path: root/Documentation/gpio.txt
diff options
authorDavid Brownell <>2008-07-25 01:46:07 -0700
committerLinus Torvalds <>2008-07-25 10:53:30 -0700
commitd8f388d8dc8d4f36539dd37c1fff62cc404ea0fc (patch)
treedf8603775c889f29f8a03c77b9f7913bfd90d296 /Documentation/gpio.txt
parent8b6dd986823a8d92ed9f54baa5cef8604d9d9d44 (diff)
gpio: sysfs interface
This adds a simple sysfs interface for GPIOs. /sys/class/gpio /export ... asks the kernel to export a GPIO to userspace /unexport ... to return a GPIO to the kernel /gpioN ... for each exported GPIO #N /value ... always readable, writes fail for input GPIOs /direction ... r/w as: in, out (default low); write high, low /gpiochipN ... for each gpiochip; #N is its first GPIO /base ... (r/o) same as N /label ... (r/o) descriptive, not necessarily unique /ngpio ... (r/o) number of GPIOs; numbered N .. N+(ngpio - 1) GPIOs claimed by kernel code may be exported by its owner using a new gpio_export() call, which should be most useful for driver debugging. Such exports may optionally be done without a "direction" attribute. Userspace may ask to take over a GPIO by writing to a sysfs control file, helping to cope with incomplete board support or other "one-off" requirements that don't merit full kernel support: echo 23 > /sys/class/gpio/export ... will gpio_request(23, "sysfs") and gpio_export(23); use /sys/class/gpio/gpio-23/direction to (re)configure it, when that GPIO can be used as both input and output. echo 23 > /sys/class/gpio/unexport ... will gpio_free(23), when it was exported as above The extra D-space footprint is a few hundred bytes, except for the sysfs resources associated with each exported GPIO. The additional I-space footprint is about two thirds of the current size of gpiolib (!). Since no /dev node creation is involved, no "udev" support is needed. Related changes: * This adds a device pointer to "struct gpio_chip". When GPIO providers initialize that, sysfs gpio class devices become children of that device instead of being "virtual" devices. * The (few) gpio_chip providers which have such a device node have been updated. * Some gpio_chip drivers also needed to update their module "owner" field ... for which missing kerneldoc was added. * Some gpio_chips don't support input GPIOs. Those GPIOs are now flagged appropriately when the chip is registered. Based on previous patches, and discussion both on and off LKML. A Documentation/ABI/testing/sysfs-gpio update is ready to submit once this merges to mainline. [ a few maintenance build fixes] Signed-off-by: David Brownell <> Cc: Guennadi Liakhovetski <> Cc: Greg KH <> Cc: Kay Sievers <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
Diffstat (limited to 'Documentation/gpio.txt')
1 files changed, 118 insertions, 5 deletions
diff --git a/Documentation/gpio.txt b/Documentation/gpio.txt
index c35ca9e40d4c..8b69811a9642 100644
--- a/Documentation/gpio.txt
+++ b/Documentation/gpio.txt
@@ -347,15 +347,12 @@ necessarily be nonportable.
Dynamic definition of GPIOs is not currently standard; for example, as
a side effect of configuring an add-on board with some GPIO expanders.
-These calls are purely for kernel space, but a userspace API could be built
-on top of them.
GPIO implementor's framework (OPTIONAL)
As noted earlier, there is an optional implementation framework making it
easier for platforms to support different kinds of GPIO controller using
-the same programming interface.
+the same programming interface. This framework is called "gpiolib".
As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file
will be found there. That will list all the controllers registered through
@@ -439,4 +436,120 @@ becomes available. That may mean the device should not be registered until
calls for that GPIO can work. One way to address such dependencies is for
such gpio_chip controllers to provide setup() and teardown() callbacks to
board specific code; those board specific callbacks would register devices
-once all the necessary resources are available.
+once all the necessary resources are available, and remove them later when
+the GPIO controller device becomes unavailable.
+Sysfs Interface for Userspace (OPTIONAL)
+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 approprate 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.
+GPIO controllers have paths like /sys/class/gpio/chipchip42/ (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 gpio_export(unsigned gpio, bool direction_may_change);
+ /* reverse gpio_export() */
+ void gpio_unexport();
+After a kernel driver requests a GPIO, it may only be made available in
+the sysfs interface by gpio_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.