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