Documentation/gpio/sysfs.txt - third_party/linux - Git at Google

 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.
	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.