^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 1) GPIO Sysfs Interface for Userspace
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 2) ==================================
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 3)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 4) .. warning::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 5)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 6) THIS ABI IS DEPRECATED, THE ABI DOCUMENTATION HAS BEEN MOVED TO
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 7) Documentation/ABI/obsolete/sysfs-gpio AND NEW USERSPACE CONSUMERS
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 8) ARE SUPPOSED TO USE THE CHARACTER DEVICE ABI. THIS OLD SYSFS ABI WILL
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 9) NOT BE DEVELOPED (NO NEW FEATURES), IT WILL JUST BE MAINTAINED.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 10)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 11) Refer to the examples in tools/gpio/* for an introduction to the new
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 12) character device ABI. Also see the userspace header in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 13) include/uapi/linux/gpio.h
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 14)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 15) The deprecated sysfs ABI
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 16) ------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 17) Platforms which use the "gpiolib" implementors framework may choose to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 18) configure a sysfs user interface to GPIOs. This is different from the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 19) debugfs interface, since it provides control over GPIO direction and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 20) value instead of just showing a gpio state summary. Plus, it could be
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 21) present on production systems without debugging support.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 22)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 23) Given appropriate hardware documentation for the system, userspace could
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 24) know for example that GPIO #23 controls the write protect line used to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 25) protect boot loader segments in flash memory. System upgrade procedures
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 26) may need to temporarily remove that protection, first importing a GPIO,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 27) then changing its output state, then updating the code before re-enabling
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 28) the write protection. In normal use, GPIO #23 would never be touched,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 29) and the kernel would have no need to know about it.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 30)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 31) Again depending on appropriate hardware documentation, on some systems
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 32) userspace GPIO can be used to determine system configuration data that
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 33) standard kernels won't know about. And for some tasks, simple userspace
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 34) GPIO drivers could be all that the system really needs.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 35)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 36) DO NOT ABUSE SYSFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 37) PLEASE READ THE DOCUMENT AT Documentation/driver-api/gpio/drivers-on-gpio.rst
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 38) TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT. REALLY.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 39)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 40) Paths in Sysfs
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 41) --------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 42) There are three kinds of entries in /sys/class/gpio:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 43)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 44) - Control interfaces used to get userspace control over GPIOs;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 45)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 46) - GPIOs themselves; and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 47)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 48) - GPIO controllers ("gpio_chip" instances).
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 49)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 50) That's in addition to standard files including the "device" symlink.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 51)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 52) The control interfaces are write-only:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 53)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 54) /sys/class/gpio/
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 55)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 56) "export" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 57) Userspace may ask the kernel to export control of
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 58) a GPIO to userspace by writing its number to this file.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 59)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 60) Example: "echo 19 > export" will create a "gpio19" node
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 61) for GPIO #19, if that's not requested by kernel code.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 62)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 63) "unexport" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 64) Reverses the effect of exporting to userspace.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 65)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 66) Example: "echo 19 > unexport" will remove a "gpio19"
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 67) node exported using the "export" file.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 68)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 69) GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 70) and have the following read/write attributes:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 71)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 72) /sys/class/gpio/gpioN/
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 73)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 74) "direction" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 75) reads as either "in" or "out". This value may
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 76) normally be written. Writing as "out" defaults to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 77) initializing the value as low. To ensure glitch free
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 78) operation, values "low" and "high" may be written to
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 79) configure the GPIO as an output with that initial value.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 80)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 81) Note that this attribute *will not exist* if the kernel
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 82) doesn't support changing the direction of a GPIO, or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 83) it was exported by kernel code that didn't explicitly
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 84) allow userspace to reconfigure this GPIO's direction.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 85)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 86) "value" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 87) reads as either 0 (low) or 1 (high). If the GPIO
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 88) is configured as an output, this value may be written;
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 89) any nonzero value is treated as high.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 90)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 91) If the pin can be configured as interrupt-generating interrupt
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 92) and if it has been configured to generate interrupts (see the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 93) description of "edge"), you can poll(2) on that file and
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 94) poll(2) will return whenever the interrupt was triggered. If
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 95) you use poll(2), set the events POLLPRI and POLLERR. If you
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 96) use select(2), set the file descriptor in exceptfds. After
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 97) poll(2) returns, either lseek(2) to the beginning of the sysfs
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 98) file and read the new value or close the file and re-open it
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 99) to read the value.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 100)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 101) "edge" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 102) reads as either "none", "rising", "falling", or
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 103) "both". Write these strings to select the signal edge(s)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 104) that will make poll(2) on the "value" file return.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 105)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 106) This file exists only if the pin can be configured as an
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 107) interrupt generating input pin.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 108)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 109) "active_low" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 110) reads as either 0 (false) or 1 (true). Write
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 111) any nonzero value to invert the value attribute both
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 112) for reading and writing. Existing and subsequent
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 113) poll(2) support configuration via the edge attribute
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 114) for "rising" and "falling" edges will follow this
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 115) setting.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 116)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 117) GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 118) controller implementing GPIOs starting at #42) and have the following
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 119) read-only attributes:
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 120)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 121) /sys/class/gpio/gpiochipN/
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 122)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 123) "base" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 124) same as N, the first GPIO managed by this chip
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 125)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 126) "label" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 127) provided for diagnostics (not always unique)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 128)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 129) "ngpio" ...
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 130) how many GPIOs this manages (N to N + ngpio - 1)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 131)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 132) Board documentation should in most cases cover what GPIOs are used for
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 133) what purposes. However, those numbers are not always stable; GPIOs on
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 134) a daughtercard might be different depending on the base board being used,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 135) or other cards in the stack. In such cases, you may need to use the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 136) gpiochip nodes (possibly in conjunction with schematics) to determine
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 137) the correct GPIO number to use for a given signal.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 138)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 139)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 140) Exporting from Kernel code
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 141) --------------------------
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 142) Kernel code can explicitly manage exports of GPIOs which have already been
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 143) requested using gpio_request()::
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 144)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 145) /* export the GPIO to userspace */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 146) int gpiod_export(struct gpio_desc *desc, bool direction_may_change);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 147)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 148) /* reverse gpio_export() */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 149) void gpiod_unexport(struct gpio_desc *desc);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 150)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 151) /* create a sysfs link to an exported GPIO node */
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 152) int gpiod_export_link(struct device *dev, const char *name,
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 153) struct gpio_desc *desc);
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 154)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 155) After a kernel driver requests a GPIO, it may only be made available in
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 156) the sysfs interface by gpiod_export(). The driver can control whether the
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 157) signal direction may change. This helps drivers prevent userspace code
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 158) from accidentally clobbering important system state.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 159)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 160) This explicit exporting can help with debugging (by making some kinds
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 161) of experiments easier), or can provide an always-there interface that's
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 162) suitable for documenting as part of a board support package.
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 163)
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 164) After the GPIO has been exported, gpiod_export_link() allows creating
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 165) symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 166) use this to provide the interface under their own device in sysfs with
^8f3ce5b39 (kx 2023-10-28 12:00:06 +0300 167) a descriptive name.
Orange Pi5 kernel
Deprecated Linux kernel 5.10.110 for OrangePi 5/5B/5+ boards
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