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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	GPIO Descriptor Consumer Interface
2	==================================
3	
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.
7	
8	
9	Guidelines for GPIOs consumers
10	==============================
11	
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:
15	
16		#include <linux/gpio/consumer.h>
17	
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.
21	
22	
23	Obtaining and Disposing GPIOs
24	=============================
25	
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:
31	
32		struct gpio_desc *gpiod_get(struct device *dev, const char *con_id,
33					    enum gpiod_flags flags)
34	
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:
37	
38		struct gpio_desc *gpiod_get_index(struct device *dev,
39						  const char *con_id, unsigned int idx,
40						  enum gpiod_flags flags)
41	
42	For a more detailed description of the con_id parameter in the DeviceTree case
43	see Documentation/gpio/board.txt
44	
45	The flags parameter is used to optionally specify a direction and initial value
46	for the GPIO. Values can be:
47	
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.
53	
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:
63	
64		struct gpio_desc *gpiod_get_optional(struct device *dev,
65						     const char *con_id,
66						     enum gpiod_flags flags)
67	
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)
72	
73	Note that gpio_get*_optional() functions (and their managed variants), unlike
74	the rest of gpiolib API, also return NULL when gpiolib support is disabled.
75	This is helpful to driver authors, since they do not need to special case
76	-ENOSYS return codes.  System integrators should however be careful to enable
77	gpiolib on systems that need it.
78	
79	For a function using multiple GPIOs all of those can be obtained with one call:
80	
81		struct gpio_descs *gpiod_get_array(struct device *dev,
82						   const char *con_id,
83						   enum gpiod_flags flags)
84	
85	This function returns a struct gpio_descs which contains an array of
86	descriptors:
87	
88		struct gpio_descs {
89			unsigned int ndescs;
90			struct gpio_desc *desc[];
91		}
92	
93	The following function returns NULL instead of -ENOENT if no GPIOs have been
94	assigned to the requested function:
95	
96		struct gpio_descs *gpiod_get_array_optional(struct device *dev,
97							    const char *con_id,
98							    enum gpiod_flags flags)
99	
100	Device-managed variants of these functions are also defined:
101	
102		struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
103						 enum gpiod_flags flags)
104	
105		struct gpio_desc *devm_gpiod_get_index(struct device *dev,
106						       const char *con_id,
107						       unsigned int idx,
108						       enum gpiod_flags flags)
109	
110		struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
111							  const char *con_id,
112							  enum gpiod_flags flags)
113	
114		struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev,
115								const char *con_id,
116								unsigned int index,
117								enum gpiod_flags flags)
118	
119		struct gpio_descs *devm_gpiod_get_array(struct device *dev,
120							const char *con_id,
121							enum gpiod_flags flags)
122	
123		struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev,
124								 const char *con_id,
125								 enum gpiod_flags flags)
126	
127	A GPIO descriptor can be disposed of using the gpiod_put() function:
128	
129		void gpiod_put(struct gpio_desc *desc)
130	
131	For an array of GPIOs this function can be used:
132	
133		void gpiod_put_array(struct gpio_descs *descs)
134	
135	It is strictly forbidden to use a descriptor after calling these functions.
136	It is also not allowed to individually release descriptors (using gpiod_put())
137	from an array acquired with gpiod_get_array().
138	
139	The device-managed variants are, unsurprisingly:
140	
141		void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
142	
143		void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs)
144	
145	
146	Using GPIOs
147	===========
148	
149	Setting Direction
150	-----------------
151	The first thing a driver must do with a GPIO is setting its direction. If no
152	direction-setting flags have been given to gpiod_get*(), this is done by
153	invoking one of the gpiod_direction_*() functions:
154	
155		int gpiod_direction_input(struct gpio_desc *desc)
156		int gpiod_direction_output(struct gpio_desc *desc, int value)
157	
158	The return value is zero for success, else a negative errno. It should be
159	checked, since the get/set calls don't return errors and since misconfiguration
160	is possible. You should normally issue these calls from a task context. However,
161	for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
162	of early board setup.
163	
164	For output GPIOs, the value provided becomes the initial output value. This
165	helps avoid signal glitching during system startup.
166	
167	A driver can also query the current direction of a GPIO:
168	
169		int gpiod_get_direction(const struct gpio_desc *desc)
170	
171	This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
172	
173	Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
174	without setting its direction first is illegal and will result in undefined
175	behavior!**
176	
177	
178	Spinlock-Safe GPIO Access
179	-------------------------
180	Most GPIO controllers can be accessed with memory read/write instructions. Those
181	don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
182	handlers and similar contexts.
183	
184	Use the following calls to access GPIOs from an atomic context:
185	
186		int gpiod_get_value(const struct gpio_desc *desc);
187		void gpiod_set_value(struct gpio_desc *desc, int value);
188	
189	The values are boolean, zero for low, nonzero for high. When reading the value
190	of an output pin, the value returned should be what's seen on the pin. That
191	won't always match the specified output value, because of issues including
192	open-drain signaling and output latencies.
193	
194	The get/set calls do not return errors because "invalid GPIO" should have been
195	reported earlier from gpiod_direction_*(). However, note that not all platforms
196	can read the value of output pins; those that can't should always return zero.
197	Also, using these calls for GPIOs that can't safely be accessed without sleeping
198	(see below) is an error.
199	
200	
201	GPIO Access That May Sleep
202	--------------------------
203	Some GPIO controllers must be accessed using message based buses like I2C or
204	SPI. Commands to read or write those GPIO values require waiting to get to the
205	head of a queue to transmit a command and get its response. This requires
206	sleeping, which can't be done from inside IRQ handlers.
207	
208	Platforms that support this type of GPIO distinguish them from other GPIOs by
209	returning nonzero from this call:
210	
211		int gpiod_cansleep(const struct gpio_desc *desc)
212	
213	To access such GPIOs, a different set of accessors is defined:
214	
215		int gpiod_get_value_cansleep(const struct gpio_desc *desc)
216		void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
217	
218	Accessing such GPIOs requires a context which may sleep, for example a threaded
219	IRQ handler, and those accessors must be used instead of spinlock-safe
220	accessors without the cansleep() name suffix.
221	
222	Other than the fact that these accessors might sleep, and will work on GPIOs
223	that can't be accessed from hardIRQ handlers, these calls act the same as the
224	spinlock-safe calls.
225	
226	
227	Active-low State and Raw GPIO Values
228	------------------------------------
229	Device drivers like to manage the logical state of a GPIO, i.e. the value their
230	device will actually receive, no matter what lies between it and the GPIO line.
231	In some cases, it might make sense to control the actual GPIO line value. The
232	following set of calls ignore the active-low property of a GPIO and work on the
233	raw line value:
234	
235		int gpiod_get_raw_value(const struct gpio_desc *desc)
236		void gpiod_set_raw_value(struct gpio_desc *desc, int value)
237		int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
238		void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
239		int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
240	
241	The active-low state of a GPIO can also be queried using the following call:
242	
243		int gpiod_is_active_low(const struct gpio_desc *desc)
244	
245	Note that these functions should only be used with great moderation ; a driver
246	should not have to care about the physical line level.
247	
248	
249	The active-low property
250	-----------------------
251	
252	As a driver should not have to care about the physical line level, all of the
253	gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with
254	the *logical* value. With this they take the active-low property into account.
255	This means that they check whether the GPIO is configured to be active-low,
256	and if so, they manipulate the passed value before the physical line level is
257	driven.
258	
259	With this, all the gpiod_set_(array)_value_xxx() functions interpret the
260	parameter "value" as "active" ("1") or "inactive" ("0"). The physical line
261	level will be driven accordingly.
262	
263	As an example, if the active-low property for a dedicated GPIO is set, and the
264	gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level
265	will be driven low.
266	
267	To summarize:
268	
269	Function (example)               active-low property  physical line
270	gpiod_set_raw_value(desc, 0);        don't care           low
271	gpiod_set_raw_value(desc, 1);        don't care           high
272	gpiod_set_value(desc, 0);       default (active-high)     low
273	gpiod_set_value(desc, 1);       default (active-high)     high
274	gpiod_set_value(desc, 0);             active-low          high
275	gpiod_set_value(desc, 1);             active-low          low
276	
277	Please note again that the set_raw/get_raw functions should be avoided as much
278	as possible, especially by drivers which should not care about the actual
279	physical line level and worry about the logical value instead.
280	
281	
282	Set multiple GPIO outputs with a single function call
283	-----------------------------------------------------
284	The following functions set the output values of an array of GPIOs:
285	
286		void gpiod_set_array_value(unsigned int array_size,
287					   struct gpio_desc **desc_array,
288					   int *value_array)
289		void gpiod_set_raw_array_value(unsigned int array_size,
290					       struct gpio_desc **desc_array,
291					       int *value_array)
292		void gpiod_set_array_value_cansleep(unsigned int array_size,
293						    struct gpio_desc **desc_array,
294						    int *value_array)
295		void gpiod_set_raw_array_value_cansleep(unsigned int array_size,
296							struct gpio_desc **desc_array,
297							int *value_array)
298	
299	The array can be an arbitrary set of GPIOs. The functions will try to set
300	GPIOs belonging to the same bank or chip simultaneously if supported by the
301	corresponding chip driver. In that case a significantly improved performance
302	can be expected. If simultaneous setting is not possible the GPIOs will be set
303	sequentially.
304	
305	The gpiod_set_array() functions take three arguments:
306		* array_size	- the number of array elements
307		* desc_array	- an array of GPIO descriptors
308		* value_array	- an array of values to assign to the GPIOs
309	
310	The descriptor array can be obtained using the gpiod_get_array() function
311	or one of its variants. If the group of descriptors returned by that function
312	matches the desired group of GPIOs, those GPIOs can be set by simply using
313	the struct gpio_descs returned by gpiod_get_array():
314	
315		struct gpio_descs *my_gpio_descs = gpiod_get_array(...);
316		gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc,
317				      my_gpio_values);
318	
319	It is also possible to set a completely arbitrary array of descriptors. The
320	descriptors may be obtained using any combination of gpiod_get() and
321	gpiod_get_array(). Afterwards the array of descriptors has to be setup
322	manually before it can be used with gpiod_set_array().
323	
324	Note that for optimal performance GPIOs belonging to the same chip should be
325	contiguous within the array of descriptors.
326	
327	
328	GPIOs mapped to IRQs
329	--------------------
330	GPIO lines can quite often be used as IRQs. You can get the IRQ number
331	corresponding to a given GPIO using the following call:
332	
333		int gpiod_to_irq(const struct gpio_desc *desc)
334	
335	It will return an IRQ number, or a negative errno code if the mapping can't be
336	done (most likely because that particular GPIO cannot be used as IRQ). It is an
337	unchecked error to use a GPIO that wasn't set up as an input using
338	gpiod_direction_input(), or to use an IRQ number that didn't originally come
339	from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
340	
341	Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
342	free_irq(). They will often be stored into IRQ resources for platform devices,
343	by the board-specific initialization code. Note that IRQ trigger options are
344	part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
345	capabilities.
346	
347	
348	GPIOs and ACPI
349	==============
350	
351	On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by
352	the _CRS configuration objects of devices.  Those resources do not provide
353	connection IDs (names) for GPIOs, so it is necessary to use an additional
354	mechanism for this purpose.
355	
356	Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object
357	which, among other things, may be used to provide connection IDs for specific
358	GPIOs described by the GpioIo()/GpioInt() resources in _CRS.  If that is the
359	case, it will be handled by the GPIO subsystem automatically.  However, if the
360	_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO
361	connection IDs need to be provided by device drivers.
362	
363	For details refer to Documentation/acpi/gpio-properties.txt
364	
365	
366	Interacting With the Legacy GPIO Subsystem
367	==========================================
368	Many kernel subsystems still handle GPIOs using the legacy integer-based
369	interface. Although it is strongly encouraged to upgrade them to the safer
370	descriptor-based API, the following two functions allow you to convert a GPIO
371	descriptor into the GPIO integer namespace and vice-versa:
372	
373		int desc_to_gpio(const struct gpio_desc *desc)
374		struct gpio_desc *gpio_to_desc(unsigned gpio)
375	
376	The GPIO number returned by desc_to_gpio() can be safely used as long as the
377	GPIO descriptor has not been freed. All the same, a GPIO number passed to
378	gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
379	descriptor is only possible after the GPIO number has been released.
380	
381	Freeing a GPIO obtained by one API with the other API is forbidden and an
382	unchecked error.
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