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Based on kernel version 4.3. Page generated on 2015-11-02 12:49 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	For a more detailed description of the con_id parameter in the DeviceTree case
43	see Documentation/gpio/board.txt
45	The flags parameter is used to optionally specify a direction and initial value
46	for the GPIO. Values can be:
48	* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set
49	  later with one of the dedicated functions.
50	* GPIOD_IN to initialize the GPIO as input.
51	* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0.
52	* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1.
54	Both functions return either a valid GPIO descriptor, or an error code checkable
55	with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
56	if and only if no GPIO has been assigned to the device/function/index triplet,
57	other error codes are used for cases where a GPIO has been assigned but an error
58	occurred while trying to acquire it. This is useful to discriminate between mere
59	errors and an absence of GPIO for optional GPIO parameters. For the common
60	pattern where a GPIO is optional, the gpiod_get_optional() and
61	gpiod_get_index_optional() functions can be used. These functions return NULL
62	instead of -ENOENT if no GPIO has been assigned to the requested function:
64		struct gpio_desc *gpiod_get_optional(struct device *dev,
65						     const char *con_id,
66						     enum gpiod_flags flags)
68		struct gpio_desc *gpiod_get_index_optional(struct device *dev,
69							   const char *con_id,
70							   unsigned int index,
71							   enum gpiod_flags flags)
73	For a function using multiple GPIOs all of those can be obtained with one call:
75		struct gpio_descs *gpiod_get_array(struct device *dev,
76						   const char *con_id,
77						   enum gpiod_flags flags)
79	This function returns a struct gpio_descs which contains an array of
80	descriptors:
82		struct gpio_descs {
83			unsigned int ndescs;
84			struct gpio_desc *desc[];
85		}
87	The following function returns NULL instead of -ENOENT if no GPIOs have been
88	assigned to the requested function:
90		struct gpio_descs *gpiod_get_array_optional(struct device *dev,
91							    const char *con_id,
92							    enum gpiod_flags flags)
94	Device-managed variants of these functions are also defined:
96		struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
97						 enum gpiod_flags flags)
99		struct gpio_desc *devm_gpiod_get_index(struct device *dev,
100						       const char *con_id,
101						       unsigned int idx,
102						       enum gpiod_flags flags)
104		struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
105							  const char *con_id,
106							  enum gpiod_flags flags)
108		struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev,
109								const char *con_id,
110								unsigned int index,
111								enum gpiod_flags flags)
113		struct gpio_descs *devm_gpiod_get_array(struct device *dev,
114							const char *con_id,
115							enum gpiod_flags flags)
117		struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev,
118								 const char *con_id,
119								 enum gpiod_flags flags)
121	A GPIO descriptor can be disposed of using the gpiod_put() function:
123		void gpiod_put(struct gpio_desc *desc)
125	For an array of GPIOs this function can be used:
127		void gpiod_put_array(struct gpio_descs *descs)
129	It is strictly forbidden to use a descriptor after calling these functions.
130	It is also not allowed to individually release descriptors (using gpiod_put())
131	from an array acquired with gpiod_get_array().
133	The device-managed variants are, unsurprisingly:
135		void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
137		void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs)
140	Using GPIOs
141	===========
143	Setting Direction
144	-----------------
145	The first thing a driver must do with a GPIO is setting its direction. If no
146	direction-setting flags have been given to gpiod_get*(), this is done by
147	invoking one of the gpiod_direction_*() functions:
149		int gpiod_direction_input(struct gpio_desc *desc)
150		int gpiod_direction_output(struct gpio_desc *desc, int value)
152	The return value is zero for success, else a negative errno. It should be
153	checked, since the get/set calls don't return errors and since misconfiguration
154	is possible. You should normally issue these calls from a task context. However,
155	for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
156	of early board setup.
158	For output GPIOs, the value provided becomes the initial output value. This
159	helps avoid signal glitching during system startup.
161	A driver can also query the current direction of a GPIO:
163		int gpiod_get_direction(const struct gpio_desc *desc)
165	This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
167	Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
168	without setting its direction first is illegal and will result in undefined
169	behavior!**
172	Spinlock-Safe GPIO Access
173	-------------------------
174	Most GPIO controllers can be accessed with memory read/write instructions. Those
175	don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
176	handlers and similar contexts.
178	Use the following calls to access GPIOs from an atomic context:
180		int gpiod_get_value(const struct gpio_desc *desc);
181		void gpiod_set_value(struct gpio_desc *desc, int value);
183	The values are boolean, zero for low, nonzero for high. When reading the value
184	of an output pin, the value returned should be what's seen on the pin. That
185	won't always match the specified output value, because of issues including
186	open-drain signaling and output latencies.
188	The get/set calls do not return errors because "invalid GPIO" should have been
189	reported earlier from gpiod_direction_*(). However, note that not all platforms
190	can read the value of output pins; those that can't should always return zero.
191	Also, using these calls for GPIOs that can't safely be accessed without sleeping
192	(see below) is an error.
195	GPIO Access That May Sleep
196	--------------------------
197	Some GPIO controllers must be accessed using message based buses like I2C or
198	SPI. Commands to read or write those GPIO values require waiting to get to the
199	head of a queue to transmit a command and get its response. This requires
200	sleeping, which can't be done from inside IRQ handlers.
202	Platforms that support this type of GPIO distinguish them from other GPIOs by
203	returning nonzero from this call:
205		int gpiod_cansleep(const struct gpio_desc *desc)
207	To access such GPIOs, a different set of accessors is defined:
209		int gpiod_get_value_cansleep(const struct gpio_desc *desc)
210		void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
212	Accessing such GPIOs requires a context which may sleep, for example a threaded
213	IRQ handler, and those accessors must be used instead of spinlock-safe
214	accessors without the cansleep() name suffix.
216	Other than the fact that these accessors might sleep, and will work on GPIOs
217	that can't be accessed from hardIRQ handlers, these calls act the same as the
218	spinlock-safe calls.
221	Active-low State and Raw GPIO Values
222	------------------------------------
223	Device drivers like to manage the logical state of a GPIO, i.e. the value their
224	device will actually receive, no matter what lies between it and the GPIO line.
225	In some cases, it might make sense to control the actual GPIO line value. The
226	following set of calls ignore the active-low property of a GPIO and work on the
227	raw line value:
229		int gpiod_get_raw_value(const struct gpio_desc *desc)
230		void gpiod_set_raw_value(struct gpio_desc *desc, int value)
231		int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
232		void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
233		int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
235	The active-low state of a GPIO can also be queried using the following call:
237		int gpiod_is_active_low(const struct gpio_desc *desc)
239	Note that these functions should only be used with great moderation ; a driver
240	should not have to care about the physical line level.
243	The active-low property
244	-----------------------
246	As a driver should not have to care about the physical line level, all of the
247	gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with
248	the *logical* value. With this they take the active-low property into account.
249	This means that they check whether the GPIO is configured to be active-low,
250	and if so, they manipulate the passed value before the physical line level is
251	driven.
253	With this, all the gpiod_set_(array)_value_xxx() functions interpret the
254	parameter "value" as "active" ("1") or "inactive" ("0"). The physical line
255	level will be driven accordingly.
257	As an example, if the active-low property for a dedicated GPIO is set, and the
258	gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level
259	will be driven low.
261	To summarize:
263	Function (example)               active-low proporty  physical line
264	gpiod_set_raw_value(desc, 0);        don't care           low
265	gpiod_set_raw_value(desc, 1);        don't care           high
266	gpiod_set_value(desc, 0);       default (active-high)     low
267	gpiod_set_value(desc, 1);       default (active-high)     high
268	gpiod_set_value(desc, 0);             active-low          high
269	gpiod_set_value(desc, 1);             active-low          low
271	Please note again that the set_raw/get_raw functions should be avoided as much
272	as possible, especially by drivers which should not care about the actual
273	physical line level and worry about the logical value instead.
276	Set multiple GPIO outputs with a single function call
277	-----------------------------------------------------
278	The following functions set the output values of an array of GPIOs:
280		void gpiod_set_array_value(unsigned int array_size,
281					   struct gpio_desc **desc_array,
282					   int *value_array)
283		void gpiod_set_raw_array_value(unsigned int array_size,
284					       struct gpio_desc **desc_array,
285					       int *value_array)
286		void gpiod_set_array_value_cansleep(unsigned int array_size,
287						    struct gpio_desc **desc_array,
288						    int *value_array)
289		void gpiod_set_raw_array_value_cansleep(unsigned int array_size,
290							struct gpio_desc **desc_array,
291							int *value_array)
293	The array can be an arbitrary set of GPIOs. The functions will try to set
294	GPIOs belonging to the same bank or chip simultaneously if supported by the
295	corresponding chip driver. In that case a significantly improved performance
296	can be expected. If simultaneous setting is not possible the GPIOs will be set
297	sequentially.
299	The gpiod_set_array() functions take three arguments:
300		* array_size	- the number of array elements
301		* desc_array	- an array of GPIO descriptors
302		* value_array	- an array of values to assign to the GPIOs
304	The descriptor array can be obtained using the gpiod_get_array() function
305	or one of its variants. If the group of descriptors returned by that function
306	matches the desired group of GPIOs, those GPIOs can be set by simply using
307	the struct gpio_descs returned by gpiod_get_array():
309		struct gpio_descs *my_gpio_descs = gpiod_get_array(...);
310		gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc,
311				      my_gpio_values);
313	It is also possible to set a completely arbitrary array of descriptors. The
314	descriptors may be obtained using any combination of gpiod_get() and
315	gpiod_get_array(). Afterwards the array of descriptors has to be setup
316	manually before it can be used with gpiod_set_array().
318	Note that for optimal performance GPIOs belonging to the same chip should be
319	contiguous within the array of descriptors.
322	GPIOs mapped to IRQs
323	--------------------
324	GPIO lines can quite often be used as IRQs. You can get the IRQ number
325	corresponding to a given GPIO using the following call:
327		int gpiod_to_irq(const struct gpio_desc *desc)
329	It will return an IRQ number, or a negative errno code if the mapping can't be
330	done (most likely because that particular GPIO cannot be used as IRQ). It is an
331	unchecked error to use a GPIO that wasn't set up as an input using
332	gpiod_direction_input(), or to use an IRQ number that didn't originally come
333	from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
335	Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
336	free_irq(). They will often be stored into IRQ resources for platform devices,
337	by the board-specific initialization code. Note that IRQ trigger options are
338	part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
339	capabilities.
342	GPIOs and ACPI
343	==============
345	On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by
346	the _CRS configuration objects of devices.  Those resources do not provide
347	connection IDs (names) for GPIOs, so it is necessary to use an additional
348	mechanism for this purpose.
350	Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object
351	which, among other things, may be used to provide connection IDs for specific
352	GPIOs described by the GpioIo()/GpioInt() resources in _CRS.  If that is the
353	case, it will be handled by the GPIO subsystem automatically.  However, if the
354	_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO
355	connection IDs need to be provided by device drivers.
357	For details refer to Documentation/acpi/gpio-properties.txt
360	Interacting With the Legacy GPIO Subsystem
361	==========================================
362	Many kernel subsystems still handle GPIOs using the legacy integer-based
363	interface. Although it is strongly encouraged to upgrade them to the safer
364	descriptor-based API, the following two functions allow you to convert a GPIO
365	descriptor into the GPIO integer namespace and vice-versa:
367		int desc_to_gpio(const struct gpio_desc *desc)
368		struct gpio_desc *gpio_to_desc(unsigned gpio)
370	The GPIO number returned by desc_to_gpio() can be safely used as long as the
371	GPIO descriptor has not been freed. All the same, a GPIO number passed to
372	gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
373	descriptor is only possible after the GPIO number has been released.
375	Freeing a GPIO obtained by one API with the other API is forbidden and an
376	unchecked error.
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