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Based on kernel version 4.1. Page generated on 2015-06-28 12:12 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	The flags parameter is used to optionally specify a direction and initial value
43	for the GPIO. Values can be:
44	
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.
50	
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:
60	
61		struct gpio_desc *gpiod_get_optional(struct device *dev,
62						     const char *con_id,
63						     enum gpiod_flags flags)
64	
65		struct gpio_desc *gpiod_get_index_optional(struct device *dev,
66							   const char *con_id,
67							   unsigned int index,
68							   enum gpiod_flags flags)
69	
70	For a function using multiple GPIOs all of those can be obtained with one call:
71	
72		struct gpio_descs *gpiod_get_array(struct device *dev,
73						   const char *con_id,
74						   enum gpiod_flags flags)
75	
76	This function returns a struct gpio_descs which contains an array of
77	descriptors:
78	
79		struct gpio_descs {
80			unsigned int ndescs;
81			struct gpio_desc *desc[];
82		}
83	
84	The following function returns NULL instead of -ENOENT if no GPIOs have been
85	assigned to the requested function:
86	
87		struct gpio_descs *gpiod_get_array_optional(struct device *dev,
88							    const char *con_id,
89							    enum gpiod_flags flags)
90	
91	Device-managed variants of these functions are also defined:
92	
93		struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
94						 enum gpiod_flags flags)
95	
96		struct gpio_desc *devm_gpiod_get_index(struct device *dev,
97						       const char *con_id,
98						       unsigned int idx,
99						       enum gpiod_flags flags)
100	
101		struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
102							  const char *con_id,
103							  enum gpiod_flags flags)
104	
105		struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev,
106								const char *con_id,
107								unsigned int index,
108								enum gpiod_flags flags)
109	
110		struct gpio_descs *devm_gpiod_get_array(struct device *dev,
111							const char *con_id,
112							enum gpiod_flags flags)
113	
114		struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev,
115								 const char *con_id,
116								 enum gpiod_flags flags)
117	
118	A GPIO descriptor can be disposed of using the gpiod_put() function:
119	
120		void gpiod_put(struct gpio_desc *desc)
121	
122	For an array of GPIOs this function can be used:
123	
124		void gpiod_put_array(struct gpio_descs *descs)
125	
126	It is strictly forbidden to use a descriptor after calling these functions.
127	It is also not allowed to individually release descriptors (using gpiod_put())
128	from an array acquired with gpiod_get_array().
129	
130	The device-managed variants are, unsurprisingly:
131	
132		void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
133	
134		void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs)
135	
136	
137	Using GPIOs
138	===========
139	
140	Setting Direction
141	-----------------
142	The first thing a driver must do with a GPIO is setting its direction. If no
143	direction-setting flags have been given to gpiod_get*(), this is done by
144	invoking one of the gpiod_direction_*() functions:
145	
146		int gpiod_direction_input(struct gpio_desc *desc)
147		int gpiod_direction_output(struct gpio_desc *desc, int value)
148	
149	The return value is zero for success, else a negative errno. It should be
150	checked, since the get/set calls don't return errors and since misconfiguration
151	is possible. You should normally issue these calls from a task context. However,
152	for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
153	of early board setup.
154	
155	For output GPIOs, the value provided becomes the initial output value. This
156	helps avoid signal glitching during system startup.
157	
158	A driver can also query the current direction of a GPIO:
159	
160		int gpiod_get_direction(const struct gpio_desc *desc)
161	
162	This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
163	
164	Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
165	without setting its direction first is illegal and will result in undefined
166	behavior!**
167	
168	
169	Spinlock-Safe GPIO Access
170	-------------------------
171	Most GPIO controllers can be accessed with memory read/write instructions. Those
172	don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
173	handlers and similar contexts.
174	
175	Use the following calls to access GPIOs from an atomic context:
176	
177		int gpiod_get_value(const struct gpio_desc *desc);
178		void gpiod_set_value(struct gpio_desc *desc, int value);
179	
180	The values are boolean, zero for low, nonzero for high. When reading the value
181	of an output pin, the value returned should be what's seen on the pin. That
182	won't always match the specified output value, because of issues including
183	open-drain signaling and output latencies.
184	
185	The get/set calls do not return errors because "invalid GPIO" should have been
186	reported earlier from gpiod_direction_*(). However, note that not all platforms
187	can read the value of output pins; those that can't should always return zero.
188	Also, using these calls for GPIOs that can't safely be accessed without sleeping
189	(see below) is an error.
190	
191	
192	GPIO Access That May Sleep
193	--------------------------
194	Some GPIO controllers must be accessed using message based buses like I2C or
195	SPI. Commands to read or write those GPIO values require waiting to get to the
196	head of a queue to transmit a command and get its response. This requires
197	sleeping, which can't be done from inside IRQ handlers.
198	
199	Platforms that support this type of GPIO distinguish them from other GPIOs by
200	returning nonzero from this call:
201	
202		int gpiod_cansleep(const struct gpio_desc *desc)
203	
204	To access such GPIOs, a different set of accessors is defined:
205	
206		int gpiod_get_value_cansleep(const struct gpio_desc *desc)
207		void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
208	
209	Accessing such GPIOs requires a context which may sleep, for example a threaded
210	IRQ handler, and those accessors must be used instead of spinlock-safe
211	accessors without the cansleep() name suffix.
212	
213	Other than the fact that these accessors might sleep, and will work on GPIOs
214	that can't be accessed from hardIRQ handlers, these calls act the same as the
215	spinlock-safe calls.
216	
217	
218	Active-low State and Raw GPIO Values
219	------------------------------------
220	Device drivers like to manage the logical state of a GPIO, i.e. the value their
221	device will actually receive, no matter what lies between it and the GPIO line.
222	In some cases, it might make sense to control the actual GPIO line value. The
223	following set of calls ignore the active-low property of a GPIO and work on the
224	raw line value:
225	
226		int gpiod_get_raw_value(const struct gpio_desc *desc)
227		void gpiod_set_raw_value(struct gpio_desc *desc, int value)
228		int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
229		void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
230		int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
231	
232	The active-low state of a GPIO can also be queried using the following call:
233	
234		int gpiod_is_active_low(const struct gpio_desc *desc)
235	
236	Note that these functions should only be used with great moderation ; a driver
237	should not have to care about the physical line level.
238	
239	
240	Set multiple GPIO outputs with a single function call
241	-----------------------------------------------------
242	The following functions set the output values of an array of GPIOs:
243	
244		void gpiod_set_array(unsigned int array_size,
245				     struct gpio_desc **desc_array,
246				     int *value_array)
247		void gpiod_set_raw_array(unsigned int array_size,
248					 struct gpio_desc **desc_array,
249					 int *value_array)
250		void gpiod_set_array_cansleep(unsigned int array_size,
251					      struct gpio_desc **desc_array,
252					      int *value_array)
253		void gpiod_set_raw_array_cansleep(unsigned int array_size,
254						  struct gpio_desc **desc_array,
255						  int *value_array)
256	
257	The array can be an arbitrary set of GPIOs. The functions will try to set
258	GPIOs belonging to the same bank or chip simultaneously if supported by the
259	corresponding chip driver. In that case a significantly improved performance
260	can be expected. If simultaneous setting is not possible the GPIOs will be set
261	sequentially.
262	
263	The gpiod_set_array() functions take three arguments:
264		* array_size	- the number of array elements
265		* desc_array	- an array of GPIO descriptors
266		* value_array	- an array of values to assign to the GPIOs
267	
268	The descriptor array can be obtained using the gpiod_get_array() function
269	or one of its variants. If the group of descriptors returned by that function
270	matches the desired group of GPIOs, those GPIOs can be set by simply using
271	the struct gpio_descs returned by gpiod_get_array():
272	
273		struct gpio_descs *my_gpio_descs = gpiod_get_array(...);
274		gpiod_set_array(my_gpio_descs->ndescs, my_gpio_descs->desc,
275				my_gpio_values);
276	
277	It is also possible to set a completely arbitrary array of descriptors. The
278	descriptors may be obtained using any combination of gpiod_get() and
279	gpiod_get_array(). Afterwards the array of descriptors has to be setup
280	manually before it can be used with gpiod_set_array().
281	
282	Note that for optimal performance GPIOs belonging to the same chip should be
283	contiguous within the array of descriptors.
284	
285	
286	GPIOs mapped to IRQs
287	--------------------
288	GPIO lines can quite often be used as IRQs. You can get the IRQ number
289	corresponding to a given GPIO using the following call:
290	
291		int gpiod_to_irq(const struct gpio_desc *desc)
292	
293	It will return an IRQ number, or an negative errno code if the mapping can't be
294	done (most likely because that particular GPIO cannot be used as IRQ). It is an
295	unchecked error to use a GPIO that wasn't set up as an input using
296	gpiod_direction_input(), or to use an IRQ number that didn't originally come
297	from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
298	
299	Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
300	free_irq(). They will often be stored into IRQ resources for platform devices,
301	by the board-specific initialization code. Note that IRQ trigger options are
302	part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
303	capabilities.
304	
305	
306	GPIOs and ACPI
307	==============
308	
309	On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by
310	the _CRS configuration objects of devices.  Those resources do not provide
311	connection IDs (names) for GPIOs, so it is necessary to use an additional
312	mechanism for this purpose.
313	
314	Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object
315	which, among other things, may be used to provide connection IDs for specific
316	GPIOs described by the GpioIo()/GpioInt() resources in _CRS.  If that is the
317	case, it will be handled by the GPIO subsystem automatically.  However, if the
318	_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO
319	connection IDs need to be provided by device drivers.
320	
321	For details refer to Documentation/acpi/gpio-properties.txt
322	
323	
324	Interacting With the Legacy GPIO Subsystem
325	==========================================
326	Many kernel subsystems still handle GPIOs using the legacy integer-based
327	interface. Although it is strongly encouraged to upgrade them to the safer
328	descriptor-based API, the following two functions allow you to convert a GPIO
329	descriptor into the GPIO integer namespace and vice-versa:
330	
331		int desc_to_gpio(const struct gpio_desc *desc)
332		struct gpio_desc *gpio_to_desc(unsigned gpio)
333	
334	The GPIO number returned by desc_to_gpio() can be safely used as long as the
335	GPIO descriptor has not been freed. All the same, a GPIO number passed to
336	gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
337	descriptor is only possible after the GPIO number has been released.
338	
339	Freeing a GPIO obtained by one API with the other API is forbidden and an
340	unchecked error.
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