1 ==================================
2 GPIO Descriptor Consumer Interface
3 ==================================
4
5 This document describes the consumer interface of the GPIO framework.
6
7
8 Guidelines for GPIOs consumers
9 ==============================
10
11 Drivers that can't work without standard GPIO calls should have Kconfig entries
12 that depend on GPIOLIB or select GPIOLIB. The functions that allow a driver to
13 obtain and use GPIOs are available by including the following file::
14
15 #include <linux/gpio/consumer.h>
16
17 There are static inline stubs for all functions in the header file in the case
18 where GPIOLIB is disabled. When these stubs are called they will emit
19 warnings. These stubs are used for two use cases:
20
21 - Simple compile coverage with e.g. COMPILE_TEST - it does not matter that
22 the current platform does not enable or select GPIOLIB because we are not
23 going to execute the system anyway.
24
25 - Truly optional GPIOLIB support - where the driver does not really make use
26 of the GPIOs on certain compile-time configurations for certain systems, but
27 will use it under other compile-time configurations. In this case the
28 consumer must make sure not to call into these functions, or the user will
29 be met with console warnings that may be perceived as intimidating.
30 Combining truly optional GPIOLIB usage with calls to
31 ``[devm_]gpiod_get_optional()`` is a *bad idea*, and will result in weird
32 error messages. Use the ordinary getter functions with optional GPIOLIB:
33 some open coding of error handling should be expected when you do this.
34
35 All the functions that work with the descriptor-based GPIO interface are
36 prefixed with ``gpiod_``. The ``gpio_`` prefix is used for the legacy
37 interface. No other function in the kernel should use these prefixes. The use
38 of the legacy functions is strongly discouraged, new code should use
39 <linux/gpio/consumer.h> and descriptors exclusively.
40
41
42 Obtaining and Disposing GPIOs
43 =============================
44
45 With the descriptor-based interface, GPIOs are identified with an opaque,
46 non-forgeable handler that must be obtained through a call to one of the
47 gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the
48 device that will use the GPIO and the function the requested GPIO is supposed to
49 fulfill::
50
51 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id,
52 enum gpiod_flags flags)
53
54 If a function is implemented by using several GPIOs together (e.g. a simple LED
55 device that displays digits), an additional index argument can be specified::
56
57 struct gpio_desc *gpiod_get_index(struct device *dev,
58 const char *con_id, unsigned int idx,
59 enum gpiod_flags flags)
60
61 For a more detailed description of the con_id parameter in the DeviceTree case
62 see Documentation/driver-api/gpio/board.rst
63
64 The flags parameter is used to optionally specify a direction and initial value
65 for the GPIO. Values can be:
66
67 * GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set
68 later with one of the dedicated functions.
69 * GPIOD_IN to initialize the GPIO as input.
70 * GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0.
71 * GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1.
72 * GPIOD_OUT_LOW_OPEN_DRAIN same as GPIOD_OUT_LOW but also enforce the line
73 to be electrically used with open drain.
74 * GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line
75 to be electrically used with open drain.
76
77 Note that the initial value is *logical* and the physical line level depends on
78 whether the line is configured active high or active low (see
79 :ref:`active_low_semantics`).
80
81 The two last flags are used for use cases where open drain is mandatory, such
82 as I2C: if the line is not already configured as open drain in the mappings
83 (see board.rst), then open drain will be enforced anyway and a warning will be
84 printed that the board configuration needs to be updated to match the use case.
85
86 Both functions return either a valid GPIO descriptor, or an error code checkable
87 with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned
88 if and only if no GPIO has been assigned to the device/function/index triplet,
89 other error codes are used for cases where a GPIO has been assigned but an error
90 occurred while trying to acquire it. This is useful to discriminate between mere
91 errors and an absence of GPIO for optional GPIO parameters. For the common
92 pattern where a GPIO is optional, the gpiod_get_optional() and
93 gpiod_get_index_optional() functions can be used. These functions return NULL
94 instead of -ENOENT if no GPIO has been assigned to the requested function::
95
96 struct gpio_desc *gpiod_get_optional(struct device *dev,
97 const char *con_id,
98 enum gpiod_flags flags)
99
100 struct gpio_desc *gpiod_get_index_optional(struct device *dev,
101 const char *con_id,
102 unsigned int index,
103 enum gpiod_flags flags)
104
105 Note that gpio_get*_optional() functions (and their managed variants), unlike
106 the rest of gpiolib API, also return NULL when gpiolib support is disabled.
107 This is helpful to driver authors, since they do not need to special case
108 -ENOSYS return codes. System integrators should however be careful to enable
109 gpiolib on systems that need it.
110
111 For a function using multiple GPIOs all of those can be obtained with one call::
112
113 struct gpio_descs *gpiod_get_array(struct device *dev,
114 const char *con_id,
115 enum gpiod_flags flags)
116
117 This function returns a struct gpio_descs which contains an array of
118 descriptors. It also contains a pointer to a gpiolib private structure which,
119 if passed back to get/set array functions, may speed up I/O processing::
120
121 struct gpio_descs {
122 struct gpio_array *info;
123 unsigned int ndescs;
124 struct gpio_desc *desc[];
125 }
126
127 The following function returns NULL instead of -ENOENT if no GPIOs have been
128 assigned to the requested function::
129
130 struct gpio_descs *gpiod_get_array_optional(struct device *dev,
131 const char *con_id,
132 enum gpiod_flags flags)
133
134 Device-managed variants of these functions are also defined::
135
136 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id,
137 enum gpiod_flags flags)
138
139 struct gpio_desc *devm_gpiod_get_index(struct device *dev,
140 const char *con_id,
141 unsigned int idx,
142 enum gpiod_flags flags)
143
144 struct gpio_desc *devm_gpiod_get_optional(struct device *dev,
145 const char *con_id,
146 enum gpiod_flags flags)
147
148 struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev,
149 const char *con_id,
150 unsigned int index,
151 enum gpiod_flags flags)
152
153 struct gpio_descs *devm_gpiod_get_array(struct device *dev,
154 const char *con_id,
155 enum gpiod_flags flags)
156
157 struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev,
158 const char *con_id,
159 enum gpiod_flags flags)
160
161 A GPIO descriptor can be disposed of using the gpiod_put() function::
162
163 void gpiod_put(struct gpio_desc *desc)
164
165 For an array of GPIOs this function can be used::
166
167 void gpiod_put_array(struct gpio_descs *descs)
168
169 It is strictly forbidden to use a descriptor after calling these functions.
170 It is also not allowed to individually release descriptors (using gpiod_put())
171 from an array acquired with gpiod_get_array().
172
173 The device-managed variants are, unsurprisingly::
174
175 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc)
176
177 void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs)
178
179
180 Using GPIOs
181 ===========
182
183 Setting Direction
184 -----------------
185 The first thing a driver must do with a GPIO is setting its direction. If no
186 direction-setting flags have been given to gpiod_get*(), this is done by
187 invoking one of the gpiod_direction_*() functions::
188
189 int gpiod_direction_input(struct gpio_desc *desc)
190 int gpiod_direction_output(struct gpio_desc *desc, int value)
191
192 The return value is zero for success, else a negative errno. It should be
193 checked, since the get/set calls don't return errors and since misconfiguration
194 is possible. You should normally issue these calls from a task context. However,
195 for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part
196 of early board setup.
197
198 For output GPIOs, the value provided becomes the initial output value. This
199 helps avoid signal glitching during system startup.
200
201 A driver can also query the current direction of a GPIO::
202
203 int gpiod_get_direction(const struct gpio_desc *desc)
204
205 This function returns 0 for output, 1 for input, or an error code in case of error.
206
207 Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO
208 without setting its direction first is illegal and will result in undefined
209 behavior!**
210
211
212 Spinlock-Safe GPIO Access
213 -------------------------
214 Most GPIO controllers can be accessed with memory read/write instructions. Those
215 don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ
216 handlers and similar contexts.
217
218 Use the following calls to access GPIOs from an atomic context::
219
220 int gpiod_get_value(const struct gpio_desc *desc);
221 void gpiod_set_value(struct gpio_desc *desc, int value);
222
223 The values are boolean, zero for inactive, nonzero for active. When reading the
224 value of an output pin, the value returned should be what's seen on the pin.
225 That won't always match the specified output value, because of issues including
226 open-drain signaling and output latencies.
227
228 The get/set calls do not return errors because "invalid GPIO" should have been
229 reported earlier from gpiod_direction_*(). However, note that not all platforms
230 can read the value of output pins; those that can't should always return zero.
231 Also, using these calls for GPIOs that can't safely be accessed without sleeping
232 (see below) is an error.
233
234
235 GPIO Access That May Sleep
236 --------------------------
237 Some GPIO controllers must be accessed using message based buses like I2C or
238 SPI. Commands to read or write those GPIO values require waiting to get to the
239 head of a queue to transmit a command and get its response. This requires
240 sleeping, which can't be done from inside IRQ handlers.
241
242 Platforms that support this type of GPIO distinguish them from other GPIOs by
243 returning nonzero from this call::
244
245 int gpiod_cansleep(const struct gpio_desc *desc)
246
247 To access such GPIOs, a different set of accessors is defined::
248
249 int gpiod_get_value_cansleep(const struct gpio_desc *desc)
250 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value)
251
252 Accessing such GPIOs requires a context which may sleep, for example a threaded
253 IRQ handler, and those accessors must be used instead of spinlock-safe
254 accessors without the cansleep() name suffix.
255
256 Other than the fact that these accessors might sleep, and will work on GPIOs
257 that can't be accessed from hardIRQ handlers, these calls act the same as the
258 spinlock-safe calls.
259
260
261 .. _active_low_semantics:
262
263 The active low and open drain semantics
264 ---------------------------------------
265 As a consumer should not have to care about the physical line level, all of the
266 gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with
267 the *logical* value. With this they take the active low property into account.
268 This means that they check whether the GPIO is configured to be active low,
269 and if so, they manipulate the passed value before the physical line level is
270 driven.
271
272 The same is applicable for open drain or open source output lines: those do not
273 actively drive their output high (open drain) or low (open source), they just
274 switch their output to a high impedance value. The consumer should not need to
275 care. (For details read about open drain in driver.rst.)
276
277 With this, all the gpiod_set_(array)_value_xxx() functions interpret the
278 parameter "value" as "active" ("1") or "inactive" ("0"). The physical line
279 level will be driven accordingly.
280
281 As an example, if the active low property for a dedicated GPIO is set, and the
282 gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level
283 will be driven low.
284
285 To summarize::
286
287 Function (example) line property physical line
288 gpiod_set_raw_value(desc, 0); don't care low
289 gpiod_set_raw_value(desc, 1); don't care high
290 gpiod_set_value(desc, 0); default (active high) low
291 gpiod_set_value(desc, 1); default (active high) high
292 gpiod_set_value(desc, 0); active low high
293 gpiod_set_value(desc, 1); active low low
294 gpiod_set_value(desc, 0); open drain low
295 gpiod_set_value(desc, 1); open drain high impedance
296 gpiod_set_value(desc, 0); open source high impedance
297 gpiod_set_value(desc, 1); open source high
298
299 It is possible to override these semantics using the set_raw/get_raw functions
300 but it should be avoided as much as possible, especially by system-agnostic drivers
301 which should not need to care about the actual physical line level and worry about
302 the logical value instead.
303
304
305 Accessing raw GPIO values
306 -------------------------
307 Consumers exist that need to manage the logical state of a GPIO line, i.e. the value
308 their device will actually receive, no matter what lies between it and the GPIO
309 line.
310
311 The following set of calls ignore the active-low or open drain property of a GPIO and
312 work on the raw line value::
313
314 int gpiod_get_raw_value(const struct gpio_desc *desc)
315 void gpiod_set_raw_value(struct gpio_desc *desc, int value)
316 int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc)
317 void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
318 int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
319
320 The active low state of a GPIO can also be queried and toggled using the
321 following calls::
322
323 int gpiod_is_active_low(const struct gpio_desc *desc)
324 void gpiod_toggle_active_low(struct gpio_desc *desc)
325
326 Note that these functions should only be used with great moderation; a driver
327 should not have to care about the physical line level or open drain semantics.
328
329
330 Access multiple GPIOs with a single function call
331 -------------------------------------------------
332 The following functions get or set the values of an array of GPIOs::
333
334 int gpiod_get_array_value(unsigned int array_size,
335 struct gpio_desc **desc_array,
336 struct gpio_array *array_info,
337 unsigned long *value_bitmap);
338 int gpiod_get_raw_array_value(unsigned int array_size,
339 struct gpio_desc **desc_array,
340 struct gpio_array *array_info,
341 unsigned long *value_bitmap);
342 int gpiod_get_array_value_cansleep(unsigned int array_size,
343 struct gpio_desc **desc_array,
344 struct gpio_array *array_info,
345 unsigned long *value_bitmap);
346 int gpiod_get_raw_array_value_cansleep(unsigned int array_size,
347 struct gpio_desc **desc_array,
348 struct gpio_array *array_info,
349 unsigned long *value_bitmap);
350
351 int gpiod_set_array_value(unsigned int array_size,
352 struct gpio_desc **desc_array,
353 struct gpio_array *array_info,
354 unsigned long *value_bitmap)
355 int gpiod_set_raw_array_value(unsigned int array_size,
356 struct gpio_desc **desc_array,
357 struct gpio_array *array_info,
358 unsigned long *value_bitmap)
359 int gpiod_set_array_value_cansleep(unsigned int array_size,
360 struct gpio_desc **desc_array,
361 struct gpio_array *array_info,
362 unsigned long *value_bitmap)
363 int gpiod_set_raw_array_value_cansleep(unsigned int array_size,
364 struct gpio_desc **desc_array,
365 struct gpio_array *array_info,
366 unsigned long *value_bitmap)
367
368 The array can be an arbitrary set of GPIOs. The functions will try to access
369 GPIOs belonging to the same bank or chip simultaneously if supported by the
370 corresponding chip driver. In that case a significantly improved performance
371 can be expected. If simultaneous access is not possible the GPIOs will be
372 accessed sequentially.
373
374 The functions take four arguments:
375
376 * array_size - the number of array elements
377 * desc_array - an array of GPIO descriptors
378 * array_info - optional information obtained from gpiod_get_array()
379 * value_bitmap - a bitmap to store the GPIOs' values (get) or
380 a bitmap of values to assign to the GPIOs (set)
381
382 The descriptor array can be obtained using the gpiod_get_array() function
383 or one of its variants. If the group of descriptors returned by that function
384 matches the desired group of GPIOs, those GPIOs can be accessed by simply using
385 the struct gpio_descs returned by gpiod_get_array()::
386
387 struct gpio_descs *my_gpio_descs = gpiod_get_array(...);
388 gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc,
389 my_gpio_descs->info, my_gpio_value_bitmap);
390
391 It is also possible to access a completely arbitrary array of descriptors. The
392 descriptors may be obtained using any combination of gpiod_get() and
393 gpiod_get_array(). Afterwards the array of descriptors has to be setup
394 manually before it can be passed to one of the above functions. In that case,
395 array_info should be set to NULL.
396
397 Note that for optimal performance GPIOs belonging to the same chip should be
398 contiguous within the array of descriptors.
399
400 Still better performance may be achieved if array indexes of the descriptors
401 match hardware pin numbers of a single chip. If an array passed to a get/set
402 array function matches the one obtained from gpiod_get_array() and array_info
403 associated with the array is also passed, the function may take a fast bitmap
404 processing path, passing the value_bitmap argument directly to the respective
405 .get/set_multiple() callback of the chip. That allows for utilization of GPIO
406 banks as data I/O ports without much loss of performance.
407
408 The return value of gpiod_get_array_value() and its variants is 0 on success
409 or negative on error. Note the difference to gpiod_get_value(), which returns
410 0 or 1 on success to convey the GPIO value. With the array functions, the GPIO
411 values are stored in value_array rather than passed back as return value.
412
413
414 GPIOs mapped to IRQs
415 --------------------
416 GPIO lines can quite often be used as IRQs. You can get the IRQ number
417 corresponding to a given GPIO using the following call::
418
419 int gpiod_to_irq(const struct gpio_desc *desc)
420
421 It will return an IRQ number, or a negative errno code if the mapping can't be
422 done (most likely because that particular GPIO cannot be used as IRQ). It is an
423 unchecked error to use a GPIO that wasn't set up as an input using
424 gpiod_direction_input(), or to use an IRQ number that didn't originally come
425 from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep.
426
427 Non-error values returned from gpiod_to_irq() can be passed to request_irq() or
428 free_irq(). They will often be stored into IRQ resources for platform devices,
429 by the board-specific initialization code. Note that IRQ trigger options are
430 part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup
431 capabilities.
432
433
434 GPIOs and ACPI
435 ==============
436
437 On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by
438 the _CRS configuration objects of devices. Those resources do not provide
439 connection IDs (names) for GPIOs, so it is necessary to use an additional
440 mechanism for this purpose.
441
442 Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object
443 which, among other things, may be used to provide connection IDs for specific
444 GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the
445 case, it will be handled by the GPIO subsystem automatically. However, if the
446 _DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO
447 connection IDs need to be provided by device drivers.
448
449 For details refer to Documentation/firmware-guide/acpi/gpio-properties.rst
450
451
452 Interacting With the Legacy GPIO Subsystem
453 ==========================================
454 Many kernel subsystems and drivers still handle GPIOs using the legacy
455 integer-based interface. It is strongly recommended to update these to the new
456 gpiod interface. For cases where both interfaces need to be used, the following
457 two functions allow to convert a GPIO descriptor into the GPIO integer namespace
458 and vice-versa::
459
460 int desc_to_gpio(const struct gpio_desc *desc)
461 struct gpio_desc *gpio_to_desc(unsigned gpio)
462
463 The GPIO number returned by desc_to_gpio() can safely be used as a parameter of
464 the gpio\_*() functions for as long as the GPIO descriptor `desc` is not freed.
465 All the same, a GPIO number passed to gpio_to_desc() must first be properly
466 acquired using e.g. gpio_request_one(), and the returned GPIO descriptor is only
467 considered valid until that GPIO number is released using gpio_free().
468
469 Freeing a GPIO obtained by one API with the other API is forbidden and an
470 unchecked error.
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