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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:46 EST.

1	GPIO Sysfs Interface for Userspace
2	==================================
3	
4	Platforms which use the "gpiolib" implementors framework may choose to
5	configure a sysfs user interface to GPIOs. This is different from the
6	debugfs interface, since it provides control over GPIO direction and
7	value instead of just showing a gpio state summary. Plus, it could be
8	present on production systems without debugging support.
9	
10	Given appropriate hardware documentation for the system, userspace could
11	know for example that GPIO #23 controls the write protect line used to
12	protect boot loader segments in flash memory. System upgrade procedures
13	may need to temporarily remove that protection, first importing a GPIO,
14	then changing its output state, then updating the code before re-enabling
15	the write protection. In normal use, GPIO #23 would never be touched,
16	and the kernel would have no need to know about it.
17	
18	Again depending on appropriate hardware documentation, on some systems
19	userspace GPIO can be used to determine system configuration data that
20	standard kernels won't know about. And for some tasks, simple userspace
21	GPIO drivers could be all that the system really needs.
22	
23	DO NOT ABUSE SYSFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
24	PLEASE READ THE DOCUMENT NAMED "drivers-on-gpio.txt" IN THIS DOCUMENTATION
25	DIRECTORY TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT.
26	REALLY.
27	
28	Paths in Sysfs
29	--------------
30	There are three kinds of entries 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	
38	That's in addition to standard files including the "device" symlink.
39	
40	The 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	
55	GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
56	and 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	
99	GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
100	controller implementing GPIOs starting at #42) and have the following
101	read-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 manages (N to N + ngpio - 1)
110	
111	Board documentation should in most cases cover what GPIOs are used for
112	what purposes. However, those numbers are not always stable; GPIOs on
113	a daughtercard might be different depending on the base board being used,
114	or other cards in the stack. In such cases, you may need to use the
115	gpiochip nodes (possibly in conjunction with schematics) to determine
116	the correct GPIO number to use for a given signal.
117	
118	
119	Exporting from Kernel code
120	--------------------------
121	Kernel code can explicitly manage exports of GPIOs which have already been
122	requested 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	
134	After a kernel driver requests a GPIO, it may only be made available in
135	the sysfs interface by gpiod_export(). The driver can control whether the
136	signal direction may change. This helps drivers prevent userspace code
137	from accidentally clobbering important system state.
138	
139	This explicit exporting can help with debugging (by making some kinds
140	of experiments easier), or can provide an always-there interface that's
141	suitable for documenting as part of a board support package.
142	
143	After the GPIO has been exported, gpiod_export_link() allows creating
144	symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
145	use this to provide the interface under their own device in sysfs with
146	a descriptive name.
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