.. SPDX-License-Identifier: GPL-2.0-or-later

Configfs GPIO Simulator
=======================

The configfs GPIO Simulator (gpio-sim) provides a way to create simulated GPIO
chips for testing purposes. The lines exposed by these chips can be accessed
using the standard GPIO character device interface as well as manipulated
using sysfs attributes.

Creating simulated chips
------------------------

The gpio-sim module registers a configfs subsystem called ``'gpio-sim'``. For
details of the configfs filesystem, please refer to the configfs documentation.

The user can create a hierarchy of configfs groups and items as well as modify
values of exposed attributes. Once the chip is instantiated, this hierarchy
will be translated to appropriate device properties. The general structure is:

**Group:** ``/config/gpio-sim``

This is the top directory of the gpio-sim configfs tree.

**Group:** ``/config/gpio-sim/gpio-device``

**Attribute:** ``/config/gpio-sim/gpio-device/dev_name``

**Attribute:** ``/config/gpio-sim/gpio-device/live``

This is a directory representing a GPIO platform device. The ``'dev_name'``
attribute is read-only and allows the user-space to read the platform device
name (e.g. ``'gpio-sim.0'``). 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 simulated device and ``'0'`` to disable and tear
it down.

**Group:** ``/config/gpio-sim/gpio-device/gpio-bankX``

**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/chip_name``

**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/num_lines``

This group represents a bank of GPIOs under the top platform device. The
``'chip_name'`` attribute is read-only and allows the user-space to read the
device name of the bank device. The ``'num_lines'`` attribute allows to specify
the number of lines exposed by this bank.

**Group:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY``

**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/name``

This group represents a single line at the offset Y. The 'name' attribute
allows to set the line name as represented by the 'gpio-line-names' property.

**Item:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog``

**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog/name``

**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog/direction``

This item makes the gpio-sim module hog the associated line. The ``'name'``
attribute specifies the in-kernel consumer name to use. The ``'direction'``
attribute specifies the hog direction and must be one of: ``'input'``,
``'output-high'`` and ``'output-low'``.

Inside each bank directory, there's a set of attributes that can be used to
configure the new chip. Additionally the user can ``mkdir()`` subdirectories
inside the chip's directory that allow to pass additional configuration for
specific lines. The name of those subdirectories must take the form of:
``'line<offset>'`` (e.g. ``'line0'``, ``'line20'``, etc.) as the name will be
used by the module to assign the config to the specific line at given offset.

Once the confiuration is complete, the ``'live'`` attribute must be set to 1 in
order to instantiate the chip. It can be set back to 0 to destroy the simulated
chip. The module will synchronously wait for the new simulated device to be
successfully probed and if this doesn't happen, writing to ``'live'`` will
result in an error.

Simulated GPIO chips can also be defined in device-tree. The compatible string
must be: ``"gpio-simulator"``. Supported properties are:

  ``"gpio-sim,label"`` - chip label

Other standard GPIO properties (like ``"gpio-line-names"``, ``"ngpios"`` or
``"gpio-hog"``) are also supported. Please refer to the GPIO documentation for
details.

An example device-tree code defining a GPIO simulator:

.. code-block :: none

    gpio-sim {
        compatible = "gpio-simulator";

        bank0 {
            gpio-controller;
            #gpio-cells = <2>;
            ngpios = <16>;
            gpio-sim,label = "dt-bank0";
            gpio-line-names = "", "sim-foo", "", "sim-bar";
        };

        bank1 {
            gpio-controller;
            #gpio-cells = <2>;
            ngpios = <8>;
            gpio-sim,label = "dt-bank1";

            line3 {
                gpio-hog;
                gpios = <3 0>;
                output-high;
                line-name = "sim-hog-from-dt";
            };
        };
    };

Manipulating simulated lines
----------------------------

Each simulated GPIO chip creates a separate sysfs group under its device
directory for each exposed line
(e.g. ``/sys/devices/platform/gpio-sim.X/gpiochipY/``). The name of each group
is of the form: ``'sim_gpioX'`` where X is the offset of the line. Inside each
group there are two attibutes:

    ``pull`` - allows to read and set the current simulated pull setting for
               every line, when writing the value must be one of: ``'pull-up'``,
               ``'pull-down'``

    ``value`` - allows to read the current value of the line which may be
                different from the pull if the line is being driven from
                user-space