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Diffstat (limited to 'Documentation/gpio/sysfs.txt')
| -rw-r--r-- | Documentation/gpio/sysfs.txt | 155 |
1 files changed, 155 insertions, 0 deletions
diff --git a/Documentation/gpio/sysfs.txt b/Documentation/gpio/sysfs.txt new file mode 100644 index 0000000..c2c3a97 --- /dev/null +++ b/Documentation/gpio/sysfs.txt @@ -0,0 +1,155 @@ +GPIO Sysfs Interface for Userspace +================================== + +Platforms which use the "gpiolib" implementors framework may choose to +configure a sysfs user interface to GPIOs. This is different from the +debugfs interface, since it provides control over GPIO direction and +value instead of just showing a gpio state summary. Plus, it could be +present on production systems without debugging support. + +Given appropriate hardware documentation for the system, userspace could +know for example that GPIO #23 controls the write protect line used to +protect boot loader segments in flash memory. System upgrade procedures +may need to temporarily remove that protection, first importing a GPIO, +then changing its output state, then updating the code before re-enabling +the write protection. In normal use, GPIO #23 would never be touched, +and the kernel would have no need to know about it. + +Again depending on appropriate hardware documentation, on some systems +userspace GPIO can be used to determine system configuration data that +standard kernels won't know about. And for some tasks, simple userspace +GPIO drivers could be all that the system really needs. + +Note that standard kernel drivers exist for common "LEDs and Buttons" +GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those +instead of talking directly to the GPIOs; they integrate with kernel +frameworks better than your userspace code could. + + +Paths in Sysfs +-------------- +There are three kinds of entry in /sys/class/gpio: + + - Control interfaces used to get userspace control over GPIOs; + + - GPIOs themselves; and + + - GPIO controllers ("gpio_chip" instances). + +That's in addition to standard files including the "device" symlink. + +The control interfaces are write-only: + + /sys/class/gpio/ + + "export" ... Userspace may ask the kernel to export control of + a GPIO to userspace by writing its number to this file. + + Example: "echo 19 > export" will create a "gpio19" node + for GPIO #19, if that's not requested by kernel code. + + "unexport" ... Reverses the effect of exporting to userspace. + + Example: "echo 19 > unexport" will remove a "gpio19" + node exported using the "export" file. + +GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) +and have the following read/write attributes: + + /sys/class/gpio/gpioN/ + + "direction" ... reads as either "in" or "out". This value may + normally be written. Writing as "out" defaults to + initializing the value as low. To ensure glitch free + operation, values "low" and "high" may be written to + configure the GPIO as an output with that initial value. + + Note that this attribute *will not exist* if the kernel + doesn't support changing the direction of a GPIO, or + it was exported by kernel code that didn't explicitly + allow userspace to reconfigure this GPIO's direction. + + "value" ... reads as either 0 (low) or 1 (high). If the GPIO + is configured as an output, this value may be written; + any nonzero value is treated as high. + + If the pin can be configured as interrupt-generating interrupt + and if it has been configured to generate interrupts (see the + description of "edge"), you can poll(2) on that file and + poll(2) will return whenever the interrupt was triggered. If + you use poll(2), set the events POLLPRI and POLLERR. If you + use select(2), set the file descriptor in exceptfds. After + poll(2) returns, either lseek(2) to the beginning of the sysfs + file and read the new value or close the file and re-open it + to read the value. + + "edge" ... reads as either "none", "rising", "falling", or + "both". Write these strings to select the signal edge(s) + that will make poll(2) on the "value" file return. + + This file exists only if the pin can be configured as an + interrupt generating input pin. + + "active_low" ... reads as either 0 (false) or 1 (true). Write + any nonzero value to invert the value attribute both + for reading and writing. Existing and subsequent + poll(2) support configuration via the edge attribute + for "rising" and "falling" edges will follow this + setting. + +GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the +controller implementing GPIOs starting at #42) and have the following +read-only attributes: + + /sys/class/gpio/gpiochipN/ + + "base" ... same as N, the first GPIO managed by this chip + + "label" ... provided for diagnostics (not always unique) + + "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) + +Board documentation should in most cases cover what GPIOs are used for +what purposes. However, those numbers are not always stable; GPIOs on +a daughtercard might be different depending on the base board being used, +or other cards in the stack. In such cases, you may need to use the +gpiochip nodes (possibly in conjunction with schematics) to determine +the correct GPIO number to use for a given signal. + + +Exporting from Kernel code +-------------------------- +Kernel code can explicitly manage exports of GPIOs which have already been +requested using gpio_request(): + + /* export the GPIO to userspace */ + int gpiod_export(struct gpio_desc *desc, bool direction_may_change); + + /* reverse gpio_export() */ + void gpiod_unexport(struct gpio_desc *desc); + + /* create a sysfs link to an exported GPIO node */ + int gpiod_export_link(struct device *dev, const char *name, + struct gpio_desc *desc); + + /* change the polarity of a GPIO node in sysfs */ + int gpiod_sysfs_set_active_low(struct gpio_desc *desc, int value); + +After a kernel driver requests a GPIO, it may only be made available in +the sysfs interface by gpiod_export(). The driver can control whether the +signal direction may change. This helps drivers prevent userspace code +from accidentally clobbering important system state. + +This explicit exporting can help with debugging (by making some kinds +of experiments easier), or can provide an always-there interface that's +suitable for documenting as part of a board support package. + +After the GPIO has been exported, gpiod_export_link() allows creating +symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can +use this to provide the interface under their own device in sysfs with +a descriptive name. + +Drivers can use gpiod_sysfs_set_active_low() to hide GPIO line polarity +differences between boards from user space. Polarity change can be done both +before and after gpiod_export(), and previously enabled poll(2) support for +either rising or falling edge will be reconfigured to follow this setting. |
