1GPIO Sysfs Interface for Userspace
2==================================
3
4.. warning::
5
6  THIS ABI IS DEPRECATED, THE ABI DOCUMENTATION HAS BEEN MOVED TO
7  Documentation/ABI/obsolete/sysfs-gpio AND NEW USERSPACE CONSUMERS
8  ARE SUPPOSED TO USE THE CHARACTER DEVICE ABI. THIS OLD SYSFS ABI WILL
9  NOT BE DEVELOPED (NO NEW FEATURES), IT WILL JUST BE MAINTAINED.
10
11Refer to the examples in tools/gpio/* for an introduction to the new
12character device ABI. Also see the userspace header in
13include/uapi/linux/gpio.h
14
15The deprecated sysfs ABI
16------------------------
17Platforms which use the "gpiolib" implementors framework may choose to
18configure a sysfs user interface to GPIOs. This is different from the
19debugfs interface, since it provides control over GPIO direction and
20value instead of just showing a gpio state summary. Plus, it could be
21present on production systems without debugging support.
22
23Given appropriate hardware documentation for the system, userspace could
24know for example that GPIO #23 controls the write protect line used to
25protect boot loader segments in flash memory. System upgrade procedures
26may need to temporarily remove that protection, first importing a GPIO,
27then changing its output state, then updating the code before re-enabling
28the write protection. In normal use, GPIO #23 would never be touched,
29and the kernel would have no need to know about it.
30
31Again depending on appropriate hardware documentation, on some systems
32userspace GPIO can be used to determine system configuration data that
33standard kernels won't know about. And for some tasks, simple userspace
34GPIO drivers could be all that the system really needs.
35
36DO NOT ABUSE SYSFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
37PLEASE READ THE DOCUMENT AT Documentation/driver-api/gpio/drivers-on-gpio.rst
38TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT. REALLY.
39
40Paths in Sysfs
41--------------
42There are three kinds of entries in /sys/class/gpio:
43
44   -	Control interfaces used to get userspace control over GPIOs;
45
46   -	GPIOs themselves; and
47
48   -	GPIO controllers ("gpio_chip" instances).
49
50That's in addition to standard files including the "device" symlink.
51
52The control interfaces are write-only:
53
54    /sys/class/gpio/
55
56	"export" ...
57		Userspace may ask the kernel to export control of
58		a GPIO to userspace by writing its number to this file.
59
60		Example:  "echo 19 > export" will create a "gpio19" node
61		for GPIO #19, if that's not requested by kernel code.
62
63	"unexport" ...
64		Reverses the effect of exporting to userspace.
65
66		Example:  "echo 19 > unexport" will remove a "gpio19"
67		node exported using the "export" file.
68
69GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
70and have the following read/write attributes:
71
72    /sys/class/gpio/gpioN/
73
74	"direction" ...
75		reads as either "in" or "out". This value may
76		normally be written. Writing as "out" defaults to
77		initializing the value as low. To ensure glitch free
78		operation, values "low" and "high" may be written to
79		configure the GPIO as an output with that initial value.
80
81		Note that this attribute *will not exist* if the kernel
82		doesn't support changing the direction of a GPIO, or
83		it was exported by kernel code that didn't explicitly
84		allow userspace to reconfigure this GPIO's direction.
85
86	"value" ...
87		reads as either 0 (low) or 1 (high). If the GPIO
88		is configured as an output, this value may be written;
89		any nonzero value is treated as high.
90
91		If the pin can be configured as interrupt-generating interrupt
92		and if it has been configured to generate interrupts (see the
93		description of "edge"), you can poll(2) on that file and
94		poll(2) will return whenever the interrupt was triggered. If
95		you use poll(2), set the events POLLPRI and POLLERR. If you
96		use select(2), set the file descriptor in exceptfds. After
97		poll(2) returns, either lseek(2) to the beginning of the sysfs
98		file and read the new value or close the file and re-open it
99		to read the value.
100
101	"edge" ...
102		reads as either "none", "rising", "falling", or
103		"both". Write these strings to select the signal edge(s)
104		that will make poll(2) on the "value" file return.
105
106		This file exists only if the pin can be configured as an
107		interrupt generating input pin.
108
109	"active_low" ...
110		reads as either 0 (false) or 1 (true). Write
111		any nonzero value to invert the value attribute both
112		for reading and writing. Existing and subsequent
113		poll(2) support configuration via the edge attribute
114		for "rising" and "falling" edges will follow this
115		setting.
116
117GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
118controller implementing GPIOs starting at #42) and have the following
119read-only attributes:
120
121    /sys/class/gpio/gpiochipN/
122
123	"base" ...
124		same as N, the first GPIO managed by this chip
125
126	"label" ...
127		provided for diagnostics (not always unique)
128
129	"ngpio" ...
130		how many GPIOs this manages (N to N + ngpio - 1)
131
132Board documentation should in most cases cover what GPIOs are used for
133what purposes. However, those numbers are not always stable; GPIOs on
134a daughtercard might be different depending on the base board being used,
135or other cards in the stack. In such cases, you may need to use the
136gpiochip nodes (possibly in conjunction with schematics) to determine
137the correct GPIO number to use for a given signal.
138
139
140Exporting from Kernel code
141--------------------------
142Kernel code can explicitly manage exports of GPIOs which have already been
143requested using gpio_request()::
144
145	/* export the GPIO to userspace */
146	int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
147
148	/* reverse gpiod_export() */
149	void gpiod_unexport(struct gpio_desc *desc);
150
151	/* create a sysfs link to an exported GPIO node */
152	int gpiod_export_link(struct device *dev, const char *name,
153		      struct gpio_desc *desc);
154
155After a kernel driver requests a GPIO, it may only be made available in
156the sysfs interface by gpiod_export(). The driver can control whether the
157signal direction may change. This helps drivers prevent userspace code
158from accidentally clobbering important system state.
159
160This explicit exporting can help with debugging (by making some kinds
161of experiments easier), or can provide an always-there interface that's
162suitable for documenting as part of a board support package.
163
164After the GPIO has been exported, gpiod_export_link() allows creating
165symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
166use this to provide the interface under their own device in sysfs with
167a descriptive name.
168