From 57f0f512b273f60d52568b8c6b77e17f5636edc0 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Andr=C3=A9=20Fabian=20Silva=20Delgado?= Date: Wed, 5 Aug 2015 17:04:01 -0300 Subject: Initial import --- Documentation/gpio/consumer.txt | 340 ++++++++++++++++++++++++++++++++++++++++ 1 file changed, 340 insertions(+) create mode 100644 Documentation/gpio/consumer.txt (limited to 'Documentation/gpio/consumer.txt') diff --git a/Documentation/gpio/consumer.txt b/Documentation/gpio/consumer.txt new file mode 100644 index 000000000..c21c1313f --- /dev/null +++ b/Documentation/gpio/consumer.txt @@ -0,0 +1,340 @@ +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. The functions that allow a driver to obtain and use +GPIOs are available by including the following file: + + #include + +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. + + +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) + +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. + +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) + +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 will return either GPIOF_DIR_IN or GPIOF_DIR_OUT. + +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. + + +Active-low State and Raw GPIO Values +------------------------------------ +Device drivers like to manage the logical state of a GPIO, i.e. the value their +device will actually receive, no matter what lies between it and the GPIO line. +In some cases, it might make sense to control the actual GPIO line value. The +following set of calls ignore the active-low 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. + + +Set multiple GPIO outputs with a single function call +----------------------------------------------------- +The following functions set the output values of an array of GPIOs: + + void gpiod_set_array(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + void gpiod_set_raw_array(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + void gpiod_set_array_cansleep(unsigned int array_size, + struct gpio_desc **desc_array, + int *value_array) + void gpiod_set_raw_array_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 set +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 setting is not possible the GPIOs will be set +sequentially. + +The gpiod_set_array() functions take three arguments: + * array_size - the number of array elements + * desc_array - an array of GPIO descriptors + * value_array - an array of values to assign to the GPIOs + +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 set by simply using +the struct gpio_descs returned by gpiod_get_array(): + + struct gpio_descs *my_gpio_descs = gpiod_get_array(...); + gpiod_set_array(my_gpio_descs->ndescs, my_gpio_descs->desc, + my_gpio_values); + +It is also possible to set 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 used with gpiod_set_array(). + +Note that for optimal performance GPIOs belonging to the same chip should be +contiguous within the array of descriptors. + + +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 an 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. -- cgit v1.2.3