1================================== 2GPIO Descriptor Consumer Interface 3================================== 4 5This document describes the consumer interface of the GPIO framework. Note that 6it describes the new descriptor-based interface. For a description of the 7deprecated integer-based GPIO interface please refer to gpio-legacy.txt. 8 9 10Guidelines for GPIOs consumers 11============================== 12 13Drivers that can't work without standard GPIO calls should have Kconfig entries 14that depend on GPIOLIB or select GPIOLIB. The functions that allow a driver to 15obtain and use GPIOs are available by including the following file: 16 17 #include <linux/gpio/consumer.h> 18 19There are static inline stubs for all functions in the header file in the case 20where GPIOLIB is disabled. When these stubs are called they will emit 21warnings. These stubs are used for two use cases: 22 23- Simple compile coverage with e.g. COMPILE_TEST - it does not matter that 24 the current platform does not enable or select GPIOLIB because we are not 25 going to execute the system anyway. 26 27- Truly optional GPIOLIB support - where the driver does not really make use 28 of the GPIOs on certain compile-time configurations for certain systems, but 29 will use it under other compile-time configurations. In this case the 30 consumer must make sure not to call into these functions, or the user will 31 be met with console warnings that may be perceived as intimidating. 32 33All the functions that work with the descriptor-based GPIO interface are 34prefixed with ``gpiod_``. The ``gpio_`` prefix is used for the legacy 35interface. No other function in the kernel should use these prefixes. The use 36of the legacy functions is strongly discouraged, new code should use 37<linux/gpio/consumer.h> and descriptors exclusively. 38 39 40Obtaining and Disposing GPIOs 41============================= 42 43With the descriptor-based interface, GPIOs are identified with an opaque, 44non-forgeable handler that must be obtained through a call to one of the 45gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the 46device that will use the GPIO and the function the requested GPIO is supposed to 47fulfill:: 48 49 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id, 50 enum gpiod_flags flags) 51 52If a function is implemented by using several GPIOs together (e.g. a simple LED 53device that displays digits), an additional index argument can be specified:: 54 55 struct gpio_desc *gpiod_get_index(struct device *dev, 56 const char *con_id, unsigned int idx, 57 enum gpiod_flags flags) 58 59For a more detailed description of the con_id parameter in the DeviceTree case 60see Documentation/driver-api/gpio/board.rst 61 62The flags parameter is used to optionally specify a direction and initial value 63for the GPIO. Values can be: 64 65* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set 66 later with one of the dedicated functions. 67* GPIOD_IN to initialize the GPIO as input. 68* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0. 69* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1. 70* GPIOD_OUT_LOW_OPEN_DRAIN same as GPIOD_OUT_LOW but also enforce the line 71 to be electrically used with open drain. 72* GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line 73 to be electrically used with open drain. 74 75The two last flags are used for use cases where open drain is mandatory, such 76as I2C: if the line is not already configured as open drain in the mappings 77(see board.txt), then open drain will be enforced anyway and a warning will be 78printed that the board configuration needs to be updated to match the use case. 79 80Both functions return either a valid GPIO descriptor, or an error code checkable 81with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned 82if and only if no GPIO has been assigned to the device/function/index triplet, 83other error codes are used for cases where a GPIO has been assigned but an error 84occurred while trying to acquire it. This is useful to discriminate between mere 85errors and an absence of GPIO for optional GPIO parameters. For the common 86pattern where a GPIO is optional, the gpiod_get_optional() and 87gpiod_get_index_optional() functions can be used. These functions return NULL 88instead of -ENOENT if no GPIO has been assigned to the requested function:: 89 90 struct gpio_desc *gpiod_get_optional(struct device *dev, 91 const char *con_id, 92 enum gpiod_flags flags) 93 94 struct gpio_desc *gpiod_get_index_optional(struct device *dev, 95 const char *con_id, 96 unsigned int index, 97 enum gpiod_flags flags) 98 99Note that gpio_get*_optional() functions (and their managed variants), unlike 100the rest of gpiolib API, also return NULL when gpiolib support is disabled. 101This is helpful to driver authors, since they do not need to special case 102-ENOSYS return codes. System integrators should however be careful to enable 103gpiolib on systems that need it. 104 105For a function using multiple GPIOs all of those can be obtained with one call:: 106 107 struct gpio_descs *gpiod_get_array(struct device *dev, 108 const char *con_id, 109 enum gpiod_flags flags) 110 111This function returns a struct gpio_descs which contains an array of 112descriptors:: 113 114 struct gpio_descs { 115 unsigned int ndescs; 116 struct gpio_desc *desc[]; 117 } 118 119The following function returns NULL instead of -ENOENT if no GPIOs have been 120assigned to the requested function:: 121 122 struct gpio_descs *gpiod_get_array_optional(struct device *dev, 123 const char *con_id, 124 enum gpiod_flags flags) 125 126Device-managed variants of these functions are also defined:: 127 128 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id, 129 enum gpiod_flags flags) 130 131 struct gpio_desc *devm_gpiod_get_index(struct device *dev, 132 const char *con_id, 133 unsigned int idx, 134 enum gpiod_flags flags) 135 136 struct gpio_desc *devm_gpiod_get_optional(struct device *dev, 137 const char *con_id, 138 enum gpiod_flags flags) 139 140 struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev, 141 const char *con_id, 142 unsigned int index, 143 enum gpiod_flags flags) 144 145 struct gpio_descs *devm_gpiod_get_array(struct device *dev, 146 const char *con_id, 147 enum gpiod_flags flags) 148 149 struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev, 150 const char *con_id, 151 enum gpiod_flags flags) 152 153A GPIO descriptor can be disposed of using the gpiod_put() function:: 154 155 void gpiod_put(struct gpio_desc *desc) 156 157For an array of GPIOs this function can be used:: 158 159 void gpiod_put_array(struct gpio_descs *descs) 160 161It is strictly forbidden to use a descriptor after calling these functions. 162It is also not allowed to individually release descriptors (using gpiod_put()) 163from an array acquired with gpiod_get_array(). 164 165The device-managed variants are, unsurprisingly:: 166 167 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) 168 169 void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs) 170 171 172Using GPIOs 173=========== 174 175Setting Direction 176----------------- 177The first thing a driver must do with a GPIO is setting its direction. If no 178direction-setting flags have been given to gpiod_get*(), this is done by 179invoking one of the gpiod_direction_*() functions:: 180 181 int gpiod_direction_input(struct gpio_desc *desc) 182 int gpiod_direction_output(struct gpio_desc *desc, int value) 183 184The return value is zero for success, else a negative errno. It should be 185checked, since the get/set calls don't return errors and since misconfiguration 186is possible. You should normally issue these calls from a task context. However, 187for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part 188of early board setup. 189 190For output GPIOs, the value provided becomes the initial output value. This 191helps avoid signal glitching during system startup. 192 193A driver can also query the current direction of a GPIO:: 194 195 int gpiod_get_direction(const struct gpio_desc *desc) 196 197This function returns 0 for output, 1 for input, or an error code in case of error. 198 199Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO 200without setting its direction first is illegal and will result in undefined 201behavior!** 202 203 204Spinlock-Safe GPIO Access 205------------------------- 206Most GPIO controllers can be accessed with memory read/write instructions. Those 207don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ 208handlers and similar contexts. 209 210Use the following calls to access GPIOs from an atomic context:: 211 212 int gpiod_get_value(const struct gpio_desc *desc); 213 void gpiod_set_value(struct gpio_desc *desc, int value); 214 215The values are boolean, zero for low, nonzero for high. When reading the value 216of an output pin, the value returned should be what's seen on the pin. That 217won't always match the specified output value, because of issues including 218open-drain signaling and output latencies. 219 220The get/set calls do not return errors because "invalid GPIO" should have been 221reported earlier from gpiod_direction_*(). However, note that not all platforms 222can read the value of output pins; those that can't should always return zero. 223Also, using these calls for GPIOs that can't safely be accessed without sleeping 224(see below) is an error. 225 226 227GPIO Access That May Sleep 228-------------------------- 229Some GPIO controllers must be accessed using message based buses like I2C or 230SPI. Commands to read or write those GPIO values require waiting to get to the 231head of a queue to transmit a command and get its response. This requires 232sleeping, which can't be done from inside IRQ handlers. 233 234Platforms that support this type of GPIO distinguish them from other GPIOs by 235returning nonzero from this call:: 236 237 int gpiod_cansleep(const struct gpio_desc *desc) 238 239To access such GPIOs, a different set of accessors is defined:: 240 241 int gpiod_get_value_cansleep(const struct gpio_desc *desc) 242 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value) 243 244Accessing such GPIOs requires a context which may sleep, for example a threaded 245IRQ handler, and those accessors must be used instead of spinlock-safe 246accessors without the cansleep() name suffix. 247 248Other than the fact that these accessors might sleep, and will work on GPIOs 249that can't be accessed from hardIRQ handlers, these calls act the same as the 250spinlock-safe calls. 251 252 253The active low and open drain semantics 254--------------------------------------- 255As a consumer should not have to care about the physical line level, all of the 256gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with 257the *logical* value. With this they take the active low property into account. 258This means that they check whether the GPIO is configured to be active low, 259and if so, they manipulate the passed value before the physical line level is 260driven. 261 262The same is applicable for open drain or open source output lines: those do not 263actively drive their output high (open drain) or low (open source), they just 264switch their output to a high impedance value. The consumer should not need to 265care. (For details read about open drain in driver.txt.) 266 267With this, all the gpiod_set_(array)_value_xxx() functions interpret the 268parameter "value" as "asserted" ("1") or "de-asserted" ("0"). The physical line 269level will be driven accordingly. 270 271As an example, if the active low property for a dedicated GPIO is set, and the 272gpiod_set_(array)_value_xxx() passes "asserted" ("1"), the physical line level 273will be driven low. 274 275To summarize:: 276 277 Function (example) line property physical line 278 gpiod_set_raw_value(desc, 0); don't care low 279 gpiod_set_raw_value(desc, 1); don't care high 280 gpiod_set_value(desc, 0); default (active high) low 281 gpiod_set_value(desc, 1); default (active high) high 282 gpiod_set_value(desc, 0); active low high 283 gpiod_set_value(desc, 1); active low low 284 gpiod_set_value(desc, 0); default (active high) low 285 gpiod_set_value(desc, 1); default (active high) high 286 gpiod_set_value(desc, 0); open drain low 287 gpiod_set_value(desc, 1); open drain high impedance 288 gpiod_set_value(desc, 0); open source high impedance 289 gpiod_set_value(desc, 1); open source high 290 291It is possible to override these semantics using the set_raw/get_raw functions 292but it should be avoided as much as possible, especially by system-agnostic drivers 293which should not need to care about the actual physical line level and worry about 294the logical value instead. 295 296 297Accessing raw GPIO values 298------------------------- 299Consumers exist that need to manage the logical state of a GPIO line, i.e. the value 300their device will actually receive, no matter what lies between it and the GPIO 301line. 302 303The following set of calls ignore the active-low or open drain property of a GPIO and 304work on the raw line value:: 305 306 int gpiod_get_raw_value(const struct gpio_desc *desc) 307 void gpiod_set_raw_value(struct gpio_desc *desc, int value) 308 int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc) 309 void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value) 310 int gpiod_direction_output_raw(struct gpio_desc *desc, int value) 311 312The active low state of a GPIO can also be queried using the following call:: 313 314 int gpiod_is_active_low(const struct gpio_desc *desc) 315 316Note that these functions should only be used with great moderation; a driver 317should not have to care about the physical line level or open drain semantics. 318 319 320Access multiple GPIOs with a single function call 321------------------------------------------------- 322The following functions get or set the values of an array of GPIOs:: 323 324 int gpiod_get_array_value(unsigned int array_size, 325 struct gpio_desc **desc_array, 326 int *value_array); 327 int gpiod_get_raw_array_value(unsigned int array_size, 328 struct gpio_desc **desc_array, 329 int *value_array); 330 int gpiod_get_array_value_cansleep(unsigned int array_size, 331 struct gpio_desc **desc_array, 332 int *value_array); 333 int gpiod_get_raw_array_value_cansleep(unsigned int array_size, 334 struct gpio_desc **desc_array, 335 int *value_array); 336 337 void gpiod_set_array_value(unsigned int array_size, 338 struct gpio_desc **desc_array, 339 int *value_array) 340 void gpiod_set_raw_array_value(unsigned int array_size, 341 struct gpio_desc **desc_array, 342 int *value_array) 343 void gpiod_set_array_value_cansleep(unsigned int array_size, 344 struct gpio_desc **desc_array, 345 int *value_array) 346 void gpiod_set_raw_array_value_cansleep(unsigned int array_size, 347 struct gpio_desc **desc_array, 348 int *value_array) 349 350The array can be an arbitrary set of GPIOs. The functions will try to access 351GPIOs belonging to the same bank or chip simultaneously if supported by the 352corresponding chip driver. In that case a significantly improved performance 353can be expected. If simultaneous access is not possible the GPIOs will be 354accessed sequentially. 355 356The functions take three arguments: 357 * array_size - the number of array elements 358 * desc_array - an array of GPIO descriptors 359 * value_array - an array to store the GPIOs' values (get) or 360 an array of values to assign to the GPIOs (set) 361 362The descriptor array can be obtained using the gpiod_get_array() function 363or one of its variants. If the group of descriptors returned by that function 364matches the desired group of GPIOs, those GPIOs can be accessed by simply using 365the struct gpio_descs returned by gpiod_get_array():: 366 367 struct gpio_descs *my_gpio_descs = gpiod_get_array(...); 368 gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc, 369 my_gpio_values); 370 371It is also possible to access a completely arbitrary array of descriptors. The 372descriptors may be obtained using any combination of gpiod_get() and 373gpiod_get_array(). Afterwards the array of descriptors has to be setup 374manually before it can be passed to one of the above functions. 375 376Note that for optimal performance GPIOs belonging to the same chip should be 377contiguous within the array of descriptors. 378 379The return value of gpiod_get_array_value() and its variants is 0 on success 380or negative on error. Note the difference to gpiod_get_value(), which returns 3810 or 1 on success to convey the GPIO value. With the array functions, the GPIO 382values are stored in value_array rather than passed back as return value. 383 384 385GPIOs mapped to IRQs 386-------------------- 387GPIO lines can quite often be used as IRQs. You can get the IRQ number 388corresponding to a given GPIO using the following call:: 389 390 int gpiod_to_irq(const struct gpio_desc *desc) 391 392It will return an IRQ number, or a negative errno code if the mapping can't be 393done (most likely because that particular GPIO cannot be used as IRQ). It is an 394unchecked error to use a GPIO that wasn't set up as an input using 395gpiod_direction_input(), or to use an IRQ number that didn't originally come 396from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. 397 398Non-error values returned from gpiod_to_irq() can be passed to request_irq() or 399free_irq(). They will often be stored into IRQ resources for platform devices, 400by the board-specific initialization code. Note that IRQ trigger options are 401part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup 402capabilities. 403 404 405GPIOs and ACPI 406============== 407 408On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by 409the _CRS configuration objects of devices. Those resources do not provide 410connection IDs (names) for GPIOs, so it is necessary to use an additional 411mechanism for this purpose. 412 413Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object 414which, among other things, may be used to provide connection IDs for specific 415GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the 416case, it will be handled by the GPIO subsystem automatically. However, if the 417_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO 418connection IDs need to be provided by device drivers. 419 420For details refer to Documentation/acpi/gpio-properties.txt 421 422 423Interacting With the Legacy GPIO Subsystem 424========================================== 425Many kernel subsystems still handle GPIOs using the legacy integer-based 426interface. Although it is strongly encouraged to upgrade them to the safer 427descriptor-based API, the following two functions allow you to convert a GPIO 428descriptor into the GPIO integer namespace and vice-versa:: 429 430 int desc_to_gpio(const struct gpio_desc *desc) 431 struct gpio_desc *gpio_to_desc(unsigned gpio) 432 433The GPIO number returned by desc_to_gpio() can be safely used as long as the 434GPIO descriptor has not been freed. All the same, a GPIO number passed to 435gpio_to_desc() must have been properly acquired, and usage of the returned GPIO 436descriptor is only possible after the GPIO number has been released. 437 438Freeing a GPIO obtained by one API with the other API is forbidden and an 439unchecked error. 440