input active-high + line 1: "lock" unused input active-high diff --git a/Documentation/admin-guide/gpio/index.rst b/Documentation/admin-guide/gpio/index.rst index a244ba4..ef28386 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 -- 2.7.4