1 .. SPDX-License-Identifier: GPL-2.0-only
2
3 GPIO Aggregator
4 ===============
5
6 The GPIO Aggregator provides a mechanism to aggregate GPIOs, and expose them as
7 a new gpio_chip. This supports the following use cases.
8
9
10 Aggregating GPIOs using Sysfs
11 -----------------------------
12
13 GPIO controllers are exported to userspace using /dev/gpiochip* character
14 devices. Access control to these devices is provided by standard UNIX file
15 system permissions, on an all-or-nothing basis: either a GPIO controller is
16 accessible for a user, or it is not.
17
18 The GPIO Aggregator provides access control for a set of one or more GPIOs, by
19 aggregating them into a new gpio_chip, which can be assigned to a group or user
20 using standard UNIX file ownership and permissions. Furthermore, this
21 simplifies and hardens exporting GPIOs to a virtual machine, as the VM can just
22 grab the full GPIO controller, and no longer needs to care about which GPIOs to
23 grab and which not, reducing the attack surface.
24
25 Aggregated GPIO controllers are instantiated and destroyed by writing to
26 write-only attribute files in sysfs.
27
28 /sys/bus/platform/drivers/gpio-aggregator/
29
30 "new_device" ...
31 Userspace may ask the kernel to instantiate an aggregated GPIO
32 controller by writing a string describing the GPIOs to
33 aggregate to the "new_device" file, using the format
34
35 .. code-block:: none
36
37 [<gpioA>] [<gpiochipB> <offsets>] ...
38
39 Where:
40
41 "<gpioA>" ...
42 is a GPIO line name,
43
44 "<gpiochipB>" ...
45 is a GPIO chip label, and
46
47 "<offsets>" ...
48 is a comma-separated list of GPIO offsets and/or
49 GPIO offset ranges denoted by dashes.
50
51 Example: Instantiate a new GPIO aggregator by aggregating GPIO
52 line 19 of "e6052000.gpio" and GPIO lines 20-21 of
53 "e6050000.gpio" into a new gpio_chip:
54
55 .. code-block:: sh
56
57 $ echo 'e6052000.gpio 19 e6050000.gpio 20-21' > new_device
58
59 "delete_device" ...
60 Userspace may ask the kernel to destroy an aggregated GPIO
61 controller after use by writing its device name to the
62 "delete_device" file.
63
64 Example: Destroy the previously-created aggregated GPIO
65 controller, assumed to be "gpio-aggregator.0":
66
67 .. code-block:: sh
68
69 $ echo gpio-aggregator.0 > delete_device
70
71
72 Generic GPIO Driver
73 -------------------
74
75 The GPIO Aggregator can also be used as a generic driver for a simple
76 GPIO-operated device described in DT, without a dedicated in-kernel driver.
77 This is useful in industrial control, and is not unlike e.g. spidev, which
78 allows the user to communicate with an SPI device from userspace.
79
80 Binding a device to the GPIO Aggregator is performed either by modifying the
81 gpio-aggregator driver, or by writing to the "driver_override" file in Sysfs.
82
83 Example: If "door" is a GPIO-operated device described in DT, using its own
84 compatible value::
85
86 door {
87 compatible = "myvendor,mydoor";
88
89 gpios = <&gpio2 19 GPIO_ACTIVE_HIGH>,
90 <&gpio2 20 GPIO_ACTIVE_LOW>;
91 gpio-line-names = "open", "lock";
92 };
93
94 it can be bound to the GPIO Aggregator by either:
95
96 1. Adding its compatible value to ``gpio_aggregator_dt_ids[]``,
97 2. Binding manually using "driver_override":
98
99 .. code-block:: sh
100
101 $ echo gpio-aggregator > /sys/bus/platform/devices/door/driver_override
102 $ echo door > /sys/bus/platform/drivers/gpio-aggregator/bind
103
104 After that, a new gpiochip "door" has been created:
105
106 .. code-block:: sh
107
108 $ gpioinfo door
109 gpiochip12 - 2 lines:
110 line 0: "open" unused input active-high
111 line 1: "lock" unused input active-high
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