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Based on kernel version 4.0. Page generated on 2015-04-14 21:24 EST.

1	GPIO Descriptor Consumer Interface
2	==================================
4	This document describes the consumer interface of the GPIO framework. Note that
5	it describes the new descriptor-based interface. For a description of the
6	deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
9	Guidelines for GPIOs consumers
10	==============================
12	Drivers that can't work without standard GPIO calls should have Kconfig entries
13	that depend on GPIOLIB. The functions that allow a driver to obtain and use
14	GPIOs are available by including the following file:
16		#include <linux/gpio/consumer.h>
18	All the functions that work with the descriptor-based GPIO interface are
19	prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No
20	other function in the kernel should use these prefixes.
23	Obtaining and Disposing GPIOs
24	=============================
26	With the descriptor-based interface, GPIOs are identified with an opaque,
27	non-forgeable handler that must be obtained through a call to one of the
28	gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the
29	device that will use the GPIO and the function the requested GPIO is supposed to
30	fulfill:
32		struct gpio_desc *gpiod_get(struct device *dev, const char *con_id,
33					    enum gpiod_flags flags)
35	If a function is implemented by using several GPIOs together (e.g. a simple LED
36	device that displays digits), an additional index argument can be specified:
38		struct gpio_desc *gpiod_get_index(struct device *dev,
39						  const char *con_id, unsigned int idx,
40						  enum gpiod_flags flags)
42	The flags parameter is used to optionally specify a direction and initial value
43	for the GPIO. Values can be:
45	* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set
46	  later with one of the dedicated functions.
47	* GPIOD_IN to initialize the GPIO as input.
48	* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0.
49	* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1.
51	Both functions return either a valid GPIO descriptor, or an error code checkable
52	with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
53	if and only if no GPIO has been assigned to the device/function/index triplet,
54	other error codes are used for cases where a GPIO has been assigned but an error
55	occurred while trying to acquire it. This is useful to discriminate between mere
56	errors and an absence of GPIO for optional GPIO parameters. For the common
57	pattern where a GPIO is optional, the gpiod_get_optional() and
58	gpiod_get_index_optional() functions can be used. These functions return NULL
59	instead of -ENOENT if no GPIO has been assigned to the requested function:
62		struct gpio_desc *gpiod_get_optional(struct device *dev,
63						     const char *con_id,
64						     enum gpiod_flags flags)
66		struct gpio_desc *gpiod_get_index_optional(struct device *dev,
67							   const char *con_id,
68							   unsigned int index,
69							   enum gpiod_flags flags)
71	Device-managed variants of these functions are also defined:
73		struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
74						 enum gpiod_flags flags)
76		struct gpio_desc *devm_gpiod_get_index(struct device *dev,
77						       const char *con_id,
78						       unsigned int idx,
79						       enum gpiod_flags flags)
81		struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
82							  const char *con_id,
83							  enum gpiod_flags flags)
85		struct gpio_desc * devm_gpiod_get_index_optional(struct device *dev,
86								const char *con_id,
87								unsigned int index,
88								enum gpiod_flags flags)
90	A GPIO descriptor can be disposed of using the gpiod_put() function:
92		void gpiod_put(struct gpio_desc *desc)
94	It is strictly forbidden to use a descriptor after calling this function. The
95	device-managed variant is, unsurprisingly:
97		void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
100	Using GPIOs
101	===========
103	Setting Direction
104	-----------------
105	The first thing a driver must do with a GPIO is setting its direction. If no
106	direction-setting flags have been given to gpiod_get*(), this is done by
107	invoking one of the gpiod_direction_*() functions:
109		int gpiod_direction_input(struct gpio_desc *desc)
110		int gpiod_direction_output(struct gpio_desc *desc, int value)
112	The return value is zero for success, else a negative errno. It should be
113	checked, since the get/set calls don't return errors and since misconfiguration
114	is possible. You should normally issue these calls from a task context. However,
115	for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
116	of early board setup.
118	For output GPIOs, the value provided becomes the initial output value. This
119	helps avoid signal glitching during system startup.
121	A driver can also query the current direction of a GPIO:
123		int gpiod_get_direction(const struct gpio_desc *desc)
125	This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
127	Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
128	without setting its direction first is illegal and will result in undefined
129	behavior!**
132	Spinlock-Safe GPIO Access
133	-------------------------
134	Most GPIO controllers can be accessed with memory read/write instructions. Those
135	don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
136	handlers and similar contexts.
138	Use the following calls to access GPIOs from an atomic context:
140		int gpiod_get_value(const struct gpio_desc *desc);
141		void gpiod_set_value(struct gpio_desc *desc, int value);
143	The values are boolean, zero for low, nonzero for high. When reading the value
144	of an output pin, the value returned should be what's seen on the pin. That
145	won't always match the specified output value, because of issues including
146	open-drain signaling and output latencies.
148	The get/set calls do not return errors because "invalid GPIO" should have been
149	reported earlier from gpiod_direction_*(). However, note that not all platforms
150	can read the value of output pins; those that can't should always return zero.
151	Also, using these calls for GPIOs that can't safely be accessed without sleeping
152	(see below) is an error.
155	GPIO Access That May Sleep
156	--------------------------
157	Some GPIO controllers must be accessed using message based buses like I2C or
158	SPI. Commands to read or write those GPIO values require waiting to get to the
159	head of a queue to transmit a command and get its response. This requires
160	sleeping, which can't be done from inside IRQ handlers.
162	Platforms that support this type of GPIO distinguish them from other GPIOs by
163	returning nonzero from this call:
165		int gpiod_cansleep(const struct gpio_desc *desc)
167	To access such GPIOs, a different set of accessors is defined:
169		int gpiod_get_value_cansleep(const struct gpio_desc *desc)
170		void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
172	Accessing such GPIOs requires a context which may sleep, for example a threaded
173	IRQ handler, and those accessors must be used instead of spinlock-safe
174	accessors without the cansleep() name suffix.
176	Other than the fact that these accessors might sleep, and will work on GPIOs
177	that can't be accessed from hardIRQ handlers, these calls act the same as the
178	spinlock-safe calls.
181	Active-low State and Raw GPIO Values
182	------------------------------------
183	Device drivers like to manage the logical state of a GPIO, i.e. the value their
184	device will actually receive, no matter what lies between it and the GPIO line.
185	In some cases, it might make sense to control the actual GPIO line value. The
186	following set of calls ignore the active-low property of a GPIO and work on the
187	raw line value:
189		int gpiod_get_raw_value(const struct gpio_desc *desc)
190		void gpiod_set_raw_value(struct gpio_desc *desc, int value)
191		int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
192		void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
193		int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
195	The active-low state of a GPIO can also be queried using the following call:
197		int gpiod_is_active_low(const struct gpio_desc *desc)
199	Note that these functions should only be used with great moderation ; a driver
200	should not have to care about the physical line level.
203	Set multiple GPIO outputs with a single function call
204	-----------------------------------------------------
205	The following functions set the output values of an array of GPIOs:
207		void gpiod_set_array(unsigned int array_size,
208				     struct gpio_desc **desc_array,
209				     int *value_array)
210		void gpiod_set_raw_array(unsigned int array_size,
211					 struct gpio_desc **desc_array,
212					 int *value_array)
213		void gpiod_set_array_cansleep(unsigned int array_size,
214					      struct gpio_desc **desc_array,
215					      int *value_array)
216		void gpiod_set_raw_array_cansleep(unsigned int array_size,
217						  struct gpio_desc **desc_array,
218						  int *value_array)
220	The array can be an arbitrary set of GPIOs. The functions will try to set
221	GPIOs belonging to the same bank or chip simultaneously if supported by the
222	corresponding chip driver. In that case a significantly improved performance
223	can be expected. If simultaneous setting is not possible the GPIOs will be set
224	sequentially.
225	Note that for optimal performance GPIOs belonging to the same chip should be
226	contiguous within the array of descriptors.
229	GPIOs mapped to IRQs
230	--------------------
231	GPIO lines can quite often be used as IRQs. You can get the IRQ number
232	corresponding to a given GPIO using the following call:
234		int gpiod_to_irq(const struct gpio_desc *desc)
236	It will return an IRQ number, or an negative errno code if the mapping can't be
237	done (most likely because that particular GPIO cannot be used as IRQ). It is an
238	unchecked error to use a GPIO that wasn't set up as an input using
239	gpiod_direction_input(), or to use an IRQ number that didn't originally come
240	from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
242	Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
243	free_irq(). They will often be stored into IRQ resources for platform devices,
244	by the board-specific initialization code. Note that IRQ trigger options are
245	part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
246	capabilities.
249	GPIOs and ACPI
250	==============
252	On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by
253	the _CRS configuration objects of devices.  Those resources do not provide
254	connection IDs (names) for GPIOs, so it is necessary to use an additional
255	mechanism for this purpose.
257	Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object
258	which, among other things, may be used to provide connection IDs for specific
259	GPIOs described by the GpioIo()/GpioInt() resources in _CRS.  If that is the
260	case, it will be handled by the GPIO subsystem automatically.  However, if the
261	_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO
262	connection IDs need to be provided by device drivers.
264	For details refer to Documentation/acpi/gpio-properties.txt
267	Interacting With the Legacy GPIO Subsystem
268	==========================================
269	Many kernel subsystems still handle GPIOs using the legacy integer-based
270	interface. Although it is strongly encouraged to upgrade them to the safer
271	descriptor-based API, the following two functions allow you to convert a GPIO
272	descriptor into the GPIO integer namespace and vice-versa:
274		int desc_to_gpio(const struct gpio_desc *desc)
275		struct gpio_desc *gpio_to_desc(unsigned gpio)
277	The GPIO number returned by desc_to_gpio() can be safely used as long as the
278	GPIO descriptor has not been freed. All the same, a GPIO number passed to
279	gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
280	descriptor is only possible after the GPIO number has been released.
282	Freeing a GPIO obtained by one API with the other API is forbidden and an
283	unchecked error.
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