1GPIO Descriptor Consumer Interface 2================================== 3 4This document describes the consumer interface of the GPIO framework. Note that 5it describes the new descriptor-based interface. For a description of the 6deprecated integer-based GPIO interface please refer to gpio-legacy.txt. 7 8 9Guidelines for GPIOs consumers 10============================== 11 12Drivers that can't work without standard GPIO calls should have Kconfig entries 13that depend on GPIOLIB. The functions that allow a driver to obtain and use 14GPIOs are available by including the following file: 15 16 #include <linux/gpio/consumer.h> 17 18All the functions that work with the descriptor-based GPIO interface are 19prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No 20other function in the kernel should use these prefixes. 21 22 23Obtaining and Disposing GPIOs 24============================= 25 26With the descriptor-based interface, GPIOs are identified with an opaque, 27non-forgeable handler that must be obtained through a call to one of the 28gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the 29device that will use the GPIO and the function the requested GPIO is supposed to 30fulfill: 31 32 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id, 33 enum gpiod_flags flags) 34 35If a function is implemented by using several GPIOs together (e.g. a simple LED 36device that displays digits), an additional index argument can be specified: 37 38 struct gpio_desc *gpiod_get_index(struct device *dev, 39 const char *con_id, unsigned int idx, 40 enum gpiod_flags flags) 41 42The flags parameter is used to optionally specify a direction and initial value 43for the GPIO. Values can be: 44 45* GPIOD_ASIS or 0 to not initialize the GPIO at all. The direction must be set 46 later with one of the dedicated functions. 47* GPIOD_IN to initialize the GPIO as input. 48* GPIOD_OUT_LOW to initialize the GPIO as output with a value of 0. 49* GPIOD_OUT_HIGH to initialize the GPIO as output with a value of 1. 50 51Both functions return either a valid GPIO descriptor, or an error code checkable 52with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned 53if and only if no GPIO has been assigned to the device/function/index triplet, 54other error codes are used for cases where a GPIO has been assigned but an error 55occurred while trying to acquire it. This is useful to discriminate between mere 56errors and an absence of GPIO for optional GPIO parameters. For the common 57pattern where a GPIO is optional, the gpiod_get_optional() and 58gpiod_get_index_optional() functions can be used. These functions return NULL 59instead of -ENOENT if no GPIO has been assigned to the requested function: 60 61 62 struct gpio_desc *gpiod_get_optional(struct device *dev, 63 const char *con_id, 64 enum gpiod_flags flags) 65 66 struct gpio_desc *gpiod_get_index_optional(struct device *dev, 67 const char *con_id, 68 unsigned int index, 69 enum gpiod_flags flags) 70 71Device-managed variants of these functions are also defined: 72 73 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id, 74 enum gpiod_flags flags) 75 76 struct gpio_desc *devm_gpiod_get_index(struct device *dev, 77 const char *con_id, 78 unsigned int idx, 79 enum gpiod_flags flags) 80 81 struct gpio_desc *devm_gpiod_get_optional(struct device *dev, 82 const char *con_id, 83 enum gpiod_flags flags) 84 85 struct gpio_desc * devm_gpiod_get_index_optional(struct device *dev, 86 const char *con_id, 87 unsigned int index, 88 enum gpiod_flags flags) 89 90A GPIO descriptor can be disposed of using the gpiod_put() function: 91 92 void gpiod_put(struct gpio_desc *desc) 93 94It is strictly forbidden to use a descriptor after calling this function. The 95device-managed variant is, unsurprisingly: 96 97 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) 98 99 100Using GPIOs 101=========== 102 103Setting Direction 104----------------- 105The first thing a driver must do with a GPIO is setting its direction. If no 106direction-setting flags have been given to gpiod_get*(), this is done by 107invoking one of the gpiod_direction_*() functions: 108 109 int gpiod_direction_input(struct gpio_desc *desc) 110 int gpiod_direction_output(struct gpio_desc *desc, int value) 111 112The return value is zero for success, else a negative errno. It should be 113checked, since the get/set calls don't return errors and since misconfiguration 114is possible. You should normally issue these calls from a task context. However, 115for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part 116of early board setup. 117 118For output GPIOs, the value provided becomes the initial output value. This 119helps avoid signal glitching during system startup. 120 121A driver can also query the current direction of a GPIO: 122 123 int gpiod_get_direction(const struct gpio_desc *desc) 124 125This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT. 126 127Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO 128without setting its direction first is illegal and will result in undefined 129behavior!** 130 131 132Spinlock-Safe GPIO Access 133------------------------- 134Most GPIO controllers can be accessed with memory read/write instructions. Those 135don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ 136handlers and similar contexts. 137 138Use the following calls to access GPIOs from an atomic context: 139 140 int gpiod_get_value(const struct gpio_desc *desc); 141 void gpiod_set_value(struct gpio_desc *desc, int value); 142 143The values are boolean, zero for low, nonzero for high. When reading the value 144of an output pin, the value returned should be what's seen on the pin. That 145won't always match the specified output value, because of issues including 146open-drain signaling and output latencies. 147 148The get/set calls do not return errors because "invalid GPIO" should have been 149reported earlier from gpiod_direction_*(). However, note that not all platforms 150can read the value of output pins; those that can't should always return zero. 151Also, using these calls for GPIOs that can't safely be accessed without sleeping 152(see below) is an error. 153 154 155GPIO Access That May Sleep 156-------------------------- 157Some GPIO controllers must be accessed using message based buses like I2C or 158SPI. Commands to read or write those GPIO values require waiting to get to the 159head of a queue to transmit a command and get its response. This requires 160sleeping, which can't be done from inside IRQ handlers. 161 162Platforms that support this type of GPIO distinguish them from other GPIOs by 163returning nonzero from this call: 164 165 int gpiod_cansleep(const struct gpio_desc *desc) 166 167To access such GPIOs, a different set of accessors is defined: 168 169 int gpiod_get_value_cansleep(const struct gpio_desc *desc) 170 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value) 171 172Accessing such GPIOs requires a context which may sleep, for example a threaded 173IRQ handler, and those accessors must be used instead of spinlock-safe 174accessors without the cansleep() name suffix. 175 176Other than the fact that these accessors might sleep, and will work on GPIOs 177that can't be accessed from hardIRQ handlers, these calls act the same as the 178spinlock-safe calls. 179 180 181Active-low State and Raw GPIO Values 182------------------------------------ 183Device drivers like to manage the logical state of a GPIO, i.e. the value their 184device will actually receive, no matter what lies between it and the GPIO line. 185In some cases, it might make sense to control the actual GPIO line value. The 186following set of calls ignore the active-low property of a GPIO and work on the 187raw line value: 188 189 int gpiod_get_raw_value(const struct gpio_desc *desc) 190 void gpiod_set_raw_value(struct gpio_desc *desc, int value) 191 int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc) 192 void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value) 193 int gpiod_direction_output_raw(struct gpio_desc *desc, int value) 194 195The active-low state of a GPIO can also be queried using the following call: 196 197 int gpiod_is_active_low(const struct gpio_desc *desc) 198 199Note that these functions should only be used with great moderation ; a driver 200should not have to care about the physical line level. 201 202GPIOs mapped to IRQs 203-------------------- 204GPIO lines can quite often be used as IRQs. You can get the IRQ number 205corresponding to a given GPIO using the following call: 206 207 int gpiod_to_irq(const struct gpio_desc *desc) 208 209It will return an IRQ number, or an negative errno code if the mapping can't be 210done (most likely because that particular GPIO cannot be used as IRQ). It is an 211unchecked error to use a GPIO that wasn't set up as an input using 212gpiod_direction_input(), or to use an IRQ number that didn't originally come 213from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. 214 215Non-error values returned from gpiod_to_irq() can be passed to request_irq() or 216free_irq(). They will often be stored into IRQ resources for platform devices, 217by the board-specific initialization code. Note that IRQ trigger options are 218part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup 219capabilities. 220 221 222Interacting With the Legacy GPIO Subsystem 223========================================== 224Many kernel subsystems still handle GPIOs using the legacy integer-based 225interface. Although it is strongly encouraged to upgrade them to the safer 226descriptor-based API, the following two functions allow you to convert a GPIO 227descriptor into the GPIO integer namespace and vice-versa: 228 229 int desc_to_gpio(const struct gpio_desc *desc) 230 struct gpio_desc *gpio_to_desc(unsigned gpio) 231 232The GPIO number returned by desc_to_gpio() can be safely used as long as the 233GPIO descriptor has not been freed. All the same, a GPIO number passed to 234gpio_to_desc() must have been properly acquired, and usage of the returned GPIO 235descriptor is only possible after the GPIO number has been released. 236 237Freeing a GPIO obtained by one API with the other API is forbidden and an 238unchecked error. 239