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Alexandre Courbotfd8e1982013-11-16 21:34:21 +09001GPIO Sysfs Interface for Userspace
2==================================
3
4Platforms which use the "gpiolib" implementors framework may choose to
5configure a sysfs user interface to GPIOs. This is different from the
6debugfs interface, since it provides control over GPIO direction and
7value instead of just showing a gpio state summary. Plus, it could be
8present on production systems without debugging support.
9
10Given appropriate hardware documentation for the system, userspace could
11know for example that GPIO #23 controls the write protect line used to
12protect boot loader segments in flash memory. System upgrade procedures
13may need to temporarily remove that protection, first importing a GPIO,
14then changing its output state, then updating the code before re-enabling
15the write protection. In normal use, GPIO #23 would never be touched,
16and the kernel would have no need to know about it.
17
18Again depending on appropriate hardware documentation, on some systems
19userspace GPIO can be used to determine system configuration data that
20standard kernels won't know about. And for some tasks, simple userspace
21GPIO drivers could be all that the system really needs.
22
Linus Walleijc21cde62015-07-21 11:36:57 +020023DO NOT ABUSE SYFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
24PLEASE READ THE DOCUMENT NAMED "drivers-on-gpio.txt" IN THIS DOCUMENTATION
25DIRECTORY TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT.
26REALLY.
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090027
28Paths in Sysfs
29--------------
30There are three kinds of entry in /sys/class/gpio:
31
32 - Control interfaces used to get userspace control over GPIOs;
33
34 - GPIOs themselves; and
35
36 - GPIO controllers ("gpio_chip" instances).
37
38That's in addition to standard files including the "device" symlink.
39
40The control interfaces are write-only:
41
42 /sys/class/gpio/
43
44 "export" ... Userspace may ask the kernel to export control of
45 a GPIO to userspace by writing its number to this file.
46
47 Example: "echo 19 > export" will create a "gpio19" node
48 for GPIO #19, if that's not requested by kernel code.
49
50 "unexport" ... Reverses the effect of exporting to userspace.
51
52 Example: "echo 19 > unexport" will remove a "gpio19"
53 node exported using the "export" file.
54
55GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
56and have the following read/write attributes:
57
58 /sys/class/gpio/gpioN/
59
60 "direction" ... reads as either "in" or "out". This value may
61 normally be written. Writing as "out" defaults to
62 initializing the value as low. To ensure glitch free
63 operation, values "low" and "high" may be written to
64 configure the GPIO as an output with that initial value.
65
66 Note that this attribute *will not exist* if the kernel
67 doesn't support changing the direction of a GPIO, or
68 it was exported by kernel code that didn't explicitly
69 allow userspace to reconfigure this GPIO's direction.
70
71 "value" ... reads as either 0 (low) or 1 (high). If the GPIO
72 is configured as an output, this value may be written;
73 any nonzero value is treated as high.
74
75 If the pin can be configured as interrupt-generating interrupt
76 and if it has been configured to generate interrupts (see the
77 description of "edge"), you can poll(2) on that file and
78 poll(2) will return whenever the interrupt was triggered. If
79 you use poll(2), set the events POLLPRI and POLLERR. If you
80 use select(2), set the file descriptor in exceptfds. After
81 poll(2) returns, either lseek(2) to the beginning of the sysfs
82 file and read the new value or close the file and re-open it
83 to read the value.
84
85 "edge" ... reads as either "none", "rising", "falling", or
86 "both". Write these strings to select the signal edge(s)
87 that will make poll(2) on the "value" file return.
88
89 This file exists only if the pin can be configured as an
90 interrupt generating input pin.
91
92 "active_low" ... reads as either 0 (false) or 1 (true). Write
93 any nonzero value to invert the value attribute both
94 for reading and writing. Existing and subsequent
95 poll(2) support configuration via the edge attribute
96 for "rising" and "falling" edges will follow this
97 setting.
98
99GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
100controller implementing GPIOs starting at #42) and have the following
101read-only attributes:
102
103 /sys/class/gpio/gpiochipN/
104
105 "base" ... same as N, the first GPIO managed by this chip
106
107 "label" ... provided for diagnostics (not always unique)
108
109 "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
110
111Board documentation should in most cases cover what GPIOs are used for
112what purposes. However, those numbers are not always stable; GPIOs on
113a daughtercard might be different depending on the base board being used,
114or other cards in the stack. In such cases, you may need to use the
115gpiochip nodes (possibly in conjunction with schematics) to determine
116the correct GPIO number to use for a given signal.
117
118
119Exporting from Kernel code
120--------------------------
121Kernel code can explicitly manage exports of GPIOs which have already been
122requested using gpio_request():
123
124 /* export the GPIO to userspace */
125 int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
126
127 /* reverse gpio_export() */
128 void gpiod_unexport(struct gpio_desc *desc);
129
130 /* create a sysfs link to an exported GPIO node */
131 int gpiod_export_link(struct device *dev, const char *name,
132 struct gpio_desc *desc);
133
Alexandre Courbotfd8e1982013-11-16 21:34:21 +0900134After a kernel driver requests a GPIO, it may only be made available in
135the sysfs interface by gpiod_export(). The driver can control whether the
136signal direction may change. This helps drivers prevent userspace code
137from accidentally clobbering important system state.
138
139This explicit exporting can help with debugging (by making some kinds
140of experiments easier), or can provide an always-there interface that's
141suitable for documenting as part of a board support package.
142
143After the GPIO has been exported, gpiod_export_link() allows creating
144symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
145use this to provide the interface under their own device in sysfs with
146a descriptive name.