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Alexandre Courbotfd8e1982013-11-16 21:34:21 +09001GPIO Descriptor Consumer Interface
2==================================
3
4This document describes the consumer interface of the GPIO framework. Note that
5it describes the new descriptor-based interface. For a description of the
6deprecated integer-based GPIO interface please refer to gpio-legacy.txt.
7
8
9Guidelines for GPIOs consumers
10==============================
11
12Drivers that can't work without standard GPIO calls should have Kconfig entries
13that depend on GPIOLIB. The functions that allow a driver to obtain and use
14GPIOs are available by including the following file:
15
16 #include <linux/gpio/consumer.h>
17
18All the functions that work with the descriptor-based GPIO interface are
19prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No
20other function in the kernel should use these prefixes.
21
22
23Obtaining and Disposing GPIOs
24=============================
25
26With the descriptor-based interface, GPIOs are identified with an opaque,
27non-forgeable handler that must be obtained through a call to one of the
28gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the
29device that will use the GPIO and the function the requested GPIO is supposed to
30fulfill:
31
Alexandre Courbot39b2bbe2014-07-25 23:38:36 +090032 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id,
33 enum gpiod_flags flags)
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090034
35If a function is implemented by using several GPIOs together (e.g. a simple LED
36device that displays digits), an additional index argument can be specified:
37
38 struct gpio_desc *gpiod_get_index(struct device *dev,
Alexandre Courbot39b2bbe2014-07-25 23:38:36 +090039 const char *con_id, unsigned int idx,
40 enum gpiod_flags flags)
41
42The flags parameter is used to optionally specify a direction and initial value
43for 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.
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090050
51Both functions return either a valid GPIO descriptor, or an error code checkable
Alexandre Courbot2a3cf6a2013-12-11 11:32:28 +090052with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
53if and only if no GPIO has been assigned to the device/function/index triplet,
54other error codes are used for cases where a GPIO has been assigned but an error
Carlos Garciac98be0c2014-04-04 22:31:00 -040055occurred while trying to acquire it. This is useful to discriminate between mere
Alexandre Courbot1b11a9b2014-08-18 09:39:01 -070056errors and an absence of GPIO for optional GPIO parameters. For the common
57pattern where a GPIO is optional, the gpiod_get_optional() and
58gpiod_get_index_optional() functions can be used. These functions return NULL
59instead of -ENOENT if no GPIO has been assigned to the requested function:
60
61
62 struct gpio_desc *gpiod_get_optional(struct device *dev,
63 const char *con_id,
64 enum gpiod_flags flags)
65
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)
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090070
71Device-managed variants of these functions are also defined:
72
Alexandre Courbot39b2bbe2014-07-25 23:38:36 +090073 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
74 enum gpiod_flags flags)
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090075
76 struct gpio_desc *devm_gpiod_get_index(struct device *dev,
77 const char *con_id,
Alexandre Courbot39b2bbe2014-07-25 23:38:36 +090078 unsigned int idx,
79 enum gpiod_flags flags)
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090080
Alexandre Courbot1b11a9b2014-08-18 09:39:01 -070081 struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
82 const char *con_id,
83 enum gpiod_flags flags)
84
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)
89
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090090A GPIO descriptor can be disposed of using the gpiod_put() function:
91
92 void gpiod_put(struct gpio_desc *desc)
93
94It is strictly forbidden to use a descriptor after calling this function. The
95device-managed variant is, unsurprisingly:
96
97 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
98
99
100Using GPIOs
101===========
102
103Setting Direction
104-----------------
Alexandre Courbot39b2bbe2014-07-25 23:38:36 +0900105The first thing a driver must do with a GPIO is setting its direction. If no
106direction-setting flags have been given to gpiod_get*(), this is done by
107invoking one of the gpiod_direction_*() functions:
Alexandre Courbotfd8e1982013-11-16 21:34:21 +0900108
109 int gpiod_direction_input(struct gpio_desc *desc)
110 int gpiod_direction_output(struct gpio_desc *desc, int value)
111
112The return value is zero for success, else a negative errno. It should be
113checked, since the get/set calls don't return errors and since misconfiguration
114is possible. You should normally issue these calls from a task context. However,
115for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
116of early board setup.
117
118For output GPIOs, the value provided becomes the initial output value. This
119helps avoid signal glitching during system startup.
120
121A driver can also query the current direction of a GPIO:
122
123 int gpiod_get_direction(const struct gpio_desc *desc)
124
125This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
126
127Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
128without setting its direction first is illegal and will result in undefined
129behavior!**
130
131
132Spinlock-Safe GPIO Access
133-------------------------
134Most GPIO controllers can be accessed with memory read/write instructions. Those
135don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
136handlers and similar contexts.
137
138Use the following calls to access GPIOs from an atomic context:
139
140 int gpiod_get_value(const struct gpio_desc *desc);
141 void gpiod_set_value(struct gpio_desc *desc, int value);
142
143The values are boolean, zero for low, nonzero for high. When reading the value
144of an output pin, the value returned should be what's seen on the pin. That
145won't always match the specified output value, because of issues including
146open-drain signaling and output latencies.
147
148The get/set calls do not return errors because "invalid GPIO" should have been
149reported earlier from gpiod_direction_*(). However, note that not all platforms
150can read the value of output pins; those that can't should always return zero.
151Also, using these calls for GPIOs that can't safely be accessed without sleeping
152(see below) is an error.
153
154
155GPIO Access That May Sleep
156--------------------------
157Some GPIO controllers must be accessed using message based buses like I2C or
158SPI. Commands to read or write those GPIO values require waiting to get to the
159head of a queue to transmit a command and get its response. This requires
160sleeping, which can't be done from inside IRQ handlers.
161
162Platforms that support this type of GPIO distinguish them from other GPIOs by
163returning nonzero from this call:
164
165 int gpiod_cansleep(const struct gpio_desc *desc)
166
167To access such GPIOs, a different set of accessors is defined:
168
169 int gpiod_get_value_cansleep(const struct gpio_desc *desc)
170 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
171
172Accessing such GPIOs requires a context which may sleep, for example a threaded
173IRQ handler, and those accessors must be used instead of spinlock-safe
174accessors without the cansleep() name suffix.
175
176Other than the fact that these accessors might sleep, and will work on GPIOs
177that can't be accessed from hardIRQ handlers, these calls act the same as the
178spinlock-safe calls.
179
180
181Active-low State and Raw GPIO Values
182------------------------------------
183Device drivers like to manage the logical state of a GPIO, i.e. the value their
184device will actually receive, no matter what lies between it and the GPIO line.
185In some cases, it might make sense to control the actual GPIO line value. The
186following set of calls ignore the active-low property of a GPIO and work on the
187raw line value:
188
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)
Philipp Zabelef70bbe2014-01-07 12:34:11 +0100193 int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
Alexandre Courbotfd8e1982013-11-16 21:34:21 +0900194
195The active-low state of a GPIO can also be queried using the following call:
196
197 int gpiod_is_active_low(const struct gpio_desc *desc)
198
199Note that these functions should only be used with great moderation ; a driver
200should not have to care about the physical line level.
201
202GPIOs mapped to IRQs
203--------------------
204GPIO lines can quite often be used as IRQs. You can get the IRQ number
205corresponding to a given GPIO using the following call:
206
207 int gpiod_to_irq(const struct gpio_desc *desc)
208
209It will return an IRQ number, or an negative errno code if the mapping can't be
210done (most likely because that particular GPIO cannot be used as IRQ). It is an
211unchecked error to use a GPIO that wasn't set up as an input using
212gpiod_direction_input(), or to use an IRQ number that didn't originally come
213from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
214
215Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
216free_irq(). They will often be stored into IRQ resources for platform devices,
217by the board-specific initialization code. Note that IRQ trigger options are
218part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
219capabilities.
220
221
222Interacting With the Legacy GPIO Subsystem
223==========================================
224Many kernel subsystems still handle GPIOs using the legacy integer-based
225interface. Although it is strongly encouraged to upgrade them to the safer
226descriptor-based API, the following two functions allow you to convert a GPIO
227descriptor into the GPIO integer namespace and vice-versa:
228
229 int desc_to_gpio(const struct gpio_desc *desc)
230 struct gpio_desc *gpio_to_desc(unsigned gpio)
231
232The GPIO number returned by desc_to_gpio() can be safely used as long as the
233GPIO descriptor has not been freed. All the same, a GPIO number passed to
234gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
235descriptor is only possible after the GPIO number has been released.
236
237Freeing a GPIO obtained by one API with the other API is forbidden and an
238unchecked error.