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| -rw-r--r-- | Documentation/driver-api/gpio/board.rst | 195 | ||||
| -rw-r--r-- | Documentation/driver-api/gpio/consumer.rst | 439 | ||||
| -rw-r--r-- | Documentation/driver-api/gpio/driver.rst | 429 | ||||
| -rw-r--r-- | Documentation/driver-api/gpio/drivers-on-gpio.rst | 101 | ||||
| -rw-r--r-- | Documentation/driver-api/gpio/index.rst | 48 | ||||
| -rw-r--r-- | Documentation/driver-api/gpio/intro.rst | 124 | ||||
| -rw-r--r-- | Documentation/driver-api/gpio/legacy.rst | 770 |
7 files changed, 2106 insertions, 0 deletions
diff --git a/Documentation/driver-api/gpio/board.rst b/Documentation/driver-api/gpio/board.rst new file mode 100644 index 000000000..2c112553d --- /dev/null +++ b/Documentation/driver-api/gpio/board.rst @@ -0,0 +1,195 @@ +============= +GPIO Mappings +============= + +This document explains how GPIOs can be assigned to given devices and functions. + +Note that it only applies to the new descriptor-based interface. For a +description of the deprecated integer-based GPIO interface please refer to +gpio-legacy.txt (actually, there is no real mapping possible with the old +interface; you just fetch an integer from somewhere and request the +corresponding GPIO). + +All platforms can enable the GPIO library, but if the platform strictly +requires GPIO functionality to be present, it needs to select GPIOLIB from its +Kconfig. Then, how GPIOs are mapped depends on what the platform uses to +describe its hardware layout. Currently, mappings can be defined through device +tree, ACPI, and platform data. + +Device Tree +----------- +GPIOs can easily be mapped to devices and functions in the device tree. The +exact way to do it depends on the GPIO controller providing the GPIOs, see the +device tree bindings for your controller. + +GPIOs mappings are defined in the consumer device's node, in a property named +<function>-gpios, where <function> is the function the driver will request +through gpiod_get(). For example:: + + foo_device { + compatible = "acme,foo"; + ... + led-gpios = <&gpio 15 GPIO_ACTIVE_HIGH>, /* red */ + <&gpio 16 GPIO_ACTIVE_HIGH>, /* green */ + <&gpio 17 GPIO_ACTIVE_HIGH>; /* blue */ + + power-gpios = <&gpio 1 GPIO_ACTIVE_LOW>; + }; + +Properties named <function>-gpio are also considered valid and old bindings use +it but are only supported for compatibility reasons and should not be used for +newer bindings since it has been deprecated. + +This property will make GPIOs 15, 16 and 17 available to the driver under the +"led" function, and GPIO 1 as the "power" GPIO:: + + struct gpio_desc *red, *green, *blue, *power; + + red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH); + green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH); + blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH); + + power = gpiod_get(dev, "power", GPIOD_OUT_HIGH); + +The led GPIOs will be active high, while the power GPIO will be active low (i.e. +gpiod_is_active_low(power) will be true). + +The second parameter of the gpiod_get() functions, the con_id string, has to be +the <function>-prefix of the GPIO suffixes ("gpios" or "gpio", automatically +looked up by the gpiod functions internally) used in the device tree. With above +"led-gpios" example, use the prefix without the "-" as con_id parameter: "led". + +Internally, the GPIO subsystem prefixes the GPIO suffix ("gpios" or "gpio") +with the string passed in con_id to get the resulting string +(``snprintf(... "%s-%s", con_id, gpio_suffixes[]``). + +ACPI +---- +ACPI also supports function names for GPIOs in a similar fashion to DT. +The above DT example can be converted to an equivalent ACPI description +with the help of _DSD (Device Specific Data), introduced in ACPI 5.1:: + + Device (FOO) { + Name (_CRS, ResourceTemplate () { + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {15} // red + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {16} // green + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {17} // blue + GpioIo (Exclusive, ..., IoRestrictionOutputOnly, + "\\_SB.GPI0") {1} // power + }) + + Name (_DSD, Package () { + ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"), + Package () { + Package () { + "led-gpios", + Package () { + ^FOO, 0, 0, 1, + ^FOO, 1, 0, 1, + ^FOO, 2, 0, 1, + } + }, + Package () { + "power-gpios", + Package () {^FOO, 3, 0, 0}, + }, + } + }) + } + +For more information about the ACPI GPIO bindings see +Documentation/acpi/gpio-properties.txt. + +Platform Data +------------- +Finally, GPIOs can be bound to devices and functions using platform data. Board +files that desire to do so need to include the following header:: + + #include <linux/gpio/machine.h> + +GPIOs are mapped by the means of tables of lookups, containing instances of the +gpiod_lookup structure. Two macros are defined to help declaring such mappings:: + + GPIO_LOOKUP(chip_label, chip_hwnum, con_id, flags) + GPIO_LOOKUP_IDX(chip_label, chip_hwnum, con_id, idx, flags) + +where + + - chip_label is the label of the gpiod_chip instance providing the GPIO + - chip_hwnum is the hardware number of the GPIO within the chip + - con_id is the name of the GPIO function from the device point of view. It + can be NULL, in which case it will match any function. + - idx is the index of the GPIO within the function. + - flags is defined to specify the following properties: + * GPIO_ACTIVE_HIGH - GPIO line is active high + * GPIO_ACTIVE_LOW - GPIO line is active low + * GPIO_OPEN_DRAIN - GPIO line is set up as open drain + * GPIO_OPEN_SOURCE - GPIO line is set up as open source + * GPIO_PERSISTENT - GPIO line is persistent during + suspend/resume and maintains its value + * GPIO_TRANSITORY - GPIO line is transitory and may loose its + electrical state during suspend/resume + +In the future, these flags might be extended to support more properties. + +Note that GPIO_LOOKUP() is just a shortcut to GPIO_LOOKUP_IDX() where idx = 0. + +A lookup table can then be defined as follows, with an empty entry defining its +end. The 'dev_id' field of the table is the identifier of the device that will +make use of these GPIOs. It can be NULL, in which case it will be matched for +calls to gpiod_get() with a NULL device. + +.. code-block:: c + + struct gpiod_lookup_table gpios_table = { + .dev_id = "foo.0", + .table = { + GPIO_LOOKUP_IDX("gpio.0", 15, "led", 0, GPIO_ACTIVE_HIGH), + GPIO_LOOKUP_IDX("gpio.0", 16, "led", 1, GPIO_ACTIVE_HIGH), + GPIO_LOOKUP_IDX("gpio.0", 17, "led", 2, GPIO_ACTIVE_HIGH), + GPIO_LOOKUP("gpio.0", 1, "power", GPIO_ACTIVE_LOW), + { }, + }, + }; + +And the table can be added by the board code as follows:: + + gpiod_add_lookup_table(&gpios_table); + +The driver controlling "foo.0" will then be able to obtain its GPIOs as follows:: + + struct gpio_desc *red, *green, *blue, *power; + + red = gpiod_get_index(dev, "led", 0, GPIOD_OUT_HIGH); + green = gpiod_get_index(dev, "led", 1, GPIOD_OUT_HIGH); + blue = gpiod_get_index(dev, "led", 2, GPIOD_OUT_HIGH); + + power = gpiod_get(dev, "power", GPIOD_OUT_HIGH); + +Since the "led" GPIOs are mapped as active-high, this example will switch their +signals to 1, i.e. enabling the LEDs. And for the "power" GPIO, which is mapped +as active-low, its actual signal will be 0 after this code. Contrary to the +legacy integer GPIO interface, the active-low property is handled during +mapping and is thus transparent to GPIO consumers. + +A set of functions such as gpiod_set_value() is available to work with +the new descriptor-oriented interface. + +Boards using platform data can also hog GPIO lines by defining GPIO hog tables. + +.. code-block:: c + + struct gpiod_hog gpio_hog_table[] = { + GPIO_HOG("gpio.0", 10, "foo", GPIO_ACTIVE_LOW, GPIOD_OUT_HIGH), + { } + }; + +And the table can be added to the board code as follows:: + + gpiod_add_hogs(gpio_hog_table); + +The line will be hogged as soon as the gpiochip is created or - in case the +chip was created earlier - when the hog table is registered. 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. diff --git a/Documentation/driver-api/gpio/driver.rst b/Documentation/driver-api/gpio/driver.rst new file mode 100644 index 000000000..cbe024284 --- /dev/null +++ b/Documentation/driver-api/gpio/driver.rst @@ -0,0 +1,429 @@ +================================ +GPIO Descriptor Driver Interface +================================ + +This document serves as a guide for GPIO chip drivers writers. 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. + +Each GPIO controller driver needs to include the following header, which defines +the structures used to define a GPIO driver: + + #include <linux/gpio/driver.h> + + +Internal Representation of GPIOs +================================ + +Inside a GPIO driver, individual GPIOs are identified by their hardware number, +which is a unique number between 0 and n, n being the number of GPIOs managed by +the chip. This number is purely internal: the hardware number of a particular +GPIO descriptor is never made visible outside of the driver. + +On top of this internal number, each GPIO also need to have a global number in +the integer GPIO namespace so that it can be used with the legacy GPIO +interface. Each chip must thus have a "base" number (which can be automatically +assigned), and for each GPIO the global number will be (base + hardware number). +Although the integer representation is considered deprecated, it still has many +users and thus needs to be maintained. + +So for example one platform could use numbers 32-159 for GPIOs, with a +controller defining 128 GPIOs at a "base" of 32 ; while another platform uses +numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO +controller, and on one particular board 80-95 with an FPGA. The numbers need not +be contiguous; either of those platforms could also use numbers 2000-2063 to +identify GPIOs in a bank of I2C GPIO expanders. + + +Controller Drivers: gpio_chip +============================= + +In the gpiolib framework each GPIO controller is packaged as a "struct +gpio_chip" (see linux/gpio/driver.h for its complete definition) with members +common to each controller of that type: + + - methods to establish GPIO line direction + - methods used to access GPIO line values + - method to set electrical configuration for a given GPIO line + - method to return the IRQ number associated to a given GPIO line + - flag saying whether calls to its methods may sleep + - optional line names array to identify lines + - optional debugfs dump method (showing extra state like pullup config) + - optional base number (will be automatically assigned if omitted) + - optional label for diagnostics and GPIO chip mapping using platform data + +The code implementing a gpio_chip should support multiple instances of the +controller, possibly using the driver model. That code will configure each +gpio_chip and issue ``gpiochip_add[_data]()`` or ``devm_gpiochip_add_data()``. +Removing a GPIO controller should be rare; use ``[devm_]gpiochip_remove()`` +when it is unavoidable. + +Often a gpio_chip is part of an instance-specific structure with states not +exposed by the GPIO interfaces, such as addressing, power management, and more. +Chips such as audio codecs will have complex non-GPIO states. + +Any debugfs dump method should normally ignore signals which haven't been +requested as GPIOs. They can use gpiochip_is_requested(), which returns either +NULL or the label associated with that GPIO when it was requested. + +RT_FULL: the GPIO driver should not use spinlock_t or any sleepable APIs +(like PM runtime) in its gpio_chip implementation (.get/.set and direction +control callbacks) if it is expected to call GPIO APIs from atomic context +on -RT (inside hard IRQ handlers and similar contexts). Normally this should +not be required. + + +GPIO electrical configuration +----------------------------- + +GPIOs can be configured for several electrical modes of operation by using the +.set_config() callback. Currently this API supports setting debouncing and +single-ended modes (open drain/open source). These settings are described +below. + +The .set_config() callback uses the same enumerators and configuration +semantics as the generic pin control drivers. This is not a coincidence: it is +possible to assign the .set_config() to the function gpiochip_generic_config() +which will result in pinctrl_gpio_set_config() being called and eventually +ending up in the pin control back-end "behind" the GPIO controller, usually +closer to the actual pins. This way the pin controller can manage the below +listed GPIO configurations. + +If a pin controller back-end is used, the GPIO controller or hardware +description needs to provide "GPIO ranges" mapping the GPIO line offsets to pin +numbers on the pin controller so they can properly cross-reference each other. + + +GPIOs with debounce support +--------------------------- + +Debouncing is a configuration set to a pin indicating that it is connected to +a mechanical switch or button, or similar that may bounce. Bouncing means the +line is pulled high/low quickly at very short intervals for mechanical +reasons. This can result in the value being unstable or irqs fireing repeatedly +unless the line is debounced. + +Debouncing in practice involves setting up a timer when something happens on +the line, wait a little while and then sample the line again, so see if it +still has the same value (low or high). This could also be repeated by a clever +state machine, waiting for a line to become stable. In either case, it sets +a certain number of milliseconds for debouncing, or just "on/off" if that time +is not configurable. + + +GPIOs with open drain/source support +------------------------------------ + +Open drain (CMOS) or open collector (TTL) means the line is not actively driven +high: instead you provide the drain/collector as output, so when the transistor +is not open, it will present a high-impedance (tristate) to the external rail:: + + + CMOS CONFIGURATION TTL CONFIGURATION + + ||--- out +--- out + in ----|| |/ + ||--+ in ----| + | |\ + GND GND + +This configuration is normally used as a way to achieve one of two things: + +- Level-shifting: to reach a logical level higher than that of the silicon + where the output resides. + +- inverse wire-OR on an I/O line, for example a GPIO line, making it possible + for any driving stage on the line to drive it low even if any other output + to the same line is simultaneously driving it high. A special case of this + is driving the SCL and SCA lines of an I2C bus, which is by definition a + wire-OR bus. + +Both usecases require that the line be equipped with a pull-up resistor. This +resistor will make the line tend to high level unless one of the transistors on +the rail actively pulls it down. + +The level on the line will go as high as the VDD on the pull-up resistor, which +may be higher than the level supported by the transistor, achieving a +level-shift to the higher VDD. + +Integrated electronics often have an output driver stage in the form of a CMOS +"totem-pole" with one N-MOS and one P-MOS transistor where one of them drives +the line high and one of them drives the line low. This is called a push-pull +output. The "totem-pole" looks like so:: + + VDD + | + OD ||--+ + +--/ ---o|| P-MOS-FET + | ||--+ + IN --+ +----- out + | ||--+ + +--/ ----|| N-MOS-FET + OS ||--+ + | + GND + +The desired output signal (e.g. coming directly from some GPIO output register) +arrives at IN. The switches named "OD" and "OS" are normally closed, creating +a push-pull circuit. + +Consider the little "switches" named "OD" and "OS" that enable/disable the +P-MOS or N-MOS transistor right after the split of the input. As you can see, +either transistor will go totally numb if this switch is open. The totem-pole +is then halved and give high impedance instead of actively driving the line +high or low respectively. That is usually how software-controlled open +drain/source works. + +Some GPIO hardware come in open drain / open source configuration. Some are +hard-wired lines that will only support open drain or open source no matter +what: there is only one transistor there. Some are software-configurable: +by flipping a bit in a register the output can be configured as open drain +or open source, in practice by flicking open the switches labeled "OD" and "OS" +in the drawing above. + +By disabling the P-MOS transistor, the output can be driven between GND and +high impedance (open drain), and by disabling the N-MOS transistor, the output +can be driven between VDD and high impedance (open source). In the first case, +a pull-up resistor is needed on the outgoing rail to complete the circuit, and +in the second case, a pull-down resistor is needed on the rail. + +Hardware that supports open drain or open source or both, can implement a +special callback in the gpio_chip: .set_config() that takes a generic +pinconf packed value telling whether to configure the line as open drain, +open source or push-pull. This will happen in response to the +GPIO_OPEN_DRAIN or GPIO_OPEN_SOURCE flag set in the machine file, or coming +from other hardware descriptions. + +If this state can not be configured in hardware, i.e. if the GPIO hardware does +not support open drain/open source in hardware, the GPIO library will instead +use a trick: when a line is set as output, if the line is flagged as open +drain, and the IN output value is low, it will be driven low as usual. But +if the IN output value is set to high, it will instead *NOT* be driven high, +instead it will be switched to input, as input mode is high impedance, thus +achieveing an "open drain emulation" of sorts: electrically the behaviour will +be identical, with the exception of possible hardware glitches when switching +the mode of the line. + +For open source configuration the same principle is used, just that instead +of actively driving the line low, it is set to input. + + +GPIO drivers providing IRQs +--------------------------- +It is custom that GPIO drivers (GPIO chips) are also providing interrupts, +most often cascaded off a parent interrupt controller, and in some special +cases the GPIO logic is melded with a SoC's primary interrupt controller. + +The IRQ portions of the GPIO block are implemented using an irqchip, using +the header <linux/irq.h>. So basically such a driver is utilizing two sub- +systems simultaneously: gpio and irq. + +RT_FULL: a realtime compliant GPIO driver should not use spinlock_t or any +sleepable APIs (like PM runtime) as part of its irq_chip implementation. + +* spinlock_t should be replaced with raw_spinlock_t [1]. +* If sleepable APIs have to be used, these can be done from the .irq_bus_lock() + and .irq_bus_unlock() callbacks, as these are the only slowpath callbacks + on an irqchip. Create the callbacks if needed [2]. + +GPIO irqchips usually fall in one of two categories: + +* CHAINED GPIO irqchips: these are usually the type that is embedded on + an SoC. This means that there is a fast IRQ flow handler for the GPIOs that + gets called in a chain from the parent IRQ handler, most typically the + system interrupt controller. This means that the GPIO irqchip handler will + be called immediately from the parent irqchip, while holding the IRQs + disabled. The GPIO irqchip will then end up calling something like this + sequence in its interrupt handler:: + + static irqreturn_t foo_gpio_irq(int irq, void *data) + chained_irq_enter(...); + generic_handle_irq(...); + chained_irq_exit(...); + + Chained GPIO irqchips typically can NOT set the .can_sleep flag on + struct gpio_chip, as everything happens directly in the callbacks: no + slow bus traffic like I2C can be used. + + RT_FULL: Note, chained IRQ handlers will not be forced threaded on -RT. + As result, spinlock_t or any sleepable APIs (like PM runtime) can't be used + in chained IRQ handler. + If required (and if it can't be converted to the nested threaded GPIO irqchip) + a chained IRQ handler can be converted to generic irq handler and this way + it will be a threaded IRQ handler on -RT and a hard IRQ handler on non-RT + (for example, see [3]). + Know W/A: The generic_handle_irq() is expected to be called with IRQ disabled, + so the IRQ core will complain if it is called from an IRQ handler which is + forced to a thread. The "fake?" raw lock can be used to W/A this problem:: + + raw_spinlock_t wa_lock; + static irqreturn_t omap_gpio_irq_handler(int irq, void *gpiobank) + unsigned long wa_lock_flags; + raw_spin_lock_irqsave(&bank->wa_lock, wa_lock_flags); + generic_handle_irq(irq_find_mapping(bank->chip.irq.domain, bit)); + raw_spin_unlock_irqrestore(&bank->wa_lock, wa_lock_flags); + +* GENERIC CHAINED GPIO irqchips: these are the same as "CHAINED GPIO irqchips", + but chained IRQ handlers are not used. Instead GPIO IRQs dispatching is + performed by generic IRQ handler which is configured using request_irq(). + The GPIO irqchip will then end up calling something like this sequence in + its interrupt handler:: + + static irqreturn_t gpio_rcar_irq_handler(int irq, void *dev_id) + for each detected GPIO IRQ + generic_handle_irq(...); + + RT_FULL: Such kind of handlers will be forced threaded on -RT, as result IRQ + core will complain that generic_handle_irq() is called with IRQ enabled and + the same W/A as for "CHAINED GPIO irqchips" can be applied. + +* NESTED THREADED GPIO irqchips: these are off-chip GPIO expanders and any + other GPIO irqchip residing on the other side of a sleeping bus. Of course + such drivers that need slow bus traffic to read out IRQ status and similar, + traffic which may in turn incur other IRQs to happen, cannot be handled + in a quick IRQ handler with IRQs disabled. Instead they need to spawn a + thread and then mask the parent IRQ line until the interrupt is handled + by the driver. The hallmark of this driver is to call something like + this in its interrupt handler:: + + static irqreturn_t foo_gpio_irq(int irq, void *data) + ... + handle_nested_irq(irq); + + The hallmark of threaded GPIO irqchips is that they set the .can_sleep + flag on struct gpio_chip to true, indicating that this chip may sleep + when accessing the GPIOs. + +To help out in handling the set-up and management of GPIO irqchips and the +associated irqdomain and resource allocation callbacks, the gpiolib has +some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig +symbol: + +* gpiochip_irqchip_add(): adds a chained irqchip to a gpiochip. It will pass + the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks + need to embed the gpio_chip in its state container and obtain a pointer + to the container using container_of(). + (See Documentation/driver-model/design-patterns.txt) + +* gpiochip_irqchip_add_nested(): adds a nested irqchip to a gpiochip. + Apart from that it works exactly like the chained irqchip. + +* gpiochip_set_chained_irqchip(): sets up a chained irq handler for a + gpio_chip from a parent IRQ and passes the struct gpio_chip* as handler + data. (Notice handler data, since the irqchip data is likely used by the + parent irqchip!). + +* gpiochip_set_nested_irqchip(): sets up a nested irq handler for a + gpio_chip from a parent IRQ. As the parent IRQ has usually been + explicitly requested by the driver, this does very little more than + mark all the child IRQs as having the other IRQ as parent. + +If there is a need to exclude certain GPIOs from the IRQ domain, you can +set .irq.need_valid_mask of the gpiochip before gpiochip_add_data() is +called. This allocates an .irq.valid_mask with as many bits set as there +are GPIOs in the chip. Drivers can exclude GPIOs by clearing bits from this +mask. The mask must be filled in before gpiochip_irqchip_add() or +gpiochip_irqchip_add_nested() is called. + +To use the helpers please keep the following in mind: + +- Make sure to assign all relevant members of the struct gpio_chip so that + the irqchip can initialize. E.g. .dev and .can_sleep shall be set up + properly. + +- Nominally set all handlers to handle_bad_irq() in the setup call and pass + handle_bad_irq() as flow handler parameter in gpiochip_irqchip_add() if it is + expected for GPIO driver that irqchip .set_type() callback have to be called + before using/enabling GPIO IRQ. Then set the handler to handle_level_irq() + and/or handle_edge_irq() in the irqchip .set_type() callback depending on + what your controller supports. + +It is legal for any IRQ consumer to request an IRQ from any irqchip no matter +if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and +irq_chip are orthogonal, and offering their services independent of each +other. + +gpiod_to_irq() is just a convenience function to figure out the IRQ for a +certain GPIO line and should not be relied upon to have been called before +the IRQ is used. + +So always prepare the hardware and make it ready for action in respective +callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having +been called first. + +This orthogonality leads to ambiguities that we need to solve: if there is +competition inside the subsystem which side is using the resource (a certain +GPIO line and register for example) it needs to deny certain operations and +keep track of usage inside of the gpiolib subsystem. This is why the API +below exists. + + +Locking IRQ usage +----------------- +Input GPIOs can be used as IRQ signals. When this happens, a driver is requested +to mark the GPIO as being used as an IRQ:: + + int gpiochip_lock_as_irq(struct gpio_chip *chip, unsigned int offset) + +This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock +is released:: + + void gpiochip_unlock_as_irq(struct gpio_chip *chip, unsigned int offset) + +When implementing an irqchip inside a GPIO driver, these two functions should +typically be called in the .startup() and .shutdown() callbacks from the +irqchip. + +When using the gpiolib irqchip helpers, these callback are automatically +assigned. + +Real-Time compliance for GPIO IRQ chips +--------------------------------------- + +Any provider of irqchips needs to be carefully tailored to support Real Time +preemption. It is desirable that all irqchips in the GPIO subsystem keep this +in mind and do the proper testing to assure they are real time-enabled. +So, pay attention on above " RT_FULL:" notes, please. +The following is a checklist to follow when preparing a driver for real +time-compliance: + +- ensure spinlock_t is not used as part irq_chip implementation; +- ensure that sleepable APIs are not used as part irq_chip implementation. + If sleepable APIs have to be used, these can be done from the .irq_bus_lock() + and .irq_bus_unlock() callbacks; +- Chained GPIO irqchips: ensure spinlock_t or any sleepable APIs are not used + from chained IRQ handler; +- Generic chained GPIO irqchips: take care about generic_handle_irq() calls and + apply corresponding W/A; +- Chained GPIO irqchips: get rid of chained IRQ handler and use generic irq + handler if possible :) +- regmap_mmio: Sry, but you are in trouble :( if MMIO regmap is used as for + GPIO IRQ chip implementation; +- Test your driver with the appropriate in-kernel real time test cases for both + level and edge IRQs. + + +Requesting self-owned GPIO pins +------------------------------- + +Sometimes it is useful to allow a GPIO chip driver to request its own GPIO +descriptors through the gpiolib API. Using gpio_request() for this purpose +does not help since it pins the module to the kernel forever (it calls +try_module_get()). A GPIO driver can use the following functions instead +to request and free descriptors without being pinned to the kernel forever:: + + struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc, + const char *label) + + void gpiochip_free_own_desc(struct gpio_desc *desc) + +Descriptors requested with gpiochip_request_own_desc() must be released with +gpiochip_free_own_desc(). + +These functions must be used with care since they do not affect module use +count. Do not use the functions to request gpio descriptors not owned by the +calling driver. + +* [1] http://www.spinics.net/lists/linux-omap/msg120425.html +* [2] https://lkml.org/lkml/2015/9/25/494 +* [3] https://lkml.org/lkml/2015/9/25/495 diff --git a/Documentation/driver-api/gpio/drivers-on-gpio.rst b/Documentation/driver-api/gpio/drivers-on-gpio.rst new file mode 100644 index 000000000..f3a189320 --- /dev/null +++ b/Documentation/driver-api/gpio/drivers-on-gpio.rst @@ -0,0 +1,101 @@ +============================ +Subsystem drivers using GPIO +============================ + +Note that standard kernel drivers exist for common GPIO tasks and will provide +the right in-kernel and userspace APIs/ABIs for the job, and that these +drivers can quite easily interconnect with other kernel subsystems using +hardware descriptions such as device tree or ACPI: + +- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO + lines, giving you the LED sysfs interface + +- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger, + i.e. a LED will turn on/off in response to a GPIO line going high or low + (and that LED may in turn use the leds-gpio as per above). + +- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line + can generate interrupts in response to a key press. Also supports debounce. + +- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your + GPIO line cannot generate interrupts, so it needs to be periodically polled + by a timer. + +- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with + up to three buttons by simply using GPIOs and no mouse port. You can cut the + mouse cable and connect the wires to GPIO lines or solder a mouse connector + to the lines for a more permanent solution of this type. + +- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from + an external speaker connected to a GPIO line. + +- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an + external connector status, such as a headset line for an audio driver or an + HDMI connector. It will provide a better userspace sysfs interface than GPIO. + +- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot + the system by pulling a GPIO line and will register a restart handler so + userspace can issue the right system call to restart the system. + +- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the + system down by pulling a GPIO line and will register a pm_power_off() + callback so that userspace can issue the right system call to power down the + system. + +- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock + (off/on) that uses a GPIO, and integrated with the clock subsystem. + +- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus + (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will + appear as any other I2C bus to the system and makes it possible to connect + drivers for the I2C devices on the bus like any other I2C bus driver. + +- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number + of wires, at least SCK and optionally MISO, MOSI and chip select lines) using + GPIO hammering (bitbang). It will appear as any other SPI bus on the system + and makes it possible to connect drivers for SPI devices on the bus like + any other SPI bus driver. For example any MMC/SD card can then be connected + to this SPI by using the mmc_spi host from the MMC/SD card subsystem. + +- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using + a GPIO line, integrating with the W1 subsystem and handling devices on + the bus like any other W1 device. + +- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the + system, connected to a GPIO line (and optionally a GPIO alarm line), + presenting all the right in-kernel and sysfs interfaces to make your system + not overheat. + +- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a + regulator providing a certain voltage by pulling a GPIO line, integrating + with the regulator subsystem and giving you all the right interfaces. + +- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer + that will periodically "ping" a hardware connected to a GPIO line by toggling + it from 1-to-0-to-1. If that hardware does not receive its "ping" + periodically, it will reset the system. + +- gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip + to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the + NAND flash MTD subsystem and provides chip access and partition parsing like + any other NAND driving hardware. + +- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio + bus, data and clock line, by bit banging two GPIO lines. It will appear as + any other serio bus to the system and makes it possible to connect drivers + for e.g. keyboards and other PS/2 protocol based devices. + +- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC + Consumer Electronics Control bus using only GPIO. It is used to communicate + with devices on the HDMI bus. + +Apart from this there are special GPIO drivers in subsystems like MMC/SD to +read card detect and write protect GPIO lines, and in the TTY serial subsystem +to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The +MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is +usually connected directly to the flash. + +Use those instead of talking directly to the GPIOs using sysfs; they integrate +with kernel frameworks better than your userspace code could. Needless to say, +just using the appropriate kernel drivers will simplify and speed up your +embedded hacking in particular by providing ready-made components. diff --git a/Documentation/driver-api/gpio/index.rst b/Documentation/driver-api/gpio/index.rst new file mode 100644 index 000000000..6a374ded1 --- /dev/null +++ b/Documentation/driver-api/gpio/index.rst @@ -0,0 +1,48 @@ +=================================== +General Purpose Input/Output (GPIO) +=================================== + +Contents: + +.. toctree:: + :maxdepth: 2 + + intro + driver + consumer + board + drivers-on-gpio + legacy + +Core +==== + +.. kernel-doc:: include/linux/gpio/driver.h + :internal: + +.. kernel-doc:: drivers/gpio/gpiolib.c + :export: + +ACPI support +============ + +.. kernel-doc:: drivers/gpio/gpiolib-acpi.c + :export: + +Device tree support +=================== + +.. kernel-doc:: drivers/gpio/gpiolib-of.c + :export: + +Device-managed API +================== + +.. kernel-doc:: drivers/gpio/devres.c + :export: + +sysfs helpers +============= + +.. kernel-doc:: drivers/gpio/gpiolib-sysfs.c + :export: diff --git a/Documentation/driver-api/gpio/intro.rst b/Documentation/driver-api/gpio/intro.rst new file mode 100644 index 000000000..74591489d --- /dev/null +++ b/Documentation/driver-api/gpio/intro.rst @@ -0,0 +1,124 @@ +============ +Introduction +============ + + +GPIO Interfaces +=============== + +The documents in this directory give detailed instructions on how to access +GPIOs in drivers, and how to write a driver for a device that provides GPIOs +itself. + +Due to the history of GPIO interfaces in the kernel, there are two different +ways to obtain and use GPIOs: + + - The descriptor-based interface is the preferred way to manipulate GPIOs, + and is described by all the files in this directory excepted gpio-legacy.txt. + - The legacy integer-based interface which is considered deprecated (but still + usable for compatibility reasons) is documented in gpio-legacy.txt. + +The remainder of this document applies to the new descriptor-based interface. +gpio-legacy.txt contains the same information applied to the legacy +integer-based interface. + + +What is a GPIO? +=============== + +A "General Purpose Input/Output" (GPIO) is a flexible software-controlled +digital signal. They are provided from many kinds of chip, and are familiar +to Linux developers working with embedded and custom hardware. Each GPIO +represents a bit connected to a particular pin, or "ball" on Ball Grid Array +(BGA) packages. Board schematics show which external hardware connects to +which GPIOs. Drivers can be written generically, so that board setup code +passes such pin configuration data to drivers. + +System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every +non-dedicated pin can be configured as a GPIO; and most chips have at least +several dozen of them. Programmable logic devices (like FPGAs) can easily +provide GPIOs; multifunction chips like power managers, and audio codecs +often have a few such pins to help with pin scarcity on SOCs; and there are +also "GPIO Expander" chips that connect using the I2C or SPI serial buses. +Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS +firmware knowing how they're used). + +The exact capabilities of GPIOs vary between systems. Common options: + + - Output values are writable (high=1, low=0). Some chips also have + options about how that value is driven, so that for example only one + value might be driven, supporting "wire-OR" and similar schemes for the + other value (notably, "open drain" signaling). + + - Input values are likewise readable (1, 0). Some chips support readback + of pins configured as "output", which is very useful in such "wire-OR" + cases (to support bidirectional signaling). GPIO controllers may have + input de-glitch/debounce logic, sometimes with software controls. + + - Inputs can often be used as IRQ signals, often edge triggered but + sometimes level triggered. Such IRQs may be configurable as system + wakeup events, to wake the system from a low power state. + + - Usually a GPIO will be configurable as either input or output, as needed + by different product boards; single direction ones exist too. + + - Most GPIOs can be accessed while holding spinlocks, but those accessed + through a serial bus normally can't. Some systems support both types. + +On a given board each GPIO is used for one specific purpose like monitoring +MMC/SD card insertion/removal, detecting card write-protect status, driving +a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware +watchdog, sensing a switch, and so on. + + +Common GPIO Properties +====================== + +These properties are met through all the other documents of the GPIO interface +and it is useful to understand them, especially if you need to define GPIO +mappings. + +Active-High and Active-Low +-------------------------- +It is natural to assume that a GPIO is "active" when its output signal is 1 +("high"), and inactive when it is 0 ("low"). However in practice the signal of a +GPIO may be inverted before is reaches its destination, or a device could decide +to have different conventions about what "active" means. Such decisions should +be transparent to device drivers, therefore it is possible to define a GPIO as +being either active-high ("1" means "active", the default) or active-low ("0" +means "active") so that drivers only need to worry about the logical signal and +not about what happens at the line level. + +Open Drain and Open Source +-------------------------- +Sometimes shared signals need to use "open drain" (where only the low signal +level is actually driven), or "open source" (where only the high signal level is +driven) signaling. That term applies to CMOS transistors; "open collector" is +used for TTL. A pullup or pulldown resistor causes the high or low signal level. +This is sometimes called a "wire-AND"; or more practically, from the negative +logic (low=true) perspective this is a "wire-OR". + +One common example of an open drain signal is a shared active-low IRQ line. +Also, bidirectional data bus signals sometimes use open drain signals. + +Some GPIO controllers directly support open drain and open source outputs; many +don't. When you need open drain signaling but your hardware doesn't directly +support it, there's a common idiom you can use to emulate it with any GPIO pin +that can be used as either an input or an output: + + LOW: gpiod_direction_output(gpio, 0) ... this drives the signal and overrides + the pullup. + + HIGH: gpiod_direction_input(gpio) ... this turns off the output, so the pullup + (or some other device) controls the signal. + +The same logic can be applied to emulate open source signaling, by driving the +high signal and configuring the GPIO as input for low. This open drain/open +source emulation can be handled transparently by the GPIO framework. + +If you are "driving" the signal high but gpiod_get_value(gpio) reports a low +value (after the appropriate rise time passes), you know some other component is +driving the shared signal low. That's not necessarily an error. As one common +example, that's how I2C clocks are stretched: a slave that needs a slower clock +delays the rising edge of SCK, and the I2C master adjusts its signaling rate +accordingly. diff --git a/Documentation/driver-api/gpio/legacy.rst b/Documentation/driver-api/gpio/legacy.rst new file mode 100644 index 000000000..5e9421e05 --- /dev/null +++ b/Documentation/driver-api/gpio/legacy.rst @@ -0,0 +1,770 @@ +====================== +Legacy GPIO Interfaces +====================== + +This provides an overview of GPIO access conventions on Linux. + +These calls use the gpio_* naming prefix. No other calls should use that +prefix, or the related __gpio_* prefix. + + +What is a GPIO? +=============== +A "General Purpose Input/Output" (GPIO) is a flexible software-controlled +digital signal. They are provided from many kinds of chip, and are familiar +to Linux developers working with embedded and custom hardware. Each GPIO +represents a bit connected to a particular pin, or "ball" on Ball Grid Array +(BGA) packages. Board schematics show which external hardware connects to +which GPIOs. Drivers can be written generically, so that board setup code +passes such pin configuration data to drivers. + +System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every +non-dedicated pin can be configured as a GPIO; and most chips have at least +several dozen of them. Programmable logic devices (like FPGAs) can easily +provide GPIOs; multifunction chips like power managers, and audio codecs +often have a few such pins to help with pin scarcity on SOCs; and there are +also "GPIO Expander" chips that connect using the I2C or SPI serial busses. +Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS +firmware knowing how they're used). + +The exact capabilities of GPIOs vary between systems. Common options: + + - Output values are writable (high=1, low=0). Some chips also have + options about how that value is driven, so that for example only one + value might be driven ... supporting "wire-OR" and similar schemes + for the other value (notably, "open drain" signaling). + + - Input values are likewise readable (1, 0). Some chips support readback + of pins configured as "output", which is very useful in such "wire-OR" + cases (to support bidirectional signaling). GPIO controllers may have + input de-glitch/debounce logic, sometimes with software controls. + + - Inputs can often be used as IRQ signals, often edge triggered but + sometimes level triggered. Such IRQs may be configurable as system + wakeup events, to wake the system from a low power state. + + - Usually a GPIO will be configurable as either input or output, as needed + by different product boards; single direction ones exist too. + + - Most GPIOs can be accessed while holding spinlocks, but those accessed + through a serial bus normally can't. Some systems support both types. + +On a given board each GPIO is used for one specific purpose like monitoring +MMC/SD card insertion/removal, detecting card writeprotect status, driving +a LED, configuring a transceiver, bitbanging a serial bus, poking a hardware +watchdog, sensing a switch, and so on. + + +GPIO conventions +================ +Note that this is called a "convention" because you don't need to do it this +way, and it's no crime if you don't. There **are** cases where portability +is not the main issue; GPIOs are often used for the kind of board-specific +glue logic that may even change between board revisions, and can't ever be +used on a board that's wired differently. Only least-common-denominator +functionality can be very portable. Other features are platform-specific, +and that can be critical for glue logic. + +Plus, this doesn't require any implementation framework, just an interface. +One platform might implement it as simple inline functions accessing chip +registers; another might implement it by delegating through abstractions +used for several very different kinds of GPIO controller. (There is some +optional code supporting such an implementation strategy, described later +in this document, but drivers acting as clients to the GPIO interface must +not care how it's implemented.) + +That said, if the convention is supported on their platform, drivers should +use it when possible. Platforms must select GPIOLIB if GPIO functionality +is strictly required. Drivers that can't work without +standard GPIO calls should have Kconfig entries which depend on GPIOLIB. The +GPIO calls are available, either as "real code" or as optimized-away stubs, +when drivers use the include file: + + #include <linux/gpio.h> + +If you stick to this convention then it'll be easier for other developers to +see what your code is doing, and help maintain it. + +Note that these operations include I/O barriers on platforms which need to +use them; drivers don't need to add them explicitly. + + +Identifying GPIOs +----------------- +GPIOs are identified by unsigned integers in the range 0..MAX_INT. That +reserves "negative" numbers for other purposes like marking signals as +"not available on this board", or indicating faults. Code that doesn't +touch the underlying hardware treats these integers as opaque cookies. + +Platforms define how they use those integers, and usually #define symbols +for the GPIO lines so that board-specific setup code directly corresponds +to the relevant schematics. In contrast, drivers should only use GPIO +numbers passed to them from that setup code, using platform_data to hold +board-specific pin configuration data (along with other board specific +data they need). That avoids portability problems. + +So for example one platform uses numbers 32-159 for GPIOs; while another +uses numbers 0..63 with one set of GPIO controllers, 64-79 with another +type of GPIO controller, and on one particular board 80-95 with an FPGA. +The numbers need not be contiguous; either of those platforms could also +use numbers 2000-2063 to identify GPIOs in a bank of I2C GPIO expanders. + +If you want to initialize a structure with an invalid GPIO number, use +some negative number (perhaps "-EINVAL"); that will never be valid. To +test if such number from such a structure could reference a GPIO, you +may use this predicate: + + int gpio_is_valid(int number); + +A number that's not valid will be rejected by calls which may request +or free GPIOs (see below). Other numbers may also be rejected; for +example, a number might be valid but temporarily unused on a given board. + +Whether a platform supports multiple GPIO controllers is a platform-specific +implementation issue, as are whether that support can leave "holes" in the space +of GPIO numbers, and whether new controllers can be added at runtime. Such issues +can affect things including whether adjacent GPIO numbers are both valid. + +Using GPIOs +----------- +The first thing a system should do with a GPIO is allocate it, using +the gpio_request() call; see later. + +One of the next things to do with a GPIO, often in board setup code when +setting up a platform_device using the GPIO, is mark its direction:: + + /* set as input or output, returning 0 or negative errno */ + int gpio_direction_input(unsigned gpio); + int gpio_direction_output(unsigned gpio, int value); + +The return value is zero for success, else a negative errno. It should +be checked, since the get/set calls don't have error returns and since +misconfiguration is possible. You should normally issue these calls from +a task context. However, for spinlock-safe GPIOs it's 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. + +For compatibility with legacy interfaces to GPIOs, setting the direction +of a GPIO implicitly requests that GPIO (see below) if it has not been +requested already. That compatibility is being removed from the optional +gpiolib framework. + +Setting the direction can fail if the GPIO number is invalid, or when +that particular GPIO can't be used in that mode. It's generally a bad +idea to rely on boot firmware to have set the direction correctly, since +it probably wasn't validated to do more than boot Linux. (Similarly, +that board setup code probably needs to multiplex that pin as a GPIO, +and configure pullups/pulldowns appropriately.) + + +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 +(nonthreaded) IRQ handlers and similar contexts. + +Use the following calls to access such GPIOs, +for which gpio_cansleep() will always return false (see below):: + + /* GPIO INPUT: return zero or nonzero */ + int gpio_get_value(unsigned gpio); + + /* GPIO OUTPUT */ + void gpio_set_value(unsigned gpio, 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 have no error returns because "invalid GPIO" should have +been reported earlier from gpio_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. + +Platform-specific implementations are encouraged to optimize the two +calls to access the GPIO value in cases where the GPIO number (and for +output, value) are constant. It's normal for them to need only a couple +of instructions in such cases (reading or writing a hardware register), +and not to need spinlocks. Such optimized calls can make bitbanging +applications a lot more efficient (in both space and time) than spending +dozens of instructions on subroutine calls. + + +GPIO access that may sleep +-------------------------- +Some GPIO controllers must be accessed using message based busses 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 (which requires a valid GPIO number, +which should have been previously allocated with gpio_request):: + + int gpio_cansleep(unsigned gpio); + +To access such GPIOs, a different set of accessors is defined:: + + /* GPIO INPUT: return zero or nonzero, might sleep */ + int gpio_get_value_cansleep(unsigned gpio); + + /* GPIO OUTPUT, might sleep */ + void gpio_set_value_cansleep(unsigned gpio, 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. + +**IN ADDITION** calls to setup and configure such GPIOs must be made +from contexts which may sleep, since they may need to access the GPIO +controller chip too (These setup calls are usually made from board +setup or driver probe/teardown code, so this is an easy constraint.):: + + gpio_direction_input() + gpio_direction_output() + gpio_request() + + ## gpio_request_one() + ## gpio_request_array() + ## gpio_free_array() + + gpio_free() + gpio_set_debounce() + + + +Claiming and Releasing GPIOs +---------------------------- +To help catch system configuration errors, two calls are defined:: + + /* request GPIO, returning 0 or negative errno. + * non-null labels may be useful for diagnostics. + */ + int gpio_request(unsigned gpio, const char *label); + + /* release previously-claimed GPIO */ + void gpio_free(unsigned gpio); + +Passing invalid GPIO numbers to gpio_request() will fail, as will requesting +GPIOs that have already been claimed with that call. The return value of +gpio_request() must be checked. You should normally issue these calls from +a task context. However, for spinlock-safe GPIOs it's OK to request GPIOs +before tasking is enabled, as part of early board setup. + +These calls serve two basic purposes. One is marking the signals which +are actually in use as GPIOs, for better diagnostics; systems may have +several hundred potential GPIOs, but often only a dozen are used on any +given board. Another is to catch conflicts, identifying errors when +(a) two or more drivers wrongly think they have exclusive use of that +signal, or (b) something wrongly believes it's safe to remove drivers +needed to manage a signal that's in active use. That is, requesting a +GPIO can serve as a kind of lock. + +Some platforms may also use knowledge about what GPIOs are active for +power management, such as by powering down unused chip sectors and, more +easily, gating off unused clocks. + +For GPIOs that use pins known to the pinctrl subsystem, that subsystem should +be informed of their use; a gpiolib driver's .request() operation may call +pinctrl_gpio_request(), and a gpiolib driver's .free() operation may call +pinctrl_gpio_free(). The pinctrl subsystem allows a pinctrl_gpio_request() +to succeed concurrently with a pin or pingroup being "owned" by a device for +pin multiplexing. + +Any programming of pin multiplexing hardware that is needed to route the +GPIO signal to the appropriate pin should occur within a GPIO driver's +.direction_input() or .direction_output() operations, and occur after any +setup of an output GPIO's value. This allows a glitch-free migration from a +pin's special function to GPIO. This is sometimes required when using a GPIO +to implement a workaround on signals typically driven by a non-GPIO HW block. + +Some platforms allow some or all GPIO signals to be routed to different pins. +Similarly, other aspects of the GPIO or pin may need to be configured, such as +pullup/pulldown. Platform software should arrange that any such details are +configured prior to gpio_request() being called for those GPIOs, e.g. using +the pinctrl subsystem's mapping table, so that GPIO users need not be aware +of these details. + +Also note that it's your responsibility to have stopped using a GPIO +before you free it. + +Considering in most cases GPIOs are actually configured right after they +are claimed, three additional calls are defined:: + + /* request a single GPIO, with initial configuration specified by + * 'flags', identical to gpio_request() wrt other arguments and + * return value + */ + int gpio_request_one(unsigned gpio, unsigned long flags, const char *label); + + /* request multiple GPIOs in a single call + */ + int gpio_request_array(struct gpio *array, size_t num); + + /* release multiple GPIOs in a single call + */ + void gpio_free_array(struct gpio *array, size_t num); + +where 'flags' is currently defined to specify the following properties: + + * GPIOF_DIR_IN - to configure direction as input + * GPIOF_DIR_OUT - to configure direction as output + + * GPIOF_INIT_LOW - as output, set initial level to LOW + * GPIOF_INIT_HIGH - as output, set initial level to HIGH + * GPIOF_OPEN_DRAIN - gpio pin is open drain type. + * GPIOF_OPEN_SOURCE - gpio pin is open source type. + + * GPIOF_EXPORT_DIR_FIXED - export gpio to sysfs, keep direction + * GPIOF_EXPORT_DIR_CHANGEABLE - also export, allow changing direction + +since GPIOF_INIT_* are only valid when configured as output, so group valid +combinations as: + + * GPIOF_IN - configure as input + * GPIOF_OUT_INIT_LOW - configured as output, initial level LOW + * GPIOF_OUT_INIT_HIGH - configured as output, initial level HIGH + +When setting the flag as GPIOF_OPEN_DRAIN then it will assume that pins is +open drain type. Such pins will not be driven to 1 in output mode. It is +require to connect pull-up on such pins. By enabling this flag, gpio lib will +make the direction to input when it is asked to set value of 1 in output mode +to make the pin HIGH. The pin is make to LOW by driving value 0 in output mode. + +When setting the flag as GPIOF_OPEN_SOURCE then it will assume that pins is +open source type. Such pins will not be driven to 0 in output mode. It is +require to connect pull-down on such pin. By enabling this flag, gpio lib will +make the direction to input when it is asked to set value of 0 in output mode +to make the pin LOW. The pin is make to HIGH by driving value 1 in output mode. + +In the future, these flags can be extended to support more properties. + +Further more, to ease the claim/release of multiple GPIOs, 'struct gpio' is +introduced to encapsulate all three fields as:: + + struct gpio { + unsigned gpio; + unsigned long flags; + const char *label; + }; + +A typical example of usage:: + + static struct gpio leds_gpios[] = { + { 32, GPIOF_OUT_INIT_HIGH, "Power LED" }, /* default to ON */ + { 33, GPIOF_OUT_INIT_LOW, "Green LED" }, /* default to OFF */ + { 34, GPIOF_OUT_INIT_LOW, "Red LED" }, /* default to OFF */ + { 35, GPIOF_OUT_INIT_LOW, "Blue LED" }, /* default to OFF */ + { ... }, + }; + + err = gpio_request_one(31, GPIOF_IN, "Reset Button"); + if (err) + ... + + err = gpio_request_array(leds_gpios, ARRAY_SIZE(leds_gpios)); + if (err) + ... + + gpio_free_array(leds_gpios, ARRAY_SIZE(leds_gpios)); + + +GPIOs mapped to IRQs +-------------------- +GPIO numbers are unsigned integers; so are IRQ numbers. These make up +two logically distinct namespaces (GPIO 0 need not use IRQ 0). You can +map between them using calls like:: + + /* map GPIO numbers to IRQ numbers */ + int gpio_to_irq(unsigned gpio); + + /* map IRQ numbers to GPIO numbers (avoid using this) */ + int irq_to_gpio(unsigned irq); + +Those return either the corresponding number in the other namespace, or +else a negative errno code if the mapping can't be done. (For example, +some GPIOs can't be used as IRQs.) It is an unchecked error to use a GPIO +number that wasn't set up as an input using gpio_direction_input(), or +to use an IRQ number that didn't originally come from gpio_to_irq(). + +These two mapping calls are expected to cost on the order of a single +addition or subtraction. They're not allowed to sleep. + +Non-error values returned from gpio_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. + +Non-error values returned from irq_to_gpio() would most commonly be used +with gpio_get_value(), for example to initialize or update driver state +when the IRQ is edge-triggered. Note that some platforms don't support +this reverse mapping, so you should avoid using it. + + +Emulating Open Drain Signals +---------------------------- +Sometimes shared signals need to use "open drain" signaling, where only the +low signal level is actually driven. (That term applies to CMOS transistors; +"open collector" is used for TTL.) A pullup resistor causes the high signal +level. This is sometimes called a "wire-AND"; or more practically, from the +negative logic (low=true) perspective this is a "wire-OR". + +One common example of an open drain signal is a shared active-low IRQ line. +Also, bidirectional data bus signals sometimes use open drain signals. + +Some GPIO controllers directly support open drain outputs; many don't. When +you need open drain signaling but your hardware doesn't directly support it, +there's a common idiom you can use to emulate it with any GPIO pin that can +be used as either an input or an output: + + LOW: gpio_direction_output(gpio, 0) ... this drives the signal + and overrides the pullup. + + HIGH: gpio_direction_input(gpio) ... this turns off the output, + so the pullup (or some other device) controls the signal. + +If you are "driving" the signal high but gpio_get_value(gpio) reports a low +value (after the appropriate rise time passes), you know some other component +is driving the shared signal low. That's not necessarily an error. As one +common example, that's how I2C clocks are stretched: a slave that needs a +slower clock delays the rising edge of SCK, and the I2C master adjusts its +signaling rate accordingly. + + +GPIO controllers and the pinctrl subsystem +------------------------------------------ + +A GPIO controller on a SOC might be tightly coupled with the pinctrl +subsystem, in the sense that the pins can be used by other functions +together with an optional gpio feature. We have already covered the +case where e.g. a GPIO controller need to reserve a pin or set the +direction of a pin by calling any of:: + + pinctrl_gpio_request() + pinctrl_gpio_free() + pinctrl_gpio_direction_input() + pinctrl_gpio_direction_output() + +But how does the pin control subsystem cross-correlate the GPIO +numbers (which are a global business) to a certain pin on a certain +pin controller? + +This is done by registering "ranges" of pins, which are essentially +cross-reference tables. These are described in +Documentation/driver-api/pinctl.rst + +While the pin allocation is totally managed by the pinctrl subsystem, +gpio (under gpiolib) is still maintained by gpio drivers. It may happen +that different pin ranges in a SoC is managed by different gpio drivers. + +This makes it logical to let gpio drivers announce their pin ranges to +the pin ctrl subsystem before it will call 'pinctrl_gpio_request' in order +to request the corresponding pin to be prepared by the pinctrl subsystem +before any gpio usage. + +For this, the gpio controller can register its pin range with pinctrl +subsystem. There are two ways of doing it currently: with or without DT. + +For with DT support refer to Documentation/devicetree/bindings/gpio/gpio.txt. + +For non-DT support, user can call gpiochip_add_pin_range() with appropriate +parameters to register a range of gpio pins with a pinctrl driver. For this +exact name string of pinctrl device has to be passed as one of the +argument to this routine. + + +What do these conventions omit? +=============================== +One of the biggest things these conventions omit is pin multiplexing, since +this is highly chip-specific and nonportable. One platform might not need +explicit multiplexing; another might have just two options for use of any +given pin; another might have eight options per pin; another might be able +to route a given GPIO to any one of several pins. (Yes, those examples all +come from systems that run Linux today.) + +Related to multiplexing is configuration and enabling of the pullups or +pulldowns integrated on some platforms. Not all platforms support them, +or support them in the same way; and any given board might use external +pullups (or pulldowns) so that the on-chip ones should not be used. +(When a circuit needs 5 kOhm, on-chip 100 kOhm resistors won't do.) +Likewise drive strength (2 mA vs 20 mA) and voltage (1.8V vs 3.3V) is a +platform-specific issue, as are models like (not) having a one-to-one +correspondence between configurable pins and GPIOs. + +There are other system-specific mechanisms that are not specified here, +like the aforementioned options for input de-glitching and wire-OR output. +Hardware may support reading or writing GPIOs in gangs, but that's usually +configuration dependent: for GPIOs sharing the same bank. (GPIOs are +commonly grouped in banks of 16 or 32, with a given SOC having several such +banks.) Some systems can trigger IRQs from output GPIOs, or read values +from pins not managed as GPIOs. Code relying on such mechanisms will +necessarily be nonportable. + +Dynamic definition of GPIOs is not currently standard; for example, as +a side effect of configuring an add-on board with some GPIO expanders. + + +GPIO implementor's framework (OPTIONAL) +======================================= +As noted earlier, there is an optional implementation framework making it +easier for platforms to support different kinds of GPIO controller using +the same programming interface. This framework is called "gpiolib". + +As a debugging aid, if debugfs is available a /sys/kernel/debug/gpio file +will be found there. That will list all the controllers registered through +this framework, and the state of the GPIOs currently in use. + + +Controller Drivers: gpio_chip +----------------------------- +In this framework each GPIO controller is packaged as a "struct gpio_chip" +with information common to each controller of that type: + + - methods to establish GPIO direction + - methods used to access GPIO values + - flag saying whether calls to its methods may sleep + - optional debugfs dump method (showing extra state like pullup config) + - label for diagnostics + +There is also per-instance data, which may come from device.platform_data: +the number of its first GPIO, and how many GPIOs it exposes. + +The code implementing a gpio_chip should support multiple instances of the +controller, possibly using the driver model. That code will configure each +gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be +rare; use gpiochip_remove() when it is unavoidable. + +Most often a gpio_chip is part of an instance-specific structure with state +not exposed by the GPIO interfaces, such as addressing, power management, +and more. Chips such as codecs will have complex non-GPIO state. + +Any debugfs dump method should normally ignore signals which haven't been +requested as GPIOs. They can use gpiochip_is_requested(), which returns +either NULL or the label associated with that GPIO when it was requested. + + +Platform Support +---------------- +To force-enable this framework, a platform's Kconfig will "select" GPIOLIB, +else it is up to the user to configure support for GPIO. + +It may also provide a custom value for ARCH_NR_GPIOS, so that it better +reflects the number of GPIOs in actual use on that platform, without +wasting static table space. (It should count both built-in/SoC GPIOs and +also ones on GPIO expanders. + +If neither of these options are selected, the platform does not support +GPIOs through GPIO-lib and the code cannot be enabled by the user. + +Trivial implementations of those functions can directly use framework +code, which always dispatches through the gpio_chip:: + + #define gpio_get_value __gpio_get_value + #define gpio_set_value __gpio_set_value + #define gpio_cansleep __gpio_cansleep + +Fancier implementations could instead define those as inline functions with +logic optimizing access to specific SOC-based GPIOs. For example, if the +referenced GPIO is the constant "12", getting or setting its value could +cost as little as two or three instructions, never sleeping. When such an +optimization is not possible those calls must delegate to the framework +code, costing at least a few dozen instructions. For bitbanged I/O, such +instruction savings can be significant. + +For SOCs, platform-specific code defines and registers gpio_chip instances +for each bank of on-chip GPIOs. Those GPIOs should be numbered/labeled to +match chip vendor documentation, and directly match board schematics. They +may well start at zero and go up to a platform-specific limit. Such GPIOs +are normally integrated into platform initialization to make them always be +available, from arch_initcall() or earlier; they can often serve as IRQs. + + +Board Support +------------- +For external GPIO controllers -- such as I2C or SPI expanders, ASICs, multi +function devices, FPGAs or CPLDs -- most often board-specific code handles +registering controller devices and ensures that their drivers know what GPIO +numbers to use with gpiochip_add(). Their numbers often start right after +platform-specific GPIOs. + +For example, board setup code could create structures identifying the range +of GPIOs that chip will expose, and passes them to each GPIO expander chip +using platform_data. Then the chip driver's probe() routine could pass that +data to gpiochip_add(). + +Initialization order can be important. For example, when a device relies on +an I2C-based GPIO, its probe() routine should only be called after that GPIO +becomes available. That may mean the device should not be registered until +calls for that GPIO can work. One way to address such dependencies is for +such gpio_chip controllers to provide setup() and teardown() callbacks to +board specific code; those board specific callbacks would register devices +once all the necessary resources are available, and remove them later when +the GPIO controller device becomes unavailable. + + +Sysfs Interface for Userspace (OPTIONAL) +======================================== +Platforms which use the "gpiolib" implementors framework may choose to +configure a sysfs user interface to GPIOs. This is different from the +debugfs interface, since it provides control over GPIO direction and +value instead of just showing a gpio state summary. Plus, it could be +present on production systems without debugging support. + +Given appropriate hardware documentation for the system, userspace could +know for example that GPIO #23 controls the write protect line used to +protect boot loader segments in flash memory. System upgrade procedures +may need to temporarily remove that protection, first importing a GPIO, +then changing its output state, then updating the code before re-enabling +the write protection. In normal use, GPIO #23 would never be touched, +and the kernel would have no need to know about it. + +Again depending on appropriate hardware documentation, on some systems +userspace GPIO can be used to determine system configuration data that +standard kernels won't know about. And for some tasks, simple userspace +GPIO drivers could be all that the system really needs. + +Note that standard kernel drivers exist for common "LEDs and Buttons" +GPIO tasks: "leds-gpio" and "gpio_keys", respectively. Use those +instead of talking directly to the GPIOs; they integrate with kernel +frameworks better than your userspace code could. + + +Paths in Sysfs +-------------- +There are three kinds of entry in /sys/class/gpio: + + - Control interfaces used to get userspace control over GPIOs; + + - GPIOs themselves; and + + - GPIO controllers ("gpio_chip" instances). + +That's in addition to standard files including the "device" symlink. + +The control interfaces are write-only: + + /sys/class/gpio/ + + "export" ... Userspace may ask the kernel to export control of + a GPIO to userspace by writing its number to this file. + + Example: "echo 19 > export" will create a "gpio19" node + for GPIO #19, if that's not requested by kernel code. + + "unexport" ... Reverses the effect of exporting to userspace. + + Example: "echo 19 > unexport" will remove a "gpio19" + node exported using the "export" file. + +GPIO signals have paths like /sys/class/gpio/gpio42/ (for GPIO #42) +and have the following read/write attributes: + + /sys/class/gpio/gpioN/ + + "direction" ... reads as either "in" or "out". This value may + normally be written. Writing as "out" defaults to + initializing the value as low. To ensure glitch free + operation, values "low" and "high" may be written to + configure the GPIO as an output with that initial value. + + Note that this attribute *will not exist* if the kernel + doesn't support changing the direction of a GPIO, or + it was exported by kernel code that didn't explicitly + allow userspace to reconfigure this GPIO's direction. + + "value" ... reads as either 0 (low) or 1 (high). If the GPIO + is configured as an output, this value may be written; + any nonzero value is treated as high. + + If the pin can be configured as interrupt-generating interrupt + and if it has been configured to generate interrupts (see the + description of "edge"), you can poll(2) on that file and + poll(2) will return whenever the interrupt was triggered. If + you use poll(2), set the events POLLPRI and POLLERR. If you + use select(2), set the file descriptor in exceptfds. After + poll(2) returns, either lseek(2) to the beginning of the sysfs + file and read the new value or close the file and re-open it + to read the value. + + "edge" ... reads as either "none", "rising", "falling", or + "both". Write these strings to select the signal edge(s) + that will make poll(2) on the "value" file return. + + This file exists only if the pin can be configured as an + interrupt generating input pin. + + "active_low" ... reads as either 0 (false) or 1 (true). Write + any nonzero value to invert the value attribute both + for reading and writing. Existing and subsequent + poll(2) support configuration via the edge attribute + for "rising" and "falling" edges will follow this + setting. + +GPIO controllers have paths like /sys/class/gpio/gpiochip42/ (for the +controller implementing GPIOs starting at #42) and have the following +read-only attributes: + + /sys/class/gpio/gpiochipN/ + + "base" ... same as N, the first GPIO managed by this chip + + "label" ... provided for diagnostics (not always unique) + + "ngpio" ... how many GPIOs this manges (N to N + ngpio - 1) + +Board documentation should in most cases cover what GPIOs are used for +what purposes. However, those numbers are not always stable; GPIOs on +a daughtercard might be different depending on the base board being used, +or other cards in the stack. In such cases, you may need to use the +gpiochip nodes (possibly in conjunction with schematics) to determine +the correct GPIO number to use for a given signal. + + +Exporting from Kernel code +-------------------------- +Kernel code can explicitly manage exports of GPIOs which have already been +requested using gpio_request():: + + /* export the GPIO to userspace */ + int gpio_export(unsigned gpio, bool direction_may_change); + + /* reverse gpio_export() */ + void gpio_unexport(); + + /* create a sysfs link to an exported GPIO node */ + int gpio_export_link(struct device *dev, const char *name, + unsigned gpio) + +After a kernel driver requests a GPIO, it may only be made available in +the sysfs interface by gpio_export(). The driver can control whether the +signal direction may change. This helps drivers prevent userspace code +from accidentally clobbering important system state. + +This explicit exporting can help with debugging (by making some kinds +of experiments easier), or can provide an always-there interface that's +suitable for documenting as part of a board support package. + +After the GPIO has been exported, gpio_export_link() allows creating +symlinks from elsewhere in sysfs to the GPIO sysfs node. Drivers can +use this to provide the interface under their own device in sysfs with +a descriptive name. + + +API Reference +============= + +The functions listed in this section are deprecated. The GPIO descriptor based +API should be used in new code. + +.. kernel-doc:: drivers/gpio/gpiolib-legacy.c + :export: |