Documentation: gpiolib: document new interface

gpiolib now exports a new descriptor-based interface which deprecates
the older integer-based one. This patch documents this new interface and
also takes the opportunity to brush-up the GPIO documentation a little
bit.

The new descriptor-based interface follows the same consumer/driver
model as many other kernel subsystems (e.g. clock, regulator), so its
documentation has similarly been splitted into different files.

The content of the former documentation has been reused whenever it
made sense; however, some of its content did not apply to the new
interface anymore and have this been removed. Likewise, new sections
like the mapping of GPIOs to devices have been written from scratch.

The deprecated legacy-based documentation is still available, untouched,
under Documentation/gpio/gpio-legacy.txt.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

6 files changed, 1436 insertions, 0 deletions

diff --git a/Documentation/gpio/board.txt b/Documentation/gpio/board.txt
new file mode 100644
index 000000000000..0d03506f2cc5
--- /dev/null
+++ b/Documentation/gpio/board.txt
@@ -0,0 +1,115 @@
+GPIO Mappings
+=============
+
+This document explains how GPIOs can be assigned to given devices and functions.
+Note that it only applies to the new descriptor-based interface. For a
+description of the deprecated integer-based GPIO interface please refer to
+gpio-legacy.txt (actually, there is no real mapping possible with the old
+interface; you just fetch an integer from somewhere and request the
+corresponding GPIO.
+
+Platforms that make use of GPIOs must select ARCH_REQUIRE_GPIOLIB (if GPIO usage
+is mandatory) or ARCH_WANT_OPTIONAL_GPIOLIB (if GPIO support can be omitted) in
+their Kconfig. Then, how GPIOs are mapped depends on what the platform uses to
+describe its hardware layout. Currently, mappings can be defined through device
+tree, ACPI, and platform data.
+
+Device Tree
+-----------
+GPIOs can easily be mapped to devices and functions in the device tree. The
+exact way to do it depends on the GPIO controller providing the GPIOs, see the
+device tree bindings for your controller.
+
+GPIOs mappings are defined in the consumer device's node, in a property named
+<function>-gpios, where <function> is the function the driver will request
+through gpiod_get(). For example:
+
+	foo_device {
+		compatible = "acme,foo";
+		...
+		led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */
+			    <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */
+			    <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */
+
+		power-gpio = <&gpio 1 GPIO_ACTIVE_LOW>;
+	};
+
+This property will make GPIOs 15, 16 and 17 available to the driver under the
+"led" function, and GPIO 1 as the "power" GPIO:
+
+	struct gpio_desc *red, *green, *blue, *power;
+
+	red = gpiod_get_index(dev, "led", 0);
+	green = gpiod_get_index(dev, "led", 1);
+	blue = gpiod_get_index(dev, "led", 2);
+
+	power = gpiod_get(dev, "power");
+
+The led GPIOs will be active-high, while the power GPIO will be active-low (i.e.
+gpiod_is_active_low(power) will be true).
+
+ACPI
+----
+ACPI does not support function names for GPIOs. Therefore, only the "idx"
+argument of gpiod_get_index() is useful to discriminate between GPIOs assigned
+to a device. The "con_id" argument can still be set for debugging purposes (it
+will appear under error messages as well as debug and sysfs nodes).
+
+Platform Data
+-------------
+Finally, GPIOs can be bound to devices and functions using platform data. Board
+files that desire to do so need to include the following header:
+
+	#include <linux/gpio/driver.h>
+
+GPIOs are mapped by the means of tables of lookups, containing instances of the
+gpiod_lookup structure. Two macros are defined to help declaring such mappings:
+
+	GPIO_LOOKUP(chip_label, chip_hwnum, dev_id, con_id, flags)
+	GPIO_LOOKUP_IDX(chip_label, chip_hwnum, dev_id, con_id, idx, flags)
+
+where
+
+  - chip_label is the label of the gpiod_chip instance providing the GPIO
+  - chip_hwnum is the hardware number of the GPIO within the chip
+  - dev_id is the identifier of the device that will make use of this GPIO. If
+	NULL, the GPIO will be available to all devices.
+  - con_id is the name of the GPIO function from the device point of view. It
+	can be NULL.
+  - idx is the index of the GPIO within the function.
+  - flags is defined to specify the following properties:
+	* GPIOF_ACTIVE_LOW	- to configure the GPIO as active-low
+	* GPIOF_OPEN_DRAIN	- GPIO pin is open drain type.
+	* GPIOF_OPEN_SOURCE	- GPIO pin is open source type.
+
+In the future, these flags might be extended to support more properties.
+
+Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0.
+
+A lookup table can then be defined as follows:
+
+	struct gpiod_lookup gpios_table[] = {
+	GPIO_LOOKUP_IDX("gpio.0", 15, "foo.0", "led", 0, GPIO_ACTIVE_HIGH),
+	GPIO_LOOKUP_IDX("gpio.0", 16, "foo.0", "led", 1, GPIO_ACTIVE_HIGH),
+	GPIO_LOOKUP_IDX("gpio.0", 17, "foo.0", "led", 2, GPIO_ACTIVE_HIGH),
+	GPIO_LOOKUP("gpio.0", 1, "foo.0", "power", GPIO_ACTIVE_LOW),
+	};
+
+And the table can be added by the board code as follows:
+
+	gpiod_add_table(gpios_table, ARRAY_SIZE(gpios_table));
+
+The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:
+
+	struct gpio_desc *red, *green, *blue, *power;
+
+	red = gpiod_get_index(dev, "led", 0);
+	green = gpiod_get_index(dev, "led", 1);
+	blue = gpiod_get_index(dev, "led", 2);
+
+	power = gpiod_get(dev, "power");
+	gpiod_direction_output(power, 1);
+
+Since the "power" GPIO is mapped as active-low, its actual signal will be 0
+after this code. Contrary to the legacy integer GPIO interface, the active-low
+property is handled during mapping and is thus transparent to GPIO consumers.
diff --git a/Documentation/gpio/consumer.txt b/Documentation/gpio/consumer.txt
new file mode 100644
index 000000000000..07c74a3765a0
--- /dev/null
+++ b/Documentation/gpio/consumer.txt
@@ -0,0 +1,197 @@
+GPIO Descriptor Consumer Interface
+==================================
+
+This document describes the consumer interface of the GPIO framework. Note that
+it describes the new descriptor-based interface. For a description of the
+deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
+
+
+Guidelines for GPIOs consumers
+==============================
+
+Drivers that can't work without standard GPIO calls should have Kconfig entries
+that depend on GPIOLIB. The functions that allow a driver to obtain and use
+GPIOs are available by including the following file:
+
+	#include <linux/gpio/consumer.h>
+
+All the functions that work with the descriptor-based GPIO interface are
+prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No
+other function in the kernel should use these prefixes.
+
+
+Obtaining and Disposing GPIOs
+=============================
+
+With the descriptor-based interface, GPIOs are identified with an opaque,
+non-forgeable handler that must be obtained through a call to one of the
+gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the
+device that will use the GPIO and the function the requested GPIO is supposed to
+fulfill:
+
+	struct gpio_desc *gpiod_get(struct device *dev, const char *con_id)
+
+If a function is implemented by using several GPIOs together (e.g. a simple LED
+device that displays digits), an additional index argument can be specified:
+
+	struct gpio_desc *gpiod_get_index(struct device *dev,
+					  const char *con_id, unsigned int idx)
+
+Both functions return either a valid GPIO descriptor, or an error code checkable
+with IS_ERR(). They will never return a NULL pointer.
+
+Device-managed variants of these functions are also defined:
+
+	struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id)
+
+	struct gpio_desc *devm_gpiod_get_index(struct device *dev,
+					       const char *con_id,
+					       unsigned int idx)
+
+A GPIO descriptor can be disposed of using the gpiod_put() function:
+
+	void gpiod_put(struct gpio_desc *desc)
+
+It is strictly forbidden to use a descriptor after calling this function. The
+device-managed variant is, unsurprisingly:
+
+	void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
+
+
+Using GPIOs
+===========
+
+Setting Direction
+-----------------
+The first thing a driver must do with a GPIO is setting its direction. This is
+done by invoking one of the gpiod_direction_*() functions:
+
+	int gpiod_direction_input(struct gpio_desc *desc)
+	int gpiod_direction_output(struct gpio_desc *desc, int value)
+
+The return value is zero for success, else a negative errno. It should be
+checked, since the get/set calls don't return errors and since misconfiguration
+is possible. You should normally issue these calls from a task context. However,
+for spinlock-safe GPIOs it is 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.
+
+A driver can also query the current direction of a GPIO:
+
+	int gpiod_get_direction(const struct gpio_desc *desc)
+
+This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
+
+Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
+without setting its direction first is illegal and will result in undefined
+behavior!**
+
+
+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 (non-threaded) IRQ
+handlers and similar contexts.
+
+Use the following calls to access GPIOs from an atomic context:
+
+	int gpiod_get_value(const struct gpio_desc *desc);
+	void gpiod_set_value(struct gpio_desc *desc, 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 do not return errors because "invalid GPIO" should have been
+reported earlier from gpiod_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.
+
+
+GPIO Access That May Sleep
+--------------------------
+Some GPIO controllers must be accessed using message based buses 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:
+
+	int gpiod_cansleep(const struct gpio_desc *desc)
+
+To access such GPIOs, a different set of accessors is defined:
+
+	int gpiod_get_value_cansleep(const struct gpio_desc *desc)
+	void gpiod_set_value_cansleep(struct gpio_desc *desc, 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.
+
+
+Active-low State and Raw GPIO Values
+------------------------------------
+Device drivers like to manage the logical state of a GPIO, i.e. the value their
+device will actually receive, no matter what lies between it and the GPIO line.
+In some cases, it might make sense to control the actual GPIO line value. The
+following set of calls ignore the active-low property of a GPIO and work on the
+raw line value:
+
+	int gpiod_get_raw_value(const struct gpio_desc *desc)
+	void gpiod_set_raw_value(struct gpio_desc *desc, int value)
+	int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
+	void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
+
+The active-low state of a GPIO can also be queried using the following call:
+
+	int gpiod_is_active_low(const struct gpio_desc *desc)
+
+Note that these functions should only be used with great moderation ; a driver
+should not have to care about the physical line level.
+
+GPIOs mapped to IRQs
+--------------------
+GPIO lines can quite often be used as IRQs. You can get the IRQ number
+corresponding to a given GPIO using the following call:
+
+	int gpiod_to_irq(const struct gpio_desc *desc)
+
+It will return an IRQ number, or an negative errno code if the mapping can't be
+done (most likely because that particular GPIO cannot be used as IRQ). It is an
+unchecked error to use a GPIO that wasn't set up as an input using
+gpiod_direction_input(), or to use an IRQ number that didn't originally come
+from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
+
+Non-error values returned from gpiod_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.
+
+
+Interacting With the Legacy GPIO Subsystem
+==========================================
+Many kernel subsystems still handle GPIOs using the legacy integer-based
+interface. Although it is strongly encouraged to upgrade them to the safer
+descriptor-based API, the following two functions allow you to convert a GPIO
+descriptor into the GPIO integer namespace and vice-versa:
+
+	int desc_to_gpio(const struct gpio_desc *desc)
+	struct gpio_desc *gpio_to_desc(unsigned gpio)
+
+The GPIO number returned by desc_to_gpio() can be safely used as long as the
+GPIO descriptor has not been freed. All the same, a GPIO number passed to
+gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
+descriptor is only possible after the GPIO number has been released.
+
+Freeing a GPIO obtained by one API with the other API is forbidden and an
+unchecked error.
diff --git a/Documentation/gpio/driver.txt b/Documentation/gpio/driver.txt
new file mode 100644
index 000000000000..9da0bfa74781
--- /dev/null
+++ b/Documentation/gpio/driver.txt
@@ -0,0 +1,75 @@
+GPIO Descriptor Driver Interface
+================================
+
+This document serves as a guide for GPIO chip drivers writers. Note that it
+describes the new descriptor-based interface. For a description of the
+deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
+
+Each GPIO controller driver needs to include the following header, which defines
+the structures used to define a GPIO driver:
+
+	#include <linux/gpio/driver.h>
+
+
+Internal Representation of GPIOs
+================================
+
+Inside a GPIO driver, individual GPIOs are identified by their hardware number,
+which is a unique number between 0 and n, n being the number of GPIOs managed by
+the chip. This number is purely internal: the hardware number of a particular
+GPIO descriptor is never made visible outside of the driver.
+
+On top of this internal number, each GPIO also need to have a global number in
+the integer GPIO namespace so that it can be used with the legacy GPIO
+interface. Each chip must thus have a "base" number (which can be automatically
+assigned), and for each GPIO the global number will be (base + hardware number).
+Although the integer representation is considered deprecated, it still has many
+users and thus needs to be maintained.
+
+So for example one platform could use numbers 32-159 for GPIOs, with a
+controller defining 128 GPIOs at a "base" of 32 ; while another platform 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.
+
+
+Controller Drivers: gpio_chip
+=============================
+
+In the gpiolib framework each GPIO controller is packaged as a "struct
+gpio_chip" (see linux/gpio/driver.h for its complete definition) with members
+common to each controller of that type:
+
+ - methods to establish GPIO direction
+ - methods used to access GPIO values
+ - method to return the IRQ number associated to a given GPIO
+ - flag saying whether calls to its methods may sleep
+ - optional debugfs dump method (showing extra state like pullup config)
+ - optional base number (will be automatically assigned if omitted)
+ - label for diagnostics and GPIOs mapping using platform data
+
+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.
+
+Locking IRQ usage
+-----------------
+Input GPIOs can be used as IRQ signals. When this happens, a driver is requested
+to mark the GPIO as being used as an IRQ:
+
+	int gpiod_lock_as_irq(struct gpio_desc *desc)
+
+This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
+is released:
+
+	void gpiod_unlock_as_irq(struct gpio_desc *desc)
diff --git a/Documentation/gpio/gpio-legacy.txt b/Documentation/gpio/gpio-legacy.txt
new file mode 100644
index 000000000000..6f83fa965b4b
--- /dev/null
+++ b/Documentation/gpio/gpio-legacy.txt
@@ -0,0 +1,775 @@
+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.
+
+
+E