Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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 or select GPIOLIB. The functions that allow a driver to 14 obtain and use GPIOs are available by including the following file: 15 16 #include <linux/gpio/consumer.h> 17 18 There are static inline stubs for all functions in the header file in the case 19 where GPIOLIB is disabled. When these stubs are called they will emit 20 warnings. These stubs are used for two use cases: 21 22 - Simple compile coverage with e.g. COMPILE_TEST - it does not matter that 23 the current platform does not enable or select GPIOLIB because we are not 24 going to execute the system anyway. 25 26 - Truly optional GPIOLIB support - where the driver does not really make use 27 of the GPIOs on certain compile-time configurations for certain systems, but 28 will use it under other compile-time configurations. In this case the 29 consumer must make sure not to call into these functions, or the user will 30 be met with console warnings that may be perceived as intimidating. 31 32 All the functions that work with the descriptor-based GPIO interface are 33 prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No 34 other function in the kernel should use these prefixes. The use of the legacy 35 functions is strongly discouraged, new code should use <linux/gpio/consumer.h> 36 and descriptors exclusively. 37 38 39 Obtaining and Disposing GPIOs 40 ============================= 41 42 With the descriptor-based interface, GPIOs are identified with an opaque, 43 non-forgeable handler that must be obtained through a call to one of the 44 gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the 45 device that will use the GPIO and the function the requested GPIO is supposed to 46 fulfill: 47 48 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id, 49 enum gpiod_flags flags) 50 51 If a function is implemented by using several GPIOs together (e.g. a simple LED 52 device that displays digits), an additional index argument can be specified: 53 54 struct gpio_desc *gpiod_get_index(struct device *dev, 55 const char *con_id, unsigned int idx, 56 enum gpiod_flags flags) 57 58 For a more detailed description of the con_id parameter in the DeviceTree case 59 see Documentation/gpio/board.txt 60 61 The flags parameter is used to optionally specify a direction and initial value 62 for the GPIO. Values can be: 63 64 * GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set 65 later with one of the dedicated functions. 66 * GPIOD_IN to initialize the GPIO as input. 67 * GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0. 68 * GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1. 69 * GPIOD_OUT_LOW_OPEN_DRAIN same as GPIOD_OUT_LOW but also enforce the line 70 to be electrically used with open drain. 71 * GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line 72 to be electrically used with open drain. 73 74 The two last flags are used for use cases where open drain is mandatory, such 75 as I2C: if the line is not already configured as open drain in the mappings 76 (see board.txt), then open drain will be enforced anyway and a warning will be 77 printed that the board configuration needs to be updated to match the use case. 78 79 Both functions return either a valid GPIO descriptor, or an error code checkable 80 with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned 81 if and only if no GPIO has been assigned to the device/function/index triplet, 82 other error codes are used for cases where a GPIO has been assigned but an error 83 occurred while trying to acquire it. This is useful to discriminate between mere 84 errors and an absence of GPIO for optional GPIO parameters. For the common 85 pattern where a GPIO is optional, the gpiod_get_optional() and 86 gpiod_get_index_optional() functions can be used. These functions return NULL 87 instead of -ENOENT if no GPIO has been assigned to the requested function: 88 89 struct gpio_desc *gpiod_get_optional(struct device *dev, 90 const char *con_id, 91 enum gpiod_flags flags) 92 93 struct gpio_desc *gpiod_get_index_optional(struct device *dev, 94 const char *con_id, 95 unsigned int index, 96 enum gpiod_flags flags) 97 98 Note that gpio_get*_optional() functions (and their managed variants), unlike 99 the rest of gpiolib API, also return NULL when gpiolib support is disabled. 100 This is helpful to driver authors, since they do not need to special case 101 -ENOSYS return codes. System integrators should however be careful to enable 102 gpiolib on systems that need it. 103 104 For a function using multiple GPIOs all of those can be obtained with one call: 105 106 struct gpio_descs *gpiod_get_array(struct device *dev, 107 const char *con_id, 108 enum gpiod_flags flags) 109 110 This function returns a struct gpio_descs which contains an array of 111 descriptors: 112 113 struct gpio_descs { 114 unsigned int ndescs; 115 struct gpio_desc *desc[]; 116 } 117 118 The following function returns NULL instead of -ENOENT if no GPIOs have been 119 assigned to the requested function: 120 121 struct gpio_descs *gpiod_get_array_optional(struct device *dev, 122 const char *con_id, 123 enum gpiod_flags flags) 124 125 Device-managed variants of these functions are also defined: 126 127 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id, 128 enum gpiod_flags flags) 129 130 struct gpio_desc *devm_gpiod_get_index(struct device *dev, 131 const char *con_id, 132 unsigned int idx, 133 enum gpiod_flags flags) 134 135 struct gpio_desc *devm_gpiod_get_optional(struct device *dev, 136 const char *con_id, 137 enum gpiod_flags flags) 138 139 struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev, 140 const char *con_id, 141 unsigned int index, 142 enum gpiod_flags flags) 143 144 struct gpio_descs *devm_gpiod_get_array(struct device *dev, 145 const char *con_id, 146 enum gpiod_flags flags) 147 148 struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev, 149 const char *con_id, 150 enum gpiod_flags flags) 151 152 A GPIO descriptor can be disposed of using the gpiod_put() function: 153 154 void gpiod_put(struct gpio_desc *desc) 155 156 For an array of GPIOs this function can be used: 157 158 void gpiod_put_array(struct gpio_descs *descs) 159 160 It is strictly forbidden to use a descriptor after calling these functions. 161 It is also not allowed to individually release descriptors (using gpiod_put()) 162 from an array acquired with gpiod_get_array(). 163 164 The device-managed variants are, unsurprisingly: 165 166 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) 167 168 void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs) 169 170 171 Using GPIOs 172 =========== 173 174 Setting Direction 175 ----------------- 176 The first thing a driver must do with a GPIO is setting its direction. If no 177 direction-setting flags have been given to gpiod_get*(), this is done by 178 invoking one of the gpiod_direction_*() functions: 179 180 int gpiod_direction_input(struct gpio_desc *desc) 181 int gpiod_direction_output(struct gpio_desc *desc, int value) 182 183 The return value is zero for success, else a negative errno. It should be 184 checked, since the get/set calls don't return errors and since misconfiguration 185 is possible. You should normally issue these calls from a task context. However, 186 for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part 187 of early board setup. 188 189 For output GPIOs, the value provided becomes the initial output value. This 190 helps avoid signal glitching during system startup. 191 192 A driver can also query the current direction of a GPIO: 193 194 int gpiod_get_direction(const struct gpio_desc *desc) 195 196 This function returns 0 for output, 1 for input, or an error code in case of error. 197 198 Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO 199 without setting its direction first is illegal and will result in undefined 200 behavior!** 201 202 203 Spinlock-Safe GPIO Access 204 ------------------------- 205 Most GPIO controllers can be accessed with memory read/write instructions. Those 206 don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ 207 handlers and similar contexts. 208 209 Use the following calls to access GPIOs from an atomic context: 210 211 int gpiod_get_value(const struct gpio_desc *desc); 212 void gpiod_set_value(struct gpio_desc *desc, int value); 213 214 The values are boolean, zero for low, nonzero for high. When reading the value 215 of an output pin, the value returned should be what's seen on the pin. That 216 won't always match the specified output value, because of issues including 217 open-drain signaling and output latencies. 218 219 The get/set calls do not return errors because "invalid GPIO" should have been 220 reported earlier from gpiod_direction_*(). However, note that not all platforms 221 can read the value of output pins; those that can't should always return zero. 222 Also, using these calls for GPIOs that can't safely be accessed without sleeping 223 (see below) is an error. 224 225 226 GPIO Access That May Sleep 227 -------------------------- 228 Some GPIO controllers must be accessed using message based buses like I2C or 229 SPI. Commands to read or write those GPIO values require waiting to get to the 230 head of a queue to transmit a command and get its response. This requires 231 sleeping, which can't be done from inside IRQ handlers. 232 233 Platforms that support this type of GPIO distinguish them from other GPIOs by 234 returning nonzero from this call: 235 236 int gpiod_cansleep(const struct gpio_desc *desc) 237 238 To access such GPIOs, a different set of accessors is defined: 239 240 int gpiod_get_value_cansleep(const struct gpio_desc *desc) 241 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value) 242 243 Accessing such GPIOs requires a context which may sleep, for example a threaded 244 IRQ handler, and those accessors must be used instead of spinlock-safe 245 accessors without the cansleep() name suffix. 246 247 Other than the fact that these accessors might sleep, and will work on GPIOs 248 that can't be accessed from hardIRQ handlers, these calls act the same as the 249 spinlock-safe calls. 250 251 252 The active low and open drain semantics 253 --------------------------------------- 254 As a consumer should not have to care about the physical line level, all of the 255 gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with 256 the *logical* value. With this they take the active low property into account. 257 This means that they check whether the GPIO is configured to be active low, 258 and if so, they manipulate the passed value before the physical line level is 259 driven. 260 261 The same is applicable for open drain or open source output lines: those do not 262 actively drive their output high (open drain) or low (open source), they just 263 switch their output to a high impedance value. The consumer should not need to 264 care. (For details read about open drain in driver.txt.) 265 266 With this, all the gpiod_set_(array)_value_xxx() functions interpret the 267 parameter "value" as "asserted" ("1") or "de-asserted" ("0"). The physical line 268 level will be driven accordingly. 269 270 As an example, if the active low property for a dedicated GPIO is set, and the 271 gpiod_set_(array)_value_xxx() passes "asserted" ("1"), the physical line level 272 will be driven low. 273 274 To summarize: 275 276 Function (example) line property physical line 277 gpiod_set_raw_value(desc, 0); don't care low 278 gpiod_set_raw_value(desc, 1); don't care high 279 gpiod_set_value(desc, 0); default (active high) low 280 gpiod_set_value(desc, 1); default (active high) high 281 gpiod_set_value(desc, 0); active low high 282 gpiod_set_value(desc, 1); active low low 283 gpiod_set_value(desc, 0); default (active high) low 284 gpiod_set_value(desc, 1); default (active high) high 285 gpiod_set_value(desc, 0); open drain low 286 gpiod_set_value(desc, 1); open drain high impedance 287 gpiod_set_value(desc, 0); open source high impedance 288 gpiod_set_value(desc, 1); open source high 289 290 It is possible to override these semantics using the *set_raw/'get_raw functions 291 but it should be avoided as much as possible, especially by system-agnostic drivers 292 which should not need to care about the actual physical line level and worry about 293 the logical value instead. 294 295 296 Accessing raw GPIO values 297 ------------------------- 298 Consumers exist that need to manage the logical state of a GPIO line, i.e. the value 299 their device will actually receive, no matter what lies between it and the GPIO 300 line. 301 302 The following set of calls ignore the active-low or open drain property of a GPIO and 303 work on the raw line value: 304 305 int gpiod_get_raw_value(const struct gpio_desc *desc) 306 void gpiod_set_raw_value(struct gpio_desc *desc, int value) 307 int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc) 308 void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value) 309 int gpiod_direction_output_raw(struct gpio_desc *desc, int value) 310 311 The active low state of a GPIO can also be queried using the following call: 312 313 int gpiod_is_active_low(const struct gpio_desc *desc) 314 315 Note that these functions should only be used with great moderation; a driver 316 should not have to care about the physical line level or open drain semantics. 317 318 319 Access multiple GPIOs with a single function call 320 ------------------------------------------------- 321 The following functions get or set the values of an array of GPIOs: 322 323 int gpiod_get_array_value(unsigned int array_size, 324 struct gpio_desc **desc_array, 325 int *value_array); 326 int gpiod_get_raw_array_value(unsigned int array_size, 327 struct gpio_desc **desc_array, 328 int *value_array); 329 int gpiod_get_array_value_cansleep(unsigned int array_size, 330 struct gpio_desc **desc_array, 331 int *value_array); 332 int gpiod_get_raw_array_value_cansleep(unsigned int array_size, 333 struct gpio_desc **desc_array, 334 int *value_array); 335 336 void gpiod_set_array_value(unsigned int array_size, 337 struct gpio_desc **desc_array, 338 int *value_array) 339 void gpiod_set_raw_array_value(unsigned int array_size, 340 struct gpio_desc **desc_array, 341 int *value_array) 342 void gpiod_set_array_value_cansleep(unsigned int array_size, 343 struct gpio_desc **desc_array, 344 int *value_array) 345 void gpiod_set_raw_array_value_cansleep(unsigned int array_size, 346 struct gpio_desc **desc_array, 347 int *value_array) 348 349 The array can be an arbitrary set of GPIOs. The functions will try to access 350 GPIOs belonging to the same bank or chip simultaneously if supported by the 351 corresponding chip driver. In that case a significantly improved performance 352 can be expected. If simultaneous access is not possible the GPIOs will be 353 accessed sequentially. 354 355 The functions take three arguments: 356 * array_size - the number of array elements 357 * desc_array - an array of GPIO descriptors 358 * value_array - an array to store the GPIOs' values (get) or 359 an array of values to assign to the GPIOs (set) 360 361 The descriptor array can be obtained using the gpiod_get_array() function 362 or one of its variants. If the group of descriptors returned by that function 363 matches the desired group of GPIOs, those GPIOs can be accessed by simply using 364 the struct gpio_descs returned by gpiod_get_array(): 365 366 struct gpio_descs *my_gpio_descs = gpiod_get_array(...); 367 gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc, 368 my_gpio_values); 369 370 It is also possible to access a completely arbitrary array of descriptors. The 371 descriptors may be obtained using any combination of gpiod_get() and 372 gpiod_get_array(). Afterwards the array of descriptors has to be setup 373 manually before it can be passed to one of the above functions. 374 375 Note that for optimal performance GPIOs belonging to the same chip should be 376 contiguous within the array of descriptors. 377 378 The return value of gpiod_get_array_value() and its variants is 0 on success 379 or negative on error. Note the difference to gpiod_get_value(), which returns 380 0 or 1 on success to convey the GPIO value. With the array functions, the GPIO 381 values are stored in value_array rather than passed back as return value. 382 383 384 GPIOs mapped to IRQs 385 -------------------- 386 GPIO lines can quite often be used as IRQs. You can get the IRQ number 387 corresponding to a given GPIO using the following call: 388 389 int gpiod_to_irq(const struct gpio_desc *desc) 390 391 It will return an IRQ number, or a negative errno code if the mapping can't be 392 done (most likely because that particular GPIO cannot be used as IRQ). It is an 393 unchecked error to use a GPIO that wasn't set up as an input using 394 gpiod_direction_input(), or to use an IRQ number that didn't originally come 395 from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. 396 397 Non-error values returned from gpiod_to_irq() can be passed to request_irq() or 398 free_irq(). They will often be stored into IRQ resources for platform devices, 399 by the board-specific initialization code. Note that IRQ trigger options are 400 part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup 401 capabilities. 402 403 404 GPIOs and ACPI 405 ============== 406 407 On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by 408 the _CRS configuration objects of devices. Those resources do not provide 409 connection IDs (names) for GPIOs, so it is necessary to use an additional 410 mechanism for this purpose. 411 412 Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object 413 which, among other things, may be used to provide connection IDs for specific 414 GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the 415 case, it will be handled by the GPIO subsystem automatically. However, if the 416 _DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO 417 connection IDs need to be provided by device drivers. 418 419 For details refer to Documentation/acpi/gpio-properties.txt 420 421 422 Interacting With the Legacy GPIO Subsystem 423 ========================================== 424 Many kernel subsystems still handle GPIOs using the legacy integer-based 425 interface. Although it is strongly encouraged to upgrade them to the safer 426 descriptor-based API, the following two functions allow you to convert a GPIO 427 descriptor into the GPIO integer namespace and vice-versa: 428 429 int desc_to_gpio(const struct gpio_desc *desc) 430 struct gpio_desc *gpio_to_desc(unsigned gpio) 431 432 The GPIO number returned by desc_to_gpio() can be safely used as long as the 433 GPIO descriptor has not been freed. All the same, a GPIO number passed to 434 gpio_to_desc() must have been properly acquired, and usage of the returned GPIO 435 descriptor is only possible after the GPIO number has been released. 436 437 Freeing a GPIO obtained by one API with the other API is forbidden and an 438 unchecked error.