The numbers need not be contiguous; either of those platforms could also
use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders.
+If you want to initialize a structure with an invalid GPIO number, use
+some negative number (perhaps "-EINVAL"); that will never be valid. To
+test if a number could reference a GPIO, you may use this predicate:
+
+ int gpio_is_valid(int number);
+
+A number that's not valid will be rejected by calls which may request
+or free GPIOs (see below). Other numbers may also be rejected; for
+example, a number might be valid but unused on a given board.
+
Whether a platform supports multiple GPIO controllers is currently a
platform-specific implementation issue.
needed to manage a signal that's in active use. That is, requesting a
GPIO can serve as a kind of lock.
+Some platforms may also use knowledge about what GPIOs are active for
+power management, such as by powering down unused chip sectors and, more
+easily, gating off unused clocks.
+
These two calls are optional because not not all current Linux platforms
offer such functionality in their GPIO support; a valid implementation
could return success for all gpio_request() calls. Unlike the other calls,
/* map GPIO numbers to IRQ numbers */
int gpio_to_irq(unsigned gpio);
- /* map IRQ numbers to GPIO numbers */
+ /* map IRQ numbers to GPIO numbers (avoid using this) */
int irq_to_gpio(unsigned irq);
Those return either the corresponding number in the other namespace, or
Non-error values returned from irq_to_gpio() would most commonly be used
with gpio_get_value(), for example to initialize or update driver state
-when the IRQ is edge-triggered.
+when the IRQ is edge-triggered. Note that some platforms don't support
+this reverse mapping, so you should avoid using it.
Emulating Open Drain Signals
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
Platform Support
----------------
-To support this framework, a platform's Kconfig will "select HAVE_GPIO_LIB"
+To support this framework, a platform's Kconfig will "select" either
+ARCH_REQUIRE_GPIOLIB or ARCH_WANT_OPTIONAL_GPIOLIB
and arrange that its <asm/gpio.h> includes <asm-generic/gpio.h> and defines
three functions: gpio_get_value(), gpio_set_value(), and gpio_cansleep().
They may also want to provide a custom value for ARCH_NR_GPIOS.
+ARCH_REQUIRE_GPIOLIB means that the gpio-lib code will always get compiled
+into the kernel on that architecture.
+
+ARCH_WANT_OPTIONAL_GPIOLIB means the gpio-lib code defaults to off and the user
+can enable it and build it into the kernel optionally.
+
+If neither of these options are selected, the platform does not support
+GPIOs through GPIO-lib and the code cannot be enabled by the user.
+
Trivial implementations of those functions can directly use framework
code, which always dispatches through the gpio_chip:
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.
git://ftp.safe.ca / safe / jmp / linux-2.6 / blobdiff