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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
32 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id)
33
34If a function is implemented by using several GPIOs together (e.g. a simple LED
35device 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
40Both functions return either a valid GPIO descriptor, or an error code checkable
Alexandre Courbot2a3cf6a2013-12-11 11:32:28 +090041with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
42if and only if no GPIO has been assigned to the device/function/index triplet,
43other error codes are used for cases where a GPIO has been assigned but an error
Carlos Garciac98be0c2014-04-04 22:31:00 -040044occurred while trying to acquire it. This is useful to discriminate between mere
Alexandre Courbot2a3cf6a2013-12-11 11:32:28 +090045errors and an absence of GPIO for optional GPIO parameters.
Alexandre Courbotfd8e1982013-11-16 21:34:21 +090046
47Device-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
55A GPIO descriptor can be disposed of using the gpiod_put() function:
56
57 void gpiod_put(struct gpio_desc *desc)
58
59It is strictly forbidden to use a descriptor after calling this function. The
60device-managed variant is, unsurprisingly:
61
62 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
63
64
65Using GPIOs
66===========
67
68Setting Direction
69-----------------
70The first thing a driver must do with a GPIO is setting its direction. This is
71done 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
76The return value is zero for success, else a negative errno. It should be
77checked, since the get/set calls don't return errors and since misconfiguration
78is possible. You should normally issue these calls from a task context. However,
79for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
80of early board setup.
81
82For output GPIOs, the value provided becomes the initial output value. This
83helps avoid signal glitching during system startup.
84
85A driver can also query the current direction of a GPIO:
86
87 int gpiod_get_direction(const struct gpio_desc *desc)
88
89This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT.
90
91Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
92without setting its direction first is illegal and will result in undefined
93behavior!**
94
95
96Spinlock-Safe GPIO Access
97-------------------------
98Most GPIO controllers can be accessed with memory read/write instructions. Those
99don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
100handlers and similar contexts.
101
102Use 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
107The values are boolean, zero for low, nonzero for high. When reading the value
108of an output pin, the value returned should be what's seen on the pin. That
109won't always match the specified output value, because of issues including
110open-drain signaling and output latencies.
111
112The get/set calls do not return errors because "invalid GPIO" should have been
113reported earlier from gpiod_direction_*(). However, note that not all platforms
114can read the value of output pins; those that can't should always return zero.
115Also, using these calls for GPIOs that can't safely be accessed without sleeping
116(see below) is an error.
117
118
119GPIO Access That May Sleep
120--------------------------
121Some GPIO controllers must be accessed using message based buses like I2C or
122SPI. Commands to read or write those GPIO values require waiting to get to the
123head of a queue to transmit a command and get its response. This requires
124sleeping, which can't be done from inside IRQ handlers.
125
126Platforms that support this type of GPIO distinguish them from other GPIOs by
127returning nonzero from this call:
128
129 int gpiod_cansleep(const struct gpio_desc *desc)
130
131To 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
136Accessing such GPIOs requires a context which may sleep, for example a threaded
137IRQ handler, and those accessors must be used instead of spinlock-safe
138accessors without the cansleep() name suffix.
139
140Other than the fact that these accessors might sleep, and will work on GPIOs
141that can't be accessed from hardIRQ handlers, these calls act the same as the
142spinlock-safe calls.
143
144
145Active-low State and Raw GPIO Values
146------------------------------------
147Device drivers like to manage the logical state of a GPIO, i.e. the value their
148device will actually receive, no matter what lies between it and the GPIO line.
149In some cases, it might make sense to control the actual GPIO line value. The
150following set of calls ignore the active-low property of a GPIO and work on the
151raw 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)
Philipp Zabelef70bbe2014-01-07 12:34:11 +0100157 int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
Alexandre Courbotfd8e1982013-11-16 21:34:21 +0900158
159The 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
163Note that these functions should only be used with great moderation ; a driver
164should not have to care about the physical line level.
165
166GPIOs mapped to IRQs
167--------------------
168GPIO lines can quite often be used as IRQs. You can get the IRQ number
169corresponding to a given GPIO using the following call:
170
171 int gpiod_to_irq(const struct gpio_desc *desc)
172
173It will return an IRQ number, or an negative errno code if the mapping can't be
174done (most likely because that particular GPIO cannot be used as IRQ). It is an
175unchecked error to use a GPIO that wasn't set up as an input using
176gpiod_direction_input(), or to use an IRQ number that didn't originally come
177from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
178
179Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
180free_irq(). They will often be stored into IRQ resources for platform devices,
181by the board-specific initialization code. Note that IRQ trigger options are
182part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
183capabilities.
184
185
186Interacting With the Legacy GPIO Subsystem
187==========================================
188Many kernel subsystems still handle GPIOs using the legacy integer-based
189interface. Although it is strongly encouraged to upgrade them to the safer
190descriptor-based API, the following two functions allow you to convert a GPIO
191descriptor 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
196The GPIO number returned by desc_to_gpio() can be safely used as long as the
197GPIO descriptor has not been freed. All the same, a GPIO number passed to
198gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
199descriptor is only possible after the GPIO number has been released.
200
201Freeing a GPIO obtained by one API with the other API is forbidden and an
202unchecked error.