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-GPIO Interfaces
-
-This provides an overview of GPIO access conventions on Linux.
-
-These calls use the gpio_* naming prefix. No other calls should use that
-prefix, or the related __gpio_* prefix.
-
-
-What is a GPIO?
-===============
-A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
-digital signal. They are provided from many kinds of chip, and are familiar
-to Linux developers working with embedded and custom hardware. Each GPIO
-represents a bit connected to a particular pin, or "ball" on Ball Grid Array
-(BGA) packages. Board schematics show which external hardware connects to
-which GPIOs. Drivers can be written generically, so that board setup code
-passes such pin configuration data to drivers.
-
-System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
-non-dedicated pin can be configured as a GPIO; and most chips have at least
-several dozen of them. Programmable logic devices (like FPGAs) can easily
-provide GPIOs; multifunction chips like power managers, and audio codecs
-often have a few such pins to help with pin scarcity on SOCs; and there are
-also "GPIO Expander" chips that connect using the I2C or SPI serial busses.
-Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
-firmware knowing how they're used).
-
-The exact capabilities of GPIOs vary between systems. Common options:
-
- - Output values are writable (high=1, low=0). Some chips also have
- options about how that value is driven, so that for example only one
- value might be driven ... supporting "wire-OR" and similar schemes
- for the other value (notably, "open drain" signaling).
-
- - Input values are likewise readable (1, 0). Some chips support readback
- of pins configured as "output", which is very useful in such "wire-OR"
- cases (to support bidirectional signaling). GPIO controllers may have
- input de-glitch/debounce logic, sometimes with software controls.
-
- - Inputs can often be used as IRQ signals, often edge triggered but
- sometimes level triggered. Such IRQs may be configurable as system
- wakeup events, to wake the system from a low power state.
-
- - Usually a GPIO will be configurable as either input or output, as needed
- by different product boards; single direction ones exist too.
-
- - Most GPIOs can be accessed while holding spinlocks, but those accessed
- through a serial bus normally can't. Some systems support both types.
-
-On a given board each GPIO is used for one specific purpose like monitoring
-MMC/SD card insertion/removal, detecting card writeprotect status, driving
-a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware
-watchdog, sensing a switch, and so on.
-
-
-GPIO conventions
-================
-Note that this is called a "convention" because you don't need to do it this
-way, and it's no crime if you don't. There **are** cases where portability
-is not the main issue; GPIOs are often used for the kind of board-specific
-glue logic that may even change between board revisions, and can't ever be
-used on a board that's wired differently. Only least-common-denominator
-functionality can be very portable. Other features are platform-specific,
-and that can be critical for glue logic.
-
-Plus, this doesn't require any implementation framework, just an interface.
-One platform might implement it as simple inline functions accessing chip
-registers; another might implement it by delegating through abstractions
-used for several very different kinds of GPIO controller. (There is some
-optional code supporting such an implementation strategy, described later
-in this document, but drivers acting as clients to the GPIO interface must
-not care how it's implemented.)
-
-That said, if the convention is supported on their platform, drivers should
-use it when possible. Platforms must declare GENERIC_GPIO support in their
-Kconfig (boolean true), and provide an <asm/gpio.h> file. Drivers that can't
-work without standard GPIO calls should have Kconfig entries which depend
-on GENERIC_GPIO. The GPIO calls are available, either as "real code" or as
-optimized-away stubs, when drivers use the include file:
-
- #include <linux/gpio.h>
-
-If you stick to this convention then it'll be easier for other developers to
-see what your code is doing, and help maintain it.
-
-Note that these operations include I/O barriers on platforms which need to
-use them; drivers don't need to add them explicitly.
-
-
-Identifying GPIOs
------------------
-GPIOs are identified by unsigned integers in the range 0..MAX_INT. That
-reserves "negative" numbers for other purposes like marking signals as
-"not available on this board", or indicating faults. Code that doesn't
-touch the underlying hardware treats these integers as opaque cookies.
-
-Platforms define how they use those integers, and usually #define symbols
-for the GPIO lines so that board-specific setup code directly corresponds
-to the relevant schematics. In contrast, drivers should only use GPIO
-numbers passed to them from that setup code, using platform_data to hold
-board-specific pin configuration data (along with other board specific
-data they need). That avoids portability problems.
-
-So for example one platform uses numbers 32-159 for GPIOs; while another
-uses numbers 0..63 with one set of GPIO controllers, 64-79 with another
-type of GPIO controller, and on one particular board 80-95 with an FPGA.
-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 such number from such a structure 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 temporarily unused on a given board.
-
-Whether a platform supports multiple GPIO controllers is a platform-specific
-implementation issue, as are whether that support can leave "holes" in the space
-of GPIO numbers, and whether new controllers can be added at runtime. Such issues
-can affect things including whether adjacent GPIO numbers are both valid.
-
-Using GPIOs
------------
-The first thing a system should do with a GPIO is allocate it, using
-the gpio_request() call; see later.
-
-One of the next things to do with a GPIO, often in board setup code when
-setting up a platform_device using the GPIO, is mark its direction:
-
- /* set as input or output, returning 0 or negative errno */
- int gpio_direction_input(unsigned gpio);
- int gpio_direction_output(unsigned gpio, int value);
-
-The return value is zero for success, else a negative errno. It should
-be checked, since the get/set calls don't have error returns and since
-misconfiguration is possible. You should normally issue these calls from
-a task context. However, for spinlock-safe GPIOs it's OK to use them
-before tasking is enabled, as part of early board setup.
-
-For output GPIOs, the value provided becomes the initial output value.
-This helps avoid signal glitching during system startup.
-
-For compatibility with legacy interfaces to GPIOs, setting the direction
-of a GPIO implicitly requests that GPIO (see below) if it has not been
-requested already. That compatibility is being removed from the optional
-gpiolib framework.
-
-Setting the direction can fail if the GPIO number is invalid, or when
-that particular GPIO can't be used in that mode. It's generally a bad
-idea to rely on boot firmware to have set the direction correctly, since
-it probably wasn't validated to do more than boot Linux. (Similarly,
-that board setup code probably needs to multiplex that pin as a GPIO,
-and configure pullups/pulldowns appropriately.)
-
-
-Spinlock-Safe GPIO access
--------------------------
-Most GPIO controllers can be accessed with memory read/write instructions.
-Those don't need to sleep, and can safely be done from inside hard
-(nonthreaded) IRQ handlers and similar contexts.
-
-Use the following calls to access such GPIOs,
-for which gpio_cansleep() will always return false (see below):
-
- /* GPIO INPUT: return zero or nonzero */
- int gpio_get_value(unsigned gpio);
-
- /* GPIO OUTPUT */
- void gpio_set_value(unsigned gpio, int value);
-
-The values are boolean, zero for low, nonzero for high. When reading the
-value of an output pin, the value returned should be what's seen on the
-pin ... that won't always match the specified output value, because of
-issues including open-drain signaling and output latencies.
-
-The get/set calls have no error returns because "invalid GPIO" should have
-been reported earlier from gpio_direction_*(). However, note that not all
-platforms can read the value of output pins; those that can't should always
-return zero. Also, using these calls for GPIOs that can't safely be accessed
-without sleeping (see below) is an error.
-
-Platform-specific implementations are encouraged to optimize the two
-calls to access the GPIO value in cases where the GPIO number (and for
-output, value) are constant. It's normal for them to need only a couple
-of instructions in such cases (reading or writing a hardware register),
-and not to need spinlocks. Such optimized calls can make bitbanging
-applications a lot more efficient (in both space and time) than spending
-dozens of instructions on subroutine calls.
-
-
-GPIO access that may sleep
---------------------------
-Some GPIO controllers must be accessed using message based busses like I2C
-or SPI. Commands to read or write those GPIO values require waiting to
-get to the head of a queue to transmit a command and get its response.
-This requires sleeping, which can't be done from inside IRQ handlers.
-
-Platforms that support this type of GPIO distinguish them from other GPIOs
-by returning nonzero from this call (which requires a valid GPIO number,
-which should have been previously allocated with gpio_request):
-
- int gpio_cansleep(unsigned gpio);
-
-To access such GPIOs, a different set of accessors is defined:
-
- /* GPIO INPUT: return zero or nonzero, might sleep */
- int gpio_get_value_cansleep(unsigned gpio);
-
- /* GPIO OUTPUT, might sleep */
- void gpio_set_value_cansleep(unsigned gpio, int value);
-
-
-Accessing such GPIOs requires a context which may sleep, for example
-a threaded IRQ handler, and those accessors must be used instead of
-spinlock-safe accessors without the cansleep() name suffix.
-
-Other than the fact that these accessors might sleep, and will work
-on GPIOs that can't be accessed from hardIRQ handlers, these calls act
-the same as the spinlock-safe calls.
-
- ** IN ADDITION ** calls to setup and configure such GPIOs must be made
-from contexts which may sleep, since they may need to access the GPIO
-controller chip too: (These setup calls are usually made from board
-setup or driver probe/teardown code, so this is an easy constraint.)
-
- gpio_direction_input()
- gpio_direction_output()
- gpio_request()
-
-## gpio_request_one()
-## gpio_request_array()
-## gpio_free_array()
-
- gpio_free()
- gpio_set_debounce()
-
-
-
-Claiming and Releasing GPIOs
-----------------------------
-To help catch system configuration errors, two calls are defined.
-
- /* request GPIO, returning 0 or negative errno.
- * non-null labels may be useful for diagnostics.
- */
- int gpio_request(unsigned gpio, const char *label);
-
- /* release previously-claimed GPIO */
- void gpio_free(unsigned gpio);
-
-Passing invalid GPIO numbers to gpio_request() will fail, as will requesting
-GPIOs that have already been claimed with that call. The return value of
-gpio_request() must be checked. You should normally issue these calls from
-a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs
-before tasking is enabled, as part of early board setup.
-
-These calls serve two basic purposes. One is marking the signals which
-are actually in use as GPIOs, for better diagnostics; systems may have
-several hundred potential GPIOs, but often only a dozen are used on any
-given board. Another is to catch conflicts, identifying errors when
-(a) two or more drivers wrongly think they have exclusive use of that
-signal, or (b) something wrongly believes it's safe to remove drivers
-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.
-
-For GPIOs that use pins known to the pinctrl subsystem, that subsystem should
-be informed of their use; a gpiolib driver's .request() operation may call
-pinctrl_request_gpio(), and a gpiolib driver's .free() operation may call
-pinctrl_free_gpio(). The pinctrl subsystem allows a pinctrl_request_gpio()
-to succeed concurrently with a pin or pingroup being "owned" by a device for
-pin multiplexing.
-
-Any programming of pin multiplexing hardware that is needed to route the
-GPIO signal to the appropriate pin should occur within a GPIO driver's
-.direction_input() or .direction_output() operations, and occur after any
-setup of an output GPIO's value. This allows a glitch-free migration from a
-pin's special function to GPIO. This is sometimes required when using a GPIO
-to implement a workaround on signals typically driven by a non-GPIO HW block.
-
-Some platforms allow some or all GPIO signals to be routed to different pins.
-Similarly, other aspects of the GPIO or pin may need to be configured, such as
-pullup/pulldown. Platform software should arrange that any such details are
-configured prior to gpio_request() being called for those GPIOs, e.g. using
-the pinctrl subsystem's mapping table, so that GPIO users need not be aware
-of these details.
-
-Also note that it's your responsibility to have stopped using a GPIO
-before you free it.
-
-Considering in most cases GPIOs are actually configured right after they
-are claimed, three additional calls are defined:
-
- /* request a single GPIO, with initial configuration specified by
- * 'flags', identical to gpio_request() wrt other arguments and
- * return value
- */
- int gpio_request_one(unsigned gpio, unsigned long flags, const char *label);
-
- /* request multiple GPIOs in a single call
- */
- int gpio_request_array(struct gpio *array, size_t num);
-
- /* release multiple GPIOs in a single call
- */
- void gpio_free_array(struct gpio *array, size_t num);
-
-where 'flags' is currently defined to specify the following properties:
-
- * GPIOF_DIR_IN - to configure direction as input
- * GPIOF_DIR_OUT - to configure direction as output
-
- * GPIOF_INIT_LOW - as output, set initial level to LOW
- * GPIOF_INIT_HIGH - as output, set initial level to HIGH
- * GPIOF_OPEN_DRAIN - gpio pin is open drain type.
- * GPIOF_OPEN_SOURCE - gpio pin is open source type.
-
- * GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction
- * GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction
-
-since GPIOF_INIT_* are only valid when configured as output, so group valid
-combinations as:
-
- * GPIOF_IN - configure as input
- * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW
- * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH
-
-When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is
-open drain type. Such pins will not be driven to 1 in output mode. It is
-require to connect pull-up on such pins. By enabling this flag, gpio lib will
-make the direction to input when it is asked to set value of 1 in output mode
-to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode.
-
-When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is
-open source type. Such pins will not be driven to 0 in output mode. It is
-require to connect pull-down on such pin. By enabling this flag, gpio lib will
-make the direction to input when it is asked to set value of 0 in output mode
-to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode.
-
-In the future, these flags can be extended to support more properties.
-
-Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is
-introduced to encapsulate all three fields as:
-
- struct gpio {
- unsigned gpio;
- unsigned long flags;
- const char *label;
- };
-
-A typical example of usage:
-
- static struct gpio leds_gpios[] = {
- { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */
- { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */
- { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */
- { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */
- { ... },
- };
-
- err = gpio_request_one(31, GPIOF_IN, "Reset Button");
- if (err)
- ...
-
- err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios));
- if (err)
- ...
-
- gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios));
-
-
-GPIOs mapped to IRQs
---------------------
-GPIO numbers are unsigned integers; so are IRQ numbers. These make up
-two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can
-map between them using calls like:
-
- /* map GPIO numbers to IRQ numbers */
- int gpio_to_irq(unsigned gpio);
-
- /* 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
-else a negative errno code if the mapping can't be done. (For example,
-some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO
-number that wasn't set up as an input using gpio_direction_input(), or
-to use an IRQ number that didn't originally come from gpio_to_irq().
-
-These two mapping calls are expected to cost on the order of a single
-addition or subtraction. They're not allowed to sleep.
-
-Non-error values returned from gpio_to_irq() can be passed to request_irq()
-or free_irq(). They will often be stored into IRQ resources for platform
-devices, by the board-specific initialization code. Note that IRQ trigger
-options are part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are
-system wakeup capabilities.
-
-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. Note that some platforms don't support
-this reverse mapping, so you should avoid using it.
-
-
-Emulating Open Drain Signals
-----------------------------
-Sometimes shared signals need to use "open drain" signaling, where only the
-low signal level is actually driven. (That term applies to CMOS transistors;
-"open collector" is used for TTL.) A pullup resistor causes the high signal
-level. This is sometimes called a "wire-AND"; or more practically, from the
-negative logic (low=true) perspective this is a "wire-OR".
-
-One common example of an open drain signal is a shared active-low IRQ line.
-Also, bidirectional data bus signals sometimes use open drain signals.
-
-Some GPIO controllers directly support open drain outputs; many don't. When
-you need open drain signaling but your hardware doesn't directly support it,
-there's a common idiom you can use to emulate it with any GPIO pin that can
-be used as either an input or an output:
-
- LOW: gpio_direction_output(gpio, 0) ... this drives the signal
- and overrides the pullup.
-
- HIGH: gpio_direction_input(gpio) ... this turns off the output,
- so the pullup (or some other device) controls the signal.
-
-If you are "driving" the signal high but gpio_get_value(gpio) reports a low
-value (after the appropriate rise time passes), you know some other component
-is driving the shared signal low. That's not necessarily an error. As one
-common example, that's how I2C clocks are stretched: a slave that needs a
-slower clock delays the rising edge of SCK, and the I2C master adjusts its
-signaling rate accordingly.
-
-
-What do these conventions omit?
-===============================
-One of the biggest things these conventions omit is pin multiplexing, since
-this is highly chip-specific and nonportable. One platform might not need
-explicit multiplexing; another might have just two options for use of any
-given pin; another might have eight options per pin; another might be able
-to route a given GPIO to any one of several pins. (Yes, those examples all
-come from systems that run Linux today.)
-
-Related to multiplexing is configuration and enabling of the pullups or
-pulldowns integrated on some platforms. Not all platforms support them,
-or support them in the same way; and any given board might use external
-pullups (or pulldowns) so that the on-chip ones should not be used.
-(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.)
-Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a
-platform-specific issue, as are models like (not) having a one-to-one
-correspondence between configurable pins and GPIOs.
-
-There are other system-specific mechanisms that are not specified here,
-like the aforementioned options for input de-glitching and wire-OR output.
-Hardware may support reading or writing GPIOs in gangs, but that's usually
-configuration dependent: for GPIOs sharing the same bank. (GPIOs are
-commonly grouped in banks of 16 or 32, with a given SOC having several such
-banks.) Some systems can trigger IRQs from output GPIOs, or read values
-from pins not managed as GPIOs. Code relying on such mechanisms will
-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.
-
-
-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. 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
-this framework, and the state of the GPIOs currently in use.
-
-
-Controller Drivers: gpio_chip
------------------------------
-In this framework each GPIO controller is packaged as a "struct gpio_chip"
-with information common to each controller of that type:
-
- - methods to establish GPIO direction
- - methods used to access GPIO values
- - flag saying whether calls to its methods may sleep
- - optional debugfs dump method (showing extra state like pullup config)
- - label for diagnostics
-
-There is also per-instance data, which may come from device.platform_data:
-the number of its first GPIO, and how many GPIOs it exposes.
-
-The code implementing a gpio_chip should support multiple instances of the
-controller, possibly using the driver model. That code will configure each
-gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be
-rare; use gpiochip_remove() when it is unavoidable.
-
-Most often a gpio_chip is part of an instance-specific structure with state
-not exposed by the GPIO interfaces, such as addressing, power management,
-and more. Chips such as codecs will have complex non-GPIO state.
-
-Any debugfs dump method should normally ignore signals which haven't been
-requested as GPIOs. They can use gpiochip_is_requested(), which returns
-either NULL or the label associated with that GPIO when it was requested.
-
-
-Platform Support
-----------------
-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().
-
-It may also provide a custom value for ARCH_NR_GPIOS, so that it better
-reflects the number of GPIOs in actual use on that platform, without
-wasting static table space. (It should count both built-in/SoC GPIOs and
-also ones on GPIO expanders.
-
-ARCH_REQUIRE_GPIOLIB means that the gpiolib code will always get compiled
-into the kernel on that architecture.
-
-ARCH_WANT_OPTIONAL_GPIOLIB means the gpiolib 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:
-
- #define gpio_get_value __gpio_get_value
- #define gpio_set_value __gpio_set_value
- #define gpio_cansleep __gpio_cansleep
-
-Fancier implementations could instead define those as inline functions with
-logic optimizing access to specific SOC-based GPIOs. For example, if the
-referenced GPIO is the constant "12", getting or setting its value could
-cost as little as two or three instructions, never sleeping. When such an
-optimization is not possible those calls must delegate to the framework
-code, costing at least a few dozen instructions. For bitbanged I/O, such
-instruction savings can be significant.
-
-For SOCs, platform-specific code defines and registers gpio_chip instances
-for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to
-match chip vendor documentation, and directly match board schematics. They
-may well start at zero and go up to a platform-specific limit. Such GPIOs
-are normally integrated into platform initialization to make them always be
-available, from arch_initcall() or earlier; they can often serve as IRQs.
-
-
-Board Support
--------------
-For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi
-function devices, FPGAs or CPLDs -- most often board-specific code handles
-registering controller devices and ensures that their drivers know what GPIO
-numbers to use with gpiochip_add(). Their numbers often start right after
-platform-specific GPIOs.
-
-For example, board setup code could create structures identifying the range
-of GPIOs that chip will expose, and passes them to each GPIO expander chip
-using platform_data. Then the chip driver's probe() routine could pass that
-data to gpiochip_add().
-
-Initialization order can be important. For example, when a device relies on
-an I2C-based GPIO, its probe() routine should only be called after that GPIO
-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, 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 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 gpio_export(unsigned gpio, bool direction_may_change);
-
- /* reverse gpio_export() */
- void gpio_unexport();
-
- /* create a sysfs link to an exported GPIO node */
- int gpio_export_link(struct device *dev, const char *name,
- unsigned gpio)
-
- /* change the polarity of a GPIO node in sysfs */
- int gpio_sysfs_set_active_low(unsigned gpio, int value);
-
-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.
-
-After the GPIO has been exported, gpio_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 gpio_sysfs_set_active_low() to hide GPIO line polarity
-differences between boards from user space. This only affects the
-sysfs interface. Polarity change can be done both before and after
-gpio_export(), and previously enabled poll(2) support for either
-rising or falling edge will be reconfigured to follow this setting.