Documentation/gpio/consumer.txt

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