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