1GPIO 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 enum gpiod_flags flags) 34 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, 39 const char *con_id, unsigned int idx, 40 enum gpiod_flags flags) 41 42For a more detailed description of the con_id parameter in the DeviceTree case 43see Documentation/gpio/board.txt 44 45The flags parameter is used to optionally specify a direction and initial value 46for the GPIO. Values can be: 47 48* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set 49 later with one of the dedicated functions. 50* GPIOD_IN to initialize the GPIO as input. 51* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0. 52* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1. 53 54Both functions return either a valid GPIO descriptor, or an error code checkable 55with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned 56if and only if no GPIO has been assigned to the device/function/index triplet, 57other error codes are used for cases where a GPIO has been assigned but an error 58occurred while trying to acquire it. This is useful to discriminate between mere 59errors and an absence of GPIO for optional GPIO parameters. For the common 60pattern where a GPIO is optional, the gpiod_get_optional() and 61gpiod_get_index_optional() functions can be used. These functions return NULL 62instead of -ENOENT if no GPIO has been assigned to the requested function: 63 64 struct gpio_desc *gpiod_get_optional(struct device *dev, 65 const char *con_id, 66 enum gpiod_flags flags) 67 68 struct gpio_desc *gpiod_get_index_optional(struct device *dev, 69 const char *con_id, 70 unsigned int index, 71 enum gpiod_flags flags) 72 73For a function using multiple GPIOs all of those can be obtained with one call: 74 75 struct gpio_descs *gpiod_get_array(struct device *dev, 76 const char *con_id, 77 enum gpiod_flags flags) 78 79This function returns a struct gpio_descs which contains an array of 80descriptors: 81 82 struct gpio_descs { 83 unsigned int ndescs; 84 struct gpio_desc *desc[]; 85 } 86 87The following function returns NULL instead of -ENOENT if no GPIOs have been 88assigned to the requested function: 89 90 struct gpio_descs *gpiod_get_array_optional(struct device *dev, 91 const char *con_id, 92 enum gpiod_flags flags) 93 94Device-managed variants of these functions are also defined: 95 96 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id, 97 enum gpiod_flags flags) 98 99 struct gpio_desc *devm_gpiod_get_index(struct device *dev, 100 const char *con_id, 101 unsigned int idx, 102 enum gpiod_flags flags) 103 104 struct gpio_desc *devm_gpiod_get_optional(struct device *dev, 105 const char *con_id, 106 enum gpiod_flags flags) 107 108 struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev, 109 const char *con_id, 110 unsigned int index, 111 enum gpiod_flags flags) 112 113 struct gpio_descs *devm_gpiod_get_array(struct device *dev, 114 const char *con_id, 115 enum gpiod_flags flags) 116 117 struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev, 118 const char *con_id, 119 enum gpiod_flags flags) 120 121A GPIO descriptor can be disposed of using the gpiod_put() function: 122 123 void gpiod_put(struct gpio_desc *desc) 124 125For an array of GPIOs this function can be used: 126 127 void gpiod_put_array(struct gpio_descs *descs) 128 129It is strictly forbidden to use a descriptor after calling these functions. 130It is also not allowed to individually release descriptors (using gpiod_put()) 131from an array acquired with gpiod_get_array(). 132 133The device-managed variants are, unsurprisingly: 134 135 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) 136 137 void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs) 138 139 140Using GPIOs 141=========== 142 143Setting Direction 144----------------- 145The first thing a driver must do with a GPIO is setting its direction. If no 146direction-setting flags have been given to gpiod_get*(), this is done by 147invoking one of the gpiod_direction_*() functions: 148 149 int gpiod_direction_input(struct gpio_desc *desc) 150 int gpiod_direction_output(struct gpio_desc *desc, int value) 151 152The return value is zero for success, else a negative errno. It should be 153checked, since the get/set calls don't return errors and since misconfiguration 154is possible. You should normally issue these calls from a task context. However, 155for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part 156of early board setup. 157 158For output GPIOs, the value provided becomes the initial output value. This 159helps avoid signal glitching during system startup. 160 161A driver can also query the current direction of a GPIO: 162 163 int gpiod_get_direction(const struct gpio_desc *desc) 164 165This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT. 166 167Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO 168without setting its direction first is illegal and will result in undefined 169behavior!** 170 171 172Spinlock-Safe GPIO Access 173------------------------- 174Most GPIO controllers can be accessed with memory read/write instructions. Those 175don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ 176handlers and similar contexts. 177 178Use the following calls to access GPIOs from an atomic context: 179 180 int gpiod_get_value(const struct gpio_desc *desc); 181 void gpiod_set_value(struct gpio_desc *desc, int value); 182 183The values are boolean, zero for low, nonzero for high. When reading the value 184of an output pin, the value returned should be what's seen on the pin. That 185won't always match the specified output value, because of issues including 186open-drain signaling and output latencies. 187 188The get/set calls do not return errors because "invalid GPIO" should have been 189reported earlier from gpiod_direction_*(). However, note that not all platforms 190can read the value of output pins; those that can't should always return zero. 191Also, using these calls for GPIOs that can't safely be accessed without sleeping 192(see below) is an error. 193 194 195GPIO Access That May Sleep 196-------------------------- 197Some GPIO controllers must be accessed using message based buses like I2C or 198SPI. Commands to read or write those GPIO values require waiting to get to the 199head of a queue to transmit a command and get its response. This requires 200sleeping, which can't be done from inside IRQ handlers. 201 202Platforms that support this type of GPIO distinguish them from other GPIOs by 203returning nonzero from this call: 204 205 int gpiod_cansleep(const struct gpio_desc *desc) 206 207To access such GPIOs, a different set of accessors is defined: 208 209 int gpiod_get_value_cansleep(const struct gpio_desc *desc) 210 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value) 211 212Accessing such GPIOs requires a context which may sleep, for example a threaded 213IRQ handler, and those accessors must be used instead of spinlock-safe 214accessors without the cansleep() name suffix. 215 216Other than the fact that these accessors might sleep, and will work on GPIOs 217that can't be accessed from hardIRQ handlers, these calls act the same as the 218spinlock-safe calls. 219 220 221Active-low State and Raw GPIO Values 222------------------------------------ 223Device drivers like to manage the logical state of a GPIO, i.e. the value their 224device will actually receive, no matter what lies between it and the GPIO line. 225In some cases, it might make sense to control the actual GPIO line value. The 226following set of calls ignore the active-low property of a GPIO and work on the 227raw line value: 228 229 int gpiod_get_raw_value(const struct gpio_desc *desc) 230 void gpiod_set_raw_value(struct gpio_desc *desc, int value) 231 int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc) 232 void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value) 233 int gpiod_direction_output_raw(struct gpio_desc *desc, int value) 234 235The active-low state of a GPIO can also be queried using the following call: 236 237 int gpiod_is_active_low(const struct gpio_desc *desc) 238 239Note that these functions should only be used with great moderation ; a driver 240should not have to care about the physical line level. 241 242 243The active-low property 244----------------------- 245 246As a driver should not have to care about the physical line level, all of the 247gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with 248the *logical* value. With this they take the active-low property into account. 249This means that they check whether the GPIO is configured to be active-low, 250and if so, they manipulate the passed value before the physical line level is 251driven. 252 253With this, all the gpiod_set_(array)_value_xxx() functions interpret the 254parameter "value" as "active" ("1") or "inactive" ("0"). The physical line 255level will be driven accordingly. 256 257As an example, if the active-low property for a dedicated GPIO is set, and the 258gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level 259will be driven low. 260 261To summarize: 262 263Function (example) active-low proporty physical line 264gpiod_set_raw_value(desc, 0); don't care low 265gpiod_set_raw_value(desc, 1); don't care high 266gpiod_set_value(desc, 0); default (active-high) low 267gpiod_set_value(desc, 1); default (active-high) high 268gpiod_set_value(desc, 0); active-low high 269gpiod_set_value(desc, 1); active-low low 270 271Please note again that the set_raw/get_raw functions should be avoided as much 272as possible, especially by drivers which should not care about the actual 273physical line level and worry about the logical value instead. 274 275 276Set multiple GPIO outputs with a single function call 277----------------------------------------------------- 278The following functions set the output values of an array of GPIOs: 279 280 void gpiod_set_array_value(unsigned int array_size, 281 struct gpio_desc **desc_array, 282 int *value_array) 283 void gpiod_set_raw_array_value(unsigned int array_size, 284 struct gpio_desc **desc_array, 285 int *value_array) 286 void gpiod_set_array_value_cansleep(unsigned int array_size, 287 struct gpio_desc **desc_array, 288 int *value_array) 289 void gpiod_set_raw_array_value_cansleep(unsigned int array_size, 290 struct gpio_desc **desc_array, 291 int *value_array) 292 293The array can be an arbitrary set of GPIOs. The functions will try to set 294GPIOs belonging to the same bank or chip simultaneously if supported by the 295corresponding chip driver. In that case a significantly improved performance 296can be expected. If simultaneous setting is not possible the GPIOs will be set 297sequentially. 298 299The gpiod_set_array() functions take three arguments: 300 * array_size - the number of array elements 301 * desc_array - an array of GPIO descriptors 302 * value_array - an array of values to assign to the GPIOs 303 304The descriptor array can be obtained using the gpiod_get_array() function 305or one of its variants. If the group of descriptors returned by that function 306matches the desired group of GPIOs, those GPIOs can be set by simply using 307the struct gpio_descs returned by gpiod_get_array(): 308 309 struct gpio_descs *my_gpio_descs = gpiod_get_array(...); 310 gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc, 311 my_gpio_values); 312 313It is also possible to set a completely arbitrary array of descriptors. The 314descriptors may be obtained using any combination of gpiod_get() and 315gpiod_get_array(). Afterwards the array of descriptors has to be setup 316manually before it can be used with gpiod_set_array(). 317 318Note that for optimal performance GPIOs belonging to the same chip should be 319contiguous within the array of descriptors. 320 321 322GPIOs mapped to IRQs 323-------------------- 324GPIO lines can quite often be used as IRQs. You can get the IRQ number 325corresponding to a given GPIO using the following call: 326 327 int gpiod_to_irq(const struct gpio_desc *desc) 328 329It will return an IRQ number, or a negative errno code if the mapping can't be 330done (most likely because that particular GPIO cannot be used as IRQ). It is an 331unchecked error to use a GPIO that wasn't set up as an input using 332gpiod_direction_input(), or to use an IRQ number that didn't originally come 333from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. 334 335Non-error values returned from gpiod_to_irq() can be passed to request_irq() or 336free_irq(). They will often be stored into IRQ resources for platform devices, 337by the board-specific initialization code. Note that IRQ trigger options are 338part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup 339capabilities. 340 341 342GPIOs and ACPI 343============== 344 345On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by 346the _CRS configuration objects of devices. Those resources do not provide 347connection IDs (names) for GPIOs, so it is necessary to use an additional 348mechanism for this purpose. 349 350Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object 351which, among other things, may be used to provide connection IDs for specific 352GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the 353case, it will be handled by the GPIO subsystem automatically. However, if the 354_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO 355connection IDs need to be provided by device drivers. 356 357For details refer to Documentation/acpi/gpio-properties.txt 358 359 360Interacting With the Legacy GPIO Subsystem 361========================================== 362Many kernel subsystems still handle GPIOs using the legacy integer-based 363interface. Although it is strongly encouraged to upgrade them to the safer 364descriptor-based API, the following two functions allow you to convert a GPIO 365descriptor into the GPIO integer namespace and vice-versa: 366 367 int desc_to_gpio(const struct gpio_desc *desc) 368 struct gpio_desc *gpio_to_desc(unsigned gpio) 369 370The GPIO number returned by desc_to_gpio() can be safely used as long as the 371GPIO descriptor has not been freed. All the same, a GPIO number passed to 372gpio_to_desc() must have been properly acquired, and usage of the returned GPIO 373descriptor is only possible after the GPIO number has been released. 374 375Freeing a GPIO obtained by one API with the other API is forbidden and an 376unchecked error. 377