Based on kernel version 4.0. Page generated on 2015-04-14 21:24 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 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) 70 71 Device-managed variants of these functions are also defined: 72 73 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id, 74 enum gpiod_flags flags) 75 76 struct gpio_desc *devm_gpiod_get_index(struct device *dev, 77 const char *con_id, 78 unsigned int idx, 79 enum gpiod_flags flags) 80 81 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 90 A GPIO descriptor can be disposed of using the gpiod_put() function: 91 92 void gpiod_put(struct gpio_desc *desc) 93 94 It is strictly forbidden to use a descriptor after calling this function. The 95 device-managed variant is, unsurprisingly: 96 97 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) 98 99 100 Using GPIOs 101 =========== 102 103 Setting Direction 104 ----------------- 105 The first thing a driver must do with a GPIO is setting its direction. If no 106 direction-setting flags have been given to gpiod_get*(), this is done by 107 invoking one of the gpiod_direction_*() functions: 108 109 int gpiod_direction_input(struct gpio_desc *desc) 110 int gpiod_direction_output(struct gpio_desc *desc, int value) 111 112 The return value is zero for success, else a negative errno. It should be 113 checked, since the get/set calls don't return errors and since misconfiguration 114 is possible. You should normally issue these calls from a task context. However, 115 for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part 116 of early board setup. 117 118 For output GPIOs, the value provided becomes the initial output value. This 119 helps avoid signal glitching during system startup. 120 121 A driver can also query the current direction of a GPIO: 122 123 int gpiod_get_direction(const struct gpio_desc *desc) 124 125 This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT. 126 127 Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO 128 without setting its direction first is illegal and will result in undefined 129 behavior!** 130 131 132 Spinlock-Safe GPIO Access 133 ------------------------- 134 Most GPIO controllers can be accessed with memory read/write instructions. Those 135 don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ 136 handlers and similar contexts. 137 138 Use 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); 142 143 The values are boolean, zero for low, nonzero for high. When reading the value 144 of an output pin, the value returned should be what's seen on the pin. That 145 won't always match the specified output value, because of issues including 146 open-drain signaling and output latencies. 147 148 The get/set calls do not return errors because "invalid GPIO" should have been 149 reported earlier from gpiod_direction_*(). However, note that not all platforms 150 can read the value of output pins; those that can't should always return zero. 151 Also, using these calls for GPIOs that can't safely be accessed without sleeping 152 (see below) is an error. 153 154 155 GPIO Access That May Sleep 156 -------------------------- 157 Some GPIO controllers must be accessed using message based buses like I2C or 158 SPI. Commands to read or write those GPIO values require waiting to get to the 159 head of a queue to transmit a command and get its response. This requires 160 sleeping, which can't be done from inside IRQ handlers. 161 162 Platforms that support this type of GPIO distinguish them from other GPIOs by 163 returning nonzero from this call: 164 165 int gpiod_cansleep(const struct gpio_desc *desc) 166 167 To 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) 171 172 Accessing such GPIOs requires a context which may sleep, for example a threaded 173 IRQ handler, and those accessors must be used instead of spinlock-safe 174 accessors without the cansleep() name suffix. 175 176 Other than the fact that these accessors might sleep, and will work on GPIOs 177 that can't be accessed from hardIRQ handlers, these calls act the same as the 178 spinlock-safe calls. 179 180 181 Active-low State and Raw GPIO Values 182 ------------------------------------ 183 Device drivers like to manage the logical state of a GPIO, i.e. the value their 184 device will actually receive, no matter what lies between it and the GPIO line. 185 In some cases, it might make sense to control the actual GPIO line value. The 186 following set of calls ignore the active-low property of a GPIO and work on the 187 raw 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) 193 int gpiod_direction_output_raw(struct gpio_desc *desc, int value) 194 195 The 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) 198 199 Note that these functions should only be used with great moderation ; a driver 200 should not have to care about the physical line level. 201 202 203 Set multiple GPIO outputs with a single function call 204 ----------------------------------------------------- 205 The following functions set the output values of an array of GPIOs: 206 207 void gpiod_set_array(unsigned int array_size, 208 struct gpio_desc **desc_array, 209 int *value_array) 210 void gpiod_set_raw_array(unsigned int array_size, 211 struct gpio_desc **desc_array, 212 int *value_array) 213 void gpiod_set_array_cansleep(unsigned int array_size, 214 struct gpio_desc **desc_array, 215 int *value_array) 216 void gpiod_set_raw_array_cansleep(unsigned int array_size, 217 struct gpio_desc **desc_array, 218 int *value_array) 219 220 The array can be an arbitrary set of GPIOs. The functions will try to set 221 GPIOs belonging to the same bank or chip simultaneously if supported by the 222 corresponding chip driver. In that case a significantly improved performance 223 can be expected. If simultaneous setting is not possible the GPIOs will be set 224 sequentially. 225 Note that for optimal performance GPIOs belonging to the same chip should be 226 contiguous within the array of descriptors. 227 228 229 GPIOs mapped to IRQs 230 -------------------- 231 GPIO lines can quite often be used as IRQs. You can get the IRQ number 232 corresponding to a given GPIO using the following call: 233 234 int gpiod_to_irq(const struct gpio_desc *desc) 235 236 It will return an IRQ number, or an negative errno code if the mapping can't be 237 done (most likely because that particular GPIO cannot be used as IRQ). It is an 238 unchecked error to use a GPIO that wasn't set up as an input using 239 gpiod_direction_input(), or to use an IRQ number that didn't originally come 240 from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. 241 242 Non-error values returned from gpiod_to_irq() can be passed to request_irq() or 243 free_irq(). They will often be stored into IRQ resources for platform devices, 244 by the board-specific initialization code. Note that IRQ trigger options are 245 part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup 246 capabilities. 247 248 249 GPIOs and ACPI 250 ============== 251 252 On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by 253 the _CRS configuration objects of devices. Those resources do not provide 254 connection IDs (names) for GPIOs, so it is necessary to use an additional 255 mechanism for this purpose. 256 257 Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object 258 which, among other things, may be used to provide connection IDs for specific 259 GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the 260 case, it will be handled by the GPIO subsystem automatically. However, if the 261 _DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO 262 connection IDs need to be provided by device drivers. 263 264 For details refer to Documentation/acpi/gpio-properties.txt 265 266 267 Interacting With the Legacy GPIO Subsystem 268 ========================================== 269 Many kernel subsystems still handle GPIOs using the legacy integer-based 270 interface. Although it is strongly encouraged to upgrade them to the safer 271 descriptor-based API, the following two functions allow you to convert a GPIO 272 descriptor into the GPIO integer namespace and vice-versa: 273 274 int desc_to_gpio(const struct gpio_desc *desc) 275 struct gpio_desc *gpio_to_desc(unsigned gpio) 276 277 The GPIO number returned by desc_to_gpio() can be safely used as long as the 278 GPIO descriptor has not been freed. All the same, a GPIO number passed to 279 gpio_to_desc() must have been properly acquired, and usage of the returned GPIO 280 descriptor is only possible after the GPIO number has been released. 281 282 Freeing a GPIO obtained by one API with the other API is forbidden and an 283 unchecked error.