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