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diff --git a/Documentation/driver-api/gpio/consumer.rst b/Documentation/driver-api/gpio/consumer.rst new file mode 100644 index 000000000..aa03f389d --- /dev/null +++ b/Documentation/driver-api/gpio/consumer.rst @@ -0,0 +1,439 @@ +================================== +GPIO Descriptor Consumer Interface +================================== + +This document describes the consumer interface of the GPIO framework. Note that +it describes the new descriptor-based interface. For a description of the +deprecated integer-based GPIO interface please refer to gpio-legacy.txt. + + +Guidelines for GPIOs consumers +============================== + +Drivers that can't work without standard GPIO calls should have Kconfig entries +that depend on GPIOLIB or select GPIOLIB. The functions that allow a driver to +obtain and use GPIOs are available by including the following file: + + #include <linux/gpio/consumer.h> + +There are static inline stubs for all functions in the header file in the case +where GPIOLIB is disabled. When these stubs are called they will emit +warnings. These stubs are used for two use cases: + +- Simple compile coverage with e.g. COMPILE_TEST - it does not matter that + the current platform does not enable or select GPIOLIB because we are not + going to execute the system anyway. + +- Truly optional GPIOLIB support - where the driver does not really make use + of the GPIOs on certain compile-time configurations for certain systems, but + will use it under other compile-time configurations. In this case the + consumer must make sure not to call into these functions, or the user will + be met with console warnings that may be perceived as intimidating. + +All the functions that work with the descriptor-based GPIO interface are +prefixed with ``gpiod_``. The ``gpio_`` prefix is used for the legacy +interface. No other function in the kernel should use these prefixes. The use +of the legacy functions is strongly discouraged, new code should use +<linux/gpio/consumer.h> and descriptors exclusively. + + +Obtaining and Disposing GPIOs +============================= + +With the descriptor-based interface, GPIOs are identified with an opaque, +non-forgeable handler that must be obtained through a call to one of the +gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the +device that will use the GPIO and the function the requested GPIO is supposed to +fulfill:: + + struct gpio_desc *gpiod_get(struct device *dev, const char *con_id, + enum gpiod_flags flags) + +If a function is implemented by using several GPIOs together (e.g. a simple LED +device that displays digits), an additional index argument can be specified:: + + struct gpio_desc *gpiod_get_index(struct device *dev, + const char *con_id, unsigned int idx, + enum gpiod_flags flags) + +For a more detailed description of the con_id parameter in the DeviceTree case +see Documentation/driver-api/gpio/board.rst + +The flags parameter is used to optionally specify a direction and initial value +for the GPIO. Values can be: + +* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set + later with one of the dedicated functions. +* GPIOD_IN to initialize the GPIO as input. +* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0. +* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1. +* GPIOD_OUT_LOW_OPEN_DRAIN same as GPIOD_OUT_LOW but also enforce the line + to be electrically used with open drain. +* GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line + to be electrically used with open drain. + +The two last flags are used for use cases where open drain is mandatory, such +as I2C: if the line is not already configured as open drain in the mappings +(see board.txt), then open drain will be enforced anyway and a warning will be +printed that the board configuration needs to be updated to match the use case. + +Both functions return either a valid GPIO descriptor, or an error code checkable +with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned +if and only if no GPIO has been assigned to the device/function/index triplet, +other error codes are used for cases where a GPIO has been assigned but an error +occurred while trying to acquire it. This is useful to discriminate between mere +errors and an absence of GPIO for optional GPIO parameters. For the common +pattern where a GPIO is optional, the gpiod_get_optional() and +gpiod_get_index_optional() functions can be used. These functions return NULL +instead of -ENOENT if no GPIO has been assigned to the requested function:: + + struct gpio_desc *gpiod_get_optional(struct device *dev, + const char *con_id, + enum gpiod_flags flags) + + struct gpio_desc *gpiod_get_index_optional(struct device *dev, + const char *con_id, + unsigned int index, + enum gpiod_flags flags) + +Note that gpio_get*_optional() functions (and their managed variants), unlike +the rest of gpiolib API, also return NULL when gpiolib support is disabled. +This is helpful to driver authors, since they do not need to special case +-ENOSYS return codes. System integrators should however be careful to enable +gpiolib on systems that need it. + +For a function using multiple GPIOs all of those can be obtained with one call:: + + struct gpio_descs *gpiod_get_array(struct device *dev, + const char *con_id, + enum gpiod_flags flags) + +This function returns a struct gpio_descs which contains an array of +descriptors:: + + struct gpio_descs { + unsigned int ndescs; + struct gpio_desc *desc[]; + } + +The following function returns NULL instead of -ENOENT if no GPIOs have been +assigned to the requested function:: + + struct gpio_descs *gpiod_get_array_optional(struct device *dev, + const char *con_id, + enum gpiod_flags flags) + +Device-managed variants of these functions are also defined:: + + struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id, + enum gpiod_flags flags) + + struct gpio_desc *devm_gpiod_get_index(struct device *dev, + const char *con_id, + unsigned int idx, + enum gpiod_flags flags) + + struct gpio_desc *devm_gpiod_get_optional(struct device *dev, + const char *con_id, + enum gpiod_flags flags) + + struct gpio_desc *devm_gpiod_get_index_optional(struct device *dev, + const char *con_id, + unsigned int index, + enum gpiod_flags flags) + + struct gpio_descs *devm_gpiod_get_array(struct device *dev, + const char *con_id, + enum gpiod_flags flags) + + struct gpio_descs *devm_gpiod_get_array_optional(struct device *dev, + const char *con_id, + enum gpiod_flags flags) + +A GPIO descriptor can be disposed of using the gpiod_put() function:: + + void gpiod_put(struct gpio_desc *desc) + +For an array of GPIOs this function can be used:: + + void gpiod_put_array(struct gpio_descs *descs) + +It is strictly forbidden to use a descriptor after calling these functions. +It is also not allowed to individually release descriptors (using gpiod_put()) +from an array acquired with gpiod_get_array(). + +The device-managed variants are, unsurprisingly:: + + void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) + + void devm_gpiod_put_array(struct device *dev, struct gpio_descs *descs) + + +Using GPIOs +=========== + +Setting Direction +----------------- +The first thing a driver must do with a GPIO is setting its direction. If no +direction-setting flags have been given to gpiod_get*(), this is done by +invoking one of the gpiod_direction_*() functions:: + + int gpiod_direction_input(struct gpio_desc *desc) + int gpiod_direction_output(struct gpio_desc *desc, int value) + +The return value is zero for success, else a negative errno. It should be +checked, since the get/set calls don't return errors and since misconfiguration +is possible. You should normally issue these calls from a task context. However, +for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part +of early board setup. + +For output GPIOs, the value provided becomes the initial output value. This +helps avoid signal glitching during system startup. + +A driver can also query the current direction of a GPIO:: + + int gpiod_get_direction(const struct gpio_desc *desc) + +This function returns 0 for output, 1 for input, or an error code in case of error. + +Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO +without setting its direction first is illegal and will result in undefined +behavior!** + + +Spinlock-Safe GPIO Access +------------------------- +Most GPIO controllers can be accessed with memory read/write instructions. Those +don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ +handlers and similar contexts. + +Use the following calls to access GPIOs from an atomic context:: + + int gpiod_get_value(const struct gpio_desc *desc); + void gpiod_set_value(struct gpio_desc *desc, int value); + +The values are boolean, zero for low, nonzero for high. When reading the value +of an output pin, the value returned should be what's seen on the pin. That +won't always match the specified output value, because of issues including +open-drain signaling and output latencies. + +The get/set calls do not return errors because "invalid GPIO" should have been +reported earlier from gpiod_direction_*(). However, note that not all platforms +can read the value of output pins; those that can't should always return zero. +Also, using these calls for GPIOs that can't safely be accessed without sleeping +(see below) is an error. + + +GPIO Access That May Sleep +-------------------------- +Some GPIO controllers must be accessed using message based buses like I2C or +SPI. Commands to read or write those GPIO values require waiting to get to the +head of a queue to transmit a command and get its response. This requires +sleeping, which can't be done from inside IRQ handlers. + +Platforms that support this type of GPIO distinguish them from other GPIOs by +returning nonzero from this call:: + + int gpiod_cansleep(const struct gpio_desc *desc) + +To access such GPIOs, a different set of accessors is defined:: + + int gpiod_get_value_cansleep(const struct gpio_desc *desc) + void gpiod_set_value_cansleep(struct gpio_desc *desc, int value) + +Accessing such GPIOs requires a context which may sleep, for example a threaded +IRQ handler, and those accessors must be used instead of spinlock-safe +accessors without the cansleep() name suffix. + +Other than the fact that these accessors might sleep, and will work on GPIOs +that can't be accessed from hardIRQ handlers, these calls act the same as the +spinlock-safe calls. + + +The active low and open drain semantics +--------------------------------------- +As a consumer should not have to care about the physical line level, all of the +gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with +the *logical* value. With this they take the active low property into account. +This means that they check whether the GPIO is configured to be active low, +and if so, they manipulate the passed value before the physical line level is +driven. + +The same is applicable for open drain or open source output lines: those do not +actively drive their output high (open drain) or low (open source), they just +switch their output to a high impedance value. The consumer should not need to +care. (For details read about open drain in driver.txt.) + +With this, all the gpiod_set_(array)_value_xxx() functions interpret the +parameter "value" as "asserted" ("1") or "de-asserted" ("0"). The physical line +level will be driven accordingly. + +As an example, if the active low property for a dedicated GPIO is set, and the +gpiod_set_(array)_value_xxx() passes "asserted" ("1"), the physical line level +will be driven low. + +To summarize:: + + Function (example) line property physical line + gpiod_set_raw_value(desc, 0); don't care low + gpiod_set_raw_value(desc, 1); don't care high + gpiod_set_value(desc, 0); default (active high) low + gpiod_set_value(desc, 1); default (active high) high + gpiod_set_value(desc, 0); active low high + gpiod_set_value(desc, 1); active low low + gpiod_set_value(desc, 0); default (active high) low + gpiod_set_value(desc, 1); default (active high) high + gpiod_set_value(desc, 0); open drain low + gpiod_set_value(desc, 1); open drain high impedance + gpiod_set_value(desc, 0); open source high impedance + gpiod_set_value(desc, 1); open source high + +It is possible to override these semantics using the set_raw/get_raw functions +but it should be avoided as much as possible, especially by system-agnostic drivers +which should not need to care about the actual physical line level and worry about +the logical value instead. + + +Accessing raw GPIO values +------------------------- +Consumers exist that need to manage the logical state of a GPIO line, i.e. the value +their device will actually receive, no matter what lies between it and the GPIO +line. + +The following set of calls ignore the active-low or open drain property of a GPIO and +work on the raw line value:: + + int gpiod_get_raw_value(const struct gpio_desc *desc) + void gpiod_set_raw_value(struct gpio_desc *desc, int value) + int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc) + void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value) + int gpiod_direction_output_raw(struct gpio_desc *desc, int value) + +The active low state of a GPIO can also be queried using the following call:: + + int gpiod_is_active_low(const struct gpio_desc *desc) + +Note that these functions should only be used with great moderation; a driver +should not have to care about the physical line level or open drain semantics. + + +Access multiple GPIOs with a single function call +------------------------------------------------- +The following functions get or set the values of an array of GPIOs:: + + int gpiod_get_array_value(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array); + int gpiod_get_raw_array_value(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array); + int gpiod_get_array_value_cansleep(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array); + int gpiod_get_raw_array_value_cansleep(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array); + + void gpiod_set_array_value(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + void gpiod_set_raw_array_value(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + void gpiod_set_array_value_cansleep(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + void gpiod_set_raw_array_value_cansleep(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + +The array can be an arbitrary set of GPIOs. The functions will try to access +GPIOs belonging to the same bank or chip simultaneously if supported by the +corresponding chip driver. In that case a significantly improved performance +can be expected. If simultaneous access is not possible the GPIOs will be +accessed sequentially. + +The functions take three arguments: + * array_size - the number of array elements + * desc_array - an array of GPIO descriptors + * value_array - an array to store the GPIOs' values (get) or + an array of values to assign to the GPIOs (set) + +The descriptor array can be obtained using the gpiod_get_array() function +or one of its variants. If the group of descriptors returned by that function +matches the desired group of GPIOs, those GPIOs can be accessed by simply using +the struct gpio_descs returned by gpiod_get_array():: + + struct gpio_descs *my_gpio_descs = gpiod_get_array(...); + gpiod_set_array_value(my_gpio_descs->ndescs, my_gpio_descs->desc, + my_gpio_values); + +It is also possible to access a completely arbitrary array of descriptors. The +descriptors may be obtained using any combination of gpiod_get() and +gpiod_get_array(). Afterwards the array of descriptors has to be setup +manually before it can be passed to one of the above functions. + +Note that for optimal performance GPIOs belonging to the same chip should be +contiguous within the array of descriptors. + +The return value of gpiod_get_array_value() and its variants is 0 on success +or negative on error. Note the difference to gpiod_get_value(), which returns +0 or 1 on success to convey the GPIO value. With the array functions, the GPIO +values are stored in value_array rather than passed back as return value. + + +GPIOs mapped to IRQs +-------------------- +GPIO lines can quite often be used as IRQs. You can get the IRQ number +corresponding to a given GPIO using the following call:: + + int gpiod_to_irq(const struct gpio_desc *desc) + +It will return an IRQ number, or a negative errno code if the mapping can't be +done (most likely because that particular GPIO cannot be used as IRQ). It is an +unchecked error to use a GPIO that wasn't set up as an input using +gpiod_direction_input(), or to use an IRQ number that didn't originally come +from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. + +Non-error values returned from gpiod_to_irq() can be passed to request_irq() or +free_irq(). They will often be stored into IRQ resources for platform devices, +by the board-specific initialization code. Note that IRQ trigger options are +part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup +capabilities. + + +GPIOs and ACPI +============== + +On ACPI systems, GPIOs are described by GpioIo()/GpioInt() resources listed by +the _CRS configuration objects of devices. Those resources do not provide +connection IDs (names) for GPIOs, so it is necessary to use an additional +mechanism for this purpose. + +Systems compliant with ACPI 5.1 or newer may provide a _DSD configuration object +which, among other things, may be used to provide connection IDs for specific +GPIOs described by the GpioIo()/GpioInt() resources in _CRS. If that is the +case, it will be handled by the GPIO subsystem automatically. However, if the +_DSD is not present, the mappings between GpioIo()/GpioInt() resources and GPIO +connection IDs need to be provided by device drivers. + +For details refer to Documentation/acpi/gpio-properties.txt + + +Interacting With the Legacy GPIO Subsystem +========================================== +Many kernel subsystems still handle GPIOs using the legacy integer-based +interface. Although it is strongly encouraged to upgrade them to the safer +descriptor-based API, the following two functions allow you to convert a GPIO +descriptor into the GPIO integer namespace and vice-versa:: + + int desc_to_gpio(const struct gpio_desc *desc) + struct gpio_desc *gpio_to_desc(unsigned gpio) + +The GPIO number returned by desc_to_gpio() can be safely used as long as the +GPIO descriptor has not been freed. All the same, a GPIO number passed to +gpio_to_desc() must have been properly acquired, and usage of the returned GPIO +descriptor is only possible after the GPIO number has been released. + +Freeing a GPIO obtained by one API with the other API is forbidden and an +unchecked error. |