1GPIO Sysfs Interface for Userspace 2================================== 3 4.. warning:: 5 This API is obsoleted by the chardev.rst and the ABI documentation has 6 been moved to Documentation/ABI/obsolete/sysfs-gpio. 7 8 New developments should use the chardev.rst, and existing developments are 9 encouraged to migrate as soon as possible, as this API will be removed 10 in the future. 11 12 This interface will continue to be maintained for the migration period, 13 but new features will only be added to the new API. 14 15The obsolete 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 36.. note:: 37 Do NOT abuse sysfs to control hardware that has proper kernel drivers. 38 Please read Documentation/driver-api/gpio/drivers-on-gpio.rst 39 to avoid reinventing kernel wheels in userspace. 40 41 I MEAN IT. REALLY. 42 43Paths in Sysfs 44-------------- 45There are three kinds of entries in /sys/class/gpio: 46 47 - Control interfaces used to get userspace control over GPIOs; 48 49 - GPIOs themselves; and 50 51 - GPIO controllers ("gpio_chip" instances). 52 53That's in addition to standard files including the "device" symlink. 54 55The control interfaces are write-only: 56 57 /sys/class/gpio/ 58 59 "export" ... 60 Userspace may ask the kernel to export control of 61 a GPIO to userspace by writing its number to this file. 62 63 Example: "echo 19 > export" will create a "gpio19" node 64 for GPIO #19, if that's not requested by kernel code. 65 66 "unexport" ... 67 Reverses the effect of exporting to userspace. 68 69 Example: "echo 19 > unexport" will remove a "gpio19" 70 node exported using the "export" file. 71 72GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) 73and have the following read/write attributes: 74 75 /sys/class/gpio/gpioN/ 76 77 "direction" ... 78 reads as either "in" or "out". This value may 79 normally be written. Writing as "out" defaults to 80 initializing the value as low. To ensure glitch free 81 operation, values "low" and "high" may be written to 82 configure the GPIO as an output with that initial value. 83 84 Note that this attribute *will not exist* if the kernel 85 doesn't support changing the direction of a GPIO, or 86 it was exported by kernel code that didn't explicitly 87 allow userspace to reconfigure this GPIO's direction. 88 89 "value" ... 90 reads as either 0 (inactive) or 1 (active). If the GPIO 91 is configured as an output, this value may be written; 92 any nonzero value is treated as active. 93 94 If the pin can be configured as interrupt-generating interrupt 95 and if it has been configured to generate interrupts (see the 96 description of "edge"), you can poll(2) on that file and 97 poll(2) will return whenever the interrupt was triggered. If 98 you use poll(2), set the events POLLPRI and POLLERR. If you 99 use select(2), set the file descriptor in exceptfds. After 100 poll(2) returns, either lseek(2) to the beginning of the sysfs 101 file and read the new value or close the file and re-open it 102 to read the value. 103 104 "edge" ... 105 reads as either "none", "rising", "falling", or 106 "both". Write these strings to select the signal edge(s) 107 that will make poll(2) on the "value" file return. 108 109 This file exists only if the pin can be configured as an 110 interrupt generating input pin. 111 112 "active_low" ... 113 reads as either 0 (false) or 1 (true). Write 114 any nonzero value to invert the value attribute both 115 for reading and writing. Existing and subsequent 116 poll(2) support configuration via the edge attribute 117 for "rising" and "falling" edges will follow this 118 setting. 119 120GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the 121controller implementing GPIOs starting at #42) and have the following 122read-only attributes: 123 124 /sys/class/gpio/gpiochipN/ 125 126 "base" ... 127 same as N, the first GPIO managed by this chip 128 129 "label" ... 130 provided for diagnostics (not always unique) 131 132 "ngpio" ... 133 how many GPIOs this manages (N to N + ngpio - 1) 134 135Board documentation should in most cases cover what GPIOs are used for 136what purposes. However, those numbers are not always stable; GPIOs on 137a daughtercard might be different depending on the base board being used, 138or other cards in the stack. In such cases, you may need to use the 139gpiochip nodes (possibly in conjunction with schematics) to determine 140the correct GPIO number to use for a given signal. 141 142 143Exporting from Kernel code 144-------------------------- 145Kernel code can explicitly manage exports of GPIOs which have already been 146requested using gpio_request():: 147 148 /* export the GPIO to userspace */ 149 int gpiod_export(struct gpio_desc *desc, bool direction_may_change); 150 151 /* reverse gpiod_export() */ 152 void gpiod_unexport(struct gpio_desc *desc); 153 154 /* create a sysfs link to an exported GPIO node */ 155 int gpiod_export_link(struct device *dev, const char *name, 156 struct gpio_desc *desc); 157 158After a kernel driver requests a GPIO, it may only be made available in 159the sysfs interface by gpiod_export(). The driver can control whether the 160signal direction may change. This helps drivers prevent userspace code 161from accidentally clobbering important system state. 162 163This explicit exporting can help with debugging (by making some kinds 164of experiments easier), or can provide an always-there interface that's 165suitable for documenting as part of a board support package. 166 167After the GPIO has been exported, gpiod_export_link() allows creating 168symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can 169use this to provide the interface under their own device in sysfs with 170a descriptive name. 171