1.. SPDX-License-Identifier: GPL-2.0-or-later
2
3Configfs GPIO Simulator
4=======================
5
6The configfs GPIO Simulator (gpio-sim) provides a way to create simulated GPIO
7chips for testing purposes. The lines exposed by these chips can be accessed
8using the standard GPIO character device interface as well as manipulated
9using sysfs attributes.
10
11Creating simulated chips
12------------------------
13
14The gpio-sim module registers a configfs subsystem called ``'gpio-sim'``. For
15details of the configfs filesystem, please refer to the configfs documentation.
16
17The user can create a hierarchy of configfs groups and items as well as modify
18values of exposed attributes. Once the chip is instantiated, this hierarchy
19will be translated to appropriate device properties. The general structure is:
20
21**Group:** ``/config/gpio-sim``
22
23This is the top directory of the gpio-sim configfs tree.
24
25**Group:** ``/config/gpio-sim/gpio-device``
26
27**Attribute:** ``/config/gpio-sim/gpio-device/dev_name``
28
29**Attribute:** ``/config/gpio-sim/gpio-device/live``
30
31This is a directory representing a GPIO platform device. The ``'dev_name'``
32attribute is read-only and allows the user-space to read the platform device
33name (e.g. ``'gpio-sim.0'``). The ``'live'`` attribute allows to trigger the
34actual creation of the device once it's fully configured. The accepted values
35are: ``'1'`` to enable the simulated device and ``'0'`` to disable and tear
36it down.
37
38**Group:** ``/config/gpio-sim/gpio-device/gpio-bankX``
39
40**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/chip_name``
41
42**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/num_lines``
43
44This group represents a bank of GPIOs under the top platform device. The
45``'chip_name'`` attribute is read-only and allows the user-space to read the
46device name of the bank device. The ``'num_lines'`` attribute allows to specify
47the number of lines exposed by this bank.
48
49**Group:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY``
50
51**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/name``
52
53This group represents a single line at the offset Y. The 'name' attribute
54allows to set the line name as represented by the 'gpio-line-names' property.
55
56**Item:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog``
57
58**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog/name``
59
60**Attribute:** ``/config/gpio-sim/gpio-device/gpio-bankX/lineY/hog/direction``
61
62This item makes the gpio-sim module hog the associated line. The ``'name'``
63attribute specifies the in-kernel consumer name to use. The ``'direction'``
64attribute specifies the hog direction and must be one of: ``'input'``,
65``'output-high'`` and ``'output-low'``.
66
67Inside each bank directory, there's a set of attributes that can be used to
68configure the new chip. Additionally the user can ``mkdir()`` subdirectories
69inside the chip's directory that allow to pass additional configuration for
70specific lines. The name of those subdirectories must take the form of:
71``'line<offset>'`` (e.g. ``'line0'``, ``'line20'``, etc.) as the name will be
72used by the module to assign the config to the specific line at given offset.
73
74Once the confiuration is complete, the ``'live'`` attribute must be set to 1 in
75order to instantiate the chip. It can be set back to 0 to destroy the simulated
76chip. The module will synchronously wait for the new simulated device to be
77successfully probed and if this doesn't happen, writing to ``'live'`` will
78result in an error.
79
80Simulated GPIO chips can also be defined in device-tree. The compatible string
81must be: ``"gpio-simulator"``. Supported properties are:
82
83  ``"gpio-sim,label"`` - chip label
84
85Other standard GPIO properties (like ``"gpio-line-names"``, ``"ngpios"`` or
86``"gpio-hog"``) are also supported. Please refer to the GPIO documentation for
87details.
88
89An example device-tree code defining a GPIO simulator:
90
91.. code-block :: none
92
93    gpio-sim {
94        compatible = "gpio-simulator";
95
96        bank0 {
97            gpio-controller;
98            #gpio-cells = <2>;
99            ngpios = <16>;
100            gpio-sim,label = "dt-bank0";
101            gpio-line-names = "", "sim-foo", "", "sim-bar";
102        };
103
104        bank1 {
105            gpio-controller;
106            #gpio-cells = <2>;
107            ngpios = <8>;
108            gpio-sim,label = "dt-bank1";
109
110            line3 {
111                gpio-hog;
112                gpios = <3 0>;
113                output-high;
114                line-name = "sim-hog-from-dt";
115            };
116        };
117    };
118
119Manipulating simulated lines
120----------------------------
121
122Each simulated GPIO chip creates a separate sysfs group under its device
123directory for each exposed line
124(e.g. ``/sys/devices/platform/gpio-sim.X/gpiochipY/``). The name of each group
125is of the form: ``'sim_gpioX'`` where X is the offset of the line. Inside each
126group there are two attibutes:
127
128    ``pull`` - allows to read and set the current simulated pull setting for
129               every line, when writing the value must be one of: ``'pull-up'``,
130               ``'pull-down'``
131
132    ``value`` - allows to read the current value of the line which may be
133                different from the pull if the line is being driven from
134                user-space
135