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Based on kernel version 3.16. Page generated on 2014-08-06 21:39 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	
34	If a function is implemented by using several GPIOs together (e.g. a simple LED
35	device that displays digits), an additional index argument can be specified:
36	
37		struct gpio_desc *gpiod_get_index(struct device *dev,
38						  const char *con_id, unsigned int idx)
39	
40	Both functions return either a valid GPIO descriptor, or an error code checkable
41	with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
42	if and only if no GPIO has been assigned to the device/function/index triplet,
43	other error codes are used for cases where a GPIO has been assigned but an error
44	occurred while trying to acquire it. This is useful to discriminate between mere
45	errors and an absence of GPIO for optional GPIO parameters.
46	
47	Device-managed variants of these functions are also defined:
48	
49		struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id)
50	
51		struct gpio_desc *devm_gpiod_get_index(struct device *dev,
52						       const char *con_id,
53						       unsigned int idx)
54	
55	A GPIO descriptor can be disposed of using the gpiod_put() function:
56	
57		void gpiod_put(struct gpio_desc *desc)
58	
59	It is strictly forbidden to use a descriptor after calling this function. The
60	device-managed variant is, unsurprisingly:
61	
62		void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
63	
64	
65	Using GPIOs
66	===========
67	
68	Setting Direction
69	-----------------
70	The first thing a driver must do with a GPIO is setting its direction. This is
71	done by invoking one of the gpiod_direction_*() functions:
72	
73		int gpiod_direction_input(struct gpio_desc *desc)
74		int gpiod_direction_output(struct gpio_desc *desc, int value)
75	
76	The return value is zero for success, else a negative errno. It should be
77	checked, since the get/set calls don't return errors and since misconfiguration
78	is possible. You should normally issue these calls from a task context. However,
79	for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
80	of early board setup.
81	
82	For output GPIOs, the value provided becomes the initial output value. This
83	helps avoid signal glitching during system startup.
84	
85	A driver can also query the current direction of a GPIO:
86	
87		int gpiod_get_direction(const struct gpio_desc *desc)
88	
89	This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
90	
91	Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
92	without setting its direction first is illegal and will result in undefined
93	behavior!**
94	
95	
96	Spinlock-Safe GPIO Access
97	-------------------------
98	Most GPIO controllers can be accessed with memory read/write instructions. Those
99	don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
100	handlers and similar contexts.
101	
102	Use the following calls to access GPIOs from an atomic context:
103	
104		int gpiod_get_value(const struct gpio_desc *desc);
105		void gpiod_set_value(struct gpio_desc *desc, int value);
106	
107	The values are boolean, zero for low, nonzero for high. When reading the value
108	of an output pin, the value returned should be what's seen on the pin. That
109	won't always match the specified output value, because of issues including
110	open-drain signaling and output latencies.
111	
112	The get/set calls do not return errors because "invalid GPIO" should have been
113	reported earlier from gpiod_direction_*(). However, note that not all platforms
114	can read the value of output pins; those that can't should always return zero.
115	Also, using these calls for GPIOs that can't safely be accessed without sleeping
116	(see below) is an error.
117	
118	
119	GPIO Access That May Sleep
120	--------------------------
121	Some GPIO controllers must be accessed using message based buses like I2C or
122	SPI. Commands to read or write those GPIO values require waiting to get to the
123	head of a queue to transmit a command and get its response. This requires
124	sleeping, which can't be done from inside IRQ handlers.
125	
126	Platforms that support this type of GPIO distinguish them from other GPIOs by
127	returning nonzero from this call:
128	
129		int gpiod_cansleep(const struct gpio_desc *desc)
130	
131	To access such GPIOs, a different set of accessors is defined:
132	
133		int gpiod_get_value_cansleep(const struct gpio_desc *desc)
134		void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
135	
136	Accessing such GPIOs requires a context which may sleep, for example a threaded
137	IRQ handler, and those accessors must be used instead of spinlock-safe
138	accessors without the cansleep() name suffix.
139	
140	Other than the fact that these accessors might sleep, and will work on GPIOs
141	that can't be accessed from hardIRQ handlers, these calls act the same as the
142	spinlock-safe calls.
143	
144	
145	Active-low State and Raw GPIO Values
146	------------------------------------
147	Device drivers like to manage the logical state of a GPIO, i.e. the value their
148	device will actually receive, no matter what lies between it and the GPIO line.
149	In some cases, it might make sense to control the actual GPIO line value. The
150	following set of calls ignore the active-low property of a GPIO and work on the
151	raw line value:
152	
153		int gpiod_get_raw_value(const struct gpio_desc *desc)
154		void gpiod_set_raw_value(struct gpio_desc *desc, int value)
155		int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
156		void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
157		int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
158	
159	The active-low state of a GPIO can also be queried using the following call:
160	
161		int gpiod_is_active_low(const struct gpio_desc *desc)
162	
163	Note that these functions should only be used with great moderation ; a driver
164	should not have to care about the physical line level.
165	
166	GPIOs mapped to IRQs
167	--------------------
168	GPIO lines can quite often be used as IRQs. You can get the IRQ number
169	corresponding to a given GPIO using the following call:
170	
171		int gpiod_to_irq(const struct gpio_desc *desc)
172	
173	It will return an IRQ number, or an negative errno code if the mapping can't be
174	done (most likely because that particular GPIO cannot be used as IRQ). It is an
175	unchecked error to use a GPIO that wasn't set up as an input using
176	gpiod_direction_input(), or to use an IRQ number that didn't originally come
177	from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
178	
179	Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
180	free_irq(). They will often be stored into IRQ resources for platform devices,
181	by the board-specific initialization code. Note that IRQ trigger options are
182	part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
183	capabilities.
184	
185	
186	Interacting With the Legacy GPIO Subsystem
187	==========================================
188	Many kernel subsystems still handle GPIOs using the legacy integer-based
189	interface. Although it is strongly encouraged to upgrade them to the safer
190	descriptor-based API, the following two functions allow you to convert a GPIO
191	descriptor into the GPIO integer namespace and vice-versa:
192	
193		int desc_to_gpio(const struct gpio_desc *desc)
194		struct gpio_desc *gpio_to_desc(unsigned gpio)
195	
196	The GPIO number returned by desc_to_gpio() can be safely used as long as the
197	GPIO descriptor has not been freed. All the same, a GPIO number passed to
198	gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
199	descriptor is only possible after the GPIO number has been released.
200	
201	Freeing a GPIO obtained by one API with the other API is forbidden and an
202	unchecked error.
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