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). -ENOENT will be returned
  if and only if no GPIO has been assigned to the device/function/index triplet,
  other error codes are used for cases where a GPIO has been assigned but an error
  occured while trying to acquire it. This is useful to discriminate between mere
  errors and an absence of GPIO for optional GPIO parameters.
  
  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.