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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 select ARCH_REQUIRE_GPIOLIB or
+ARCH_WANT_OPTIONAL_GPIOLIB in their Kconfig.  Drivers that can't work without
+standard GPIO calls should have Kconfig entries which depend on GPIOLIB.  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.
+
+
+GPIO controllers and the pinctrl subsystem
+------------------------------------------
+
+A GPIO controller on a SOC might be tightly coupled with the pinctrl
+subsystem, in the sense that the pins can be used by other functions
+together with an optional gpio feature. We have already covered the
+case where e.g. a GPIO controller need to reserve a pin or set the
+direction of a pin by calling any of:
+
+pinctrl_request_gpio()
+pinctrl_free_gpio()
+pinctrl_gpio_direction_input()
+pinctrl_gpio_direction_output()
+
+But how does the pin control subsystem cross-correlate the GPIO
+numbers (which are a global business) to a certain pin on a certain
+pin controller?
+
+This is done by registering "ranges" of pins, which are essentially
+cross-reference tables. These are described in
+Documentation/pinctrl.txt
+
+While the pin allocation is totally managed by the pinctrl subsystem,
+gpio (under gpiolib) is still maintained by gpio drivers. It may happen
+that different pin ranges in a SoC is managed by different gpio drivers.
+
+This makes it logical to let gpio drivers announce their pin ranges to
+the pin ctrl subsystem before it will call 'pinctrl_request_gpio' in order
+to request the corresponding pin to be prepared by the pinctrl subsystem
+before any gpio usage.
+
+For this, the gpio controller can register its pin range with pinctrl
+subsystem. There are two ways of doing it currently: with or without DT.
+
+For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt.
+
+For non-DT support, user can call gpiochip_add_pin_range() with appropriate
+parameters to register a range of gpio pins with a pinctrl driver. For this
+exact name string of pinctrl device has to be passed as one of the
+argument to this routine.
+
+
+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.