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Documentation / gpio / sysfs.txt

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Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.

1	GPIO Sysfs Interface for Userspace
2	==================================
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.
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.
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.
23	Note that standard kernel drivers exist for common "LEDs and Buttons"
24	GPIO tasks:  "leds-gpio" and "gpio_keys", respectively. Use those
25	instead of talking directly to the GPIOs; they integrate with kernel
26	frameworks better than your userspace code could.
29	Paths in Sysfs
30	--------------
31	There are three kinds of entry in /sys/class/gpio:
33	   -	Control interfaces used to get userspace control over GPIOs;
35	   -	GPIOs themselves; and
37	   -	GPIO controllers ("gpio_chip" instances).
39	That's in addition to standard files including the "device" symlink.
41	The control interfaces are write-only:
43	    /sys/class/gpio/
45	    	"export" ... Userspace may ask the kernel to export control of
46			a GPIO to userspace by writing its number to this file.
48			Example:  "echo 19 > export" will create a "gpio19" node
49			for GPIO #19, if that's not requested by kernel code.
51	    	"unexport" ... Reverses the effect of exporting to userspace.
53			Example:  "echo 19 > unexport" will remove a "gpio19"
54			node exported using the "export" file.
56	GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
57	and have the following read/write attributes:
59	    /sys/class/gpio/gpioN/
61		"direction" ... reads as either "in" or "out". This value may
62			normally be written. Writing as "out" defaults to
63			initializing the value as low. To ensure glitch free
64			operation, values "low" and "high" may be written to
65			configure the GPIO as an output with that initial value.
67			Note that this attribute *will not exist* if the kernel
68			doesn't support changing the direction of a GPIO, or
69			it was exported by kernel code that didn't explicitly
70			allow userspace to reconfigure this GPIO's direction.
72		"value" ... reads as either 0 (low) or 1 (high). If the GPIO
73			is configured as an output, this value may be written;
74			any nonzero value is treated as high.
76			If the pin can be configured as interrupt-generating interrupt
77			and if it has been configured to generate interrupts (see the
78			description of "edge"), you can poll(2) on that file and
79			poll(2) will return whenever the interrupt was triggered. If
80			you use poll(2), set the events POLLPRI and POLLERR. If you
81			use select(2), set the file descriptor in exceptfds. After
82			poll(2) returns, either lseek(2) to the beginning of the sysfs
83			file and read the new value or close the file and re-open it
84			to read the value.
86		"edge" ... reads as either "none", "rising", "falling", or
87			"both". Write these strings to select the signal edge(s)
88			that will make poll(2) on the "value" file return.
90			This file exists only if the pin can be configured as an
91			interrupt generating input pin.
93		"active_low" ... reads as either 0 (false) or 1 (true). Write
94			any nonzero value to invert the value attribute both
95			for reading and writing. Existing and subsequent
96			poll(2) support configuration via the edge attribute
97			for "rising" and "falling" edges will follow this
98			setting.
100	GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
101	controller implementing GPIOs starting at #42) and have the following
102	read-only attributes:
104	    /sys/class/gpio/gpiochipN/
106	    	"base" ... same as N, the first GPIO managed by this chip
108	    	"label" ... provided for diagnostics (not always unique)
110	    	"ngpio" ... how many GPIOs this manges (N to N + ngpio - 1)
112	Board documentation should in most cases cover what GPIOs are used for
113	what purposes. However, those numbers are not always stable; GPIOs on
114	a daughtercard might be different depending on the base board being used,
115	or other cards in the stack. In such cases, you may need to use the
116	gpiochip nodes (possibly in conjunction with schematics) to determine
117	the correct GPIO number to use for a given signal.
120	Exporting from Kernel code
121	--------------------------
122	Kernel code can explicitly manage exports of GPIOs which have already been
123	requested using gpio_request():
125		/* export the GPIO to userspace */
126		int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
128		/* reverse gpio_export() */
129		void gpiod_unexport(struct gpio_desc *desc);
131		/* create a sysfs link to an exported GPIO node */
132		int gpiod_export_link(struct device *dev, const char *name,
133			      struct gpio_desc *desc);
135	After a kernel driver requests a GPIO, it may only be made available in
136	the sysfs interface by gpiod_export(). The driver can control whether the
137	signal direction may change. This helps drivers prevent userspace code
138	from accidentally clobbering important system state.
140	This explicit exporting can help with debugging (by making some kinds
141	of experiments easier), or can provide an always-there interface that's
142	suitable for documenting as part of a board support package.
144	After the GPIO has been exported, gpiod_export_link() allows creating
145	symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
146	use this to provide the interface under their own device in sysfs with
147	a descriptive name.
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