Based on kernel version 3.16. Page generated on 2014-08-06 21:39 EST.
1 GPIO Descriptor Consumer Interface 2 ================================== 3 4 This document describes the consumer interface of the GPIO framework. Note that 5 it describes the new descriptor-based interface. For a description of the 6 deprecated integer-based GPIO interface please refer to gpio-legacy.txt. 7 8 9 Guidelines for GPIOs consumers 10 ============================== 11 12 Drivers that can't work without standard GPIO calls should have Kconfig entries 13 that depend on GPIOLIB. The functions that allow a driver to obtain and use 14 GPIOs are available by including the following file: 15 16 #include <linux/gpio/consumer.h> 17 18 All the functions that work with the descriptor-based GPIO interface are 19 prefixed with gpiod_. The gpio_ prefix is used for the legacy interface. No 20 other function in the kernel should use these prefixes. 21 22 23 Obtaining and Disposing GPIOs 24 ============================= 25 26 With the descriptor-based interface, GPIOs are identified with an opaque, 27 non-forgeable handler that must be obtained through a call to one of the 28 gpiod_get() functions. Like many other kernel subsystems, gpiod_get() takes the 29 device that will use the GPIO and the function the requested GPIO is supposed to 30 fulfill: 31 32 struct gpio_desc *gpiod_get(struct device *dev, const char *con_id) 33 34 If a function is implemented by using several GPIOs together (e.g. a simple LED 35 device that displays digits), an additional index argument can be specified: 36 37 struct gpio_desc *gpiod_get_index(struct device *dev, 38 const char *con_id, unsigned int idx) 39 40 Both functions return either a valid GPIO descriptor, or an error code checkable 41 with IS_ERR() (they will never return a NULL pointer). -ENOENT will be returned 42 if and only if no GPIO has been assigned to the device/function/index triplet, 43 other error codes are used for cases where a GPIO has been assigned but an error 44 occurred while trying to acquire it. This is useful to discriminate between mere 45 errors and an absence of GPIO for optional GPIO parameters. 46 47 Device-managed variants of these functions are also defined: 48 49 struct gpio_desc *devm_gpiod_get(struct device *dev, const char *con_id) 50 51 struct gpio_desc *devm_gpiod_get_index(struct device *dev, 52 const char *con_id, 53 unsigned int idx) 54 55 A GPIO descriptor can be disposed of using the gpiod_put() function: 56 57 void gpiod_put(struct gpio_desc *desc) 58 59 It is strictly forbidden to use a descriptor after calling this function. The 60 device-managed variant is, unsurprisingly: 61 62 void devm_gpiod_put(struct device *dev, struct gpio_desc *desc) 63 64 65 Using GPIOs 66 =========== 67 68 Setting Direction 69 ----------------- 70 The first thing a driver must do with a GPIO is setting its direction. This is 71 done by invoking one of the gpiod_direction_*() functions: 72 73 int gpiod_direction_input(struct gpio_desc *desc) 74 int gpiod_direction_output(struct gpio_desc *desc, int value) 75 76 The return value is zero for success, else a negative errno. It should be 77 checked, since the get/set calls don't return errors and since misconfiguration 78 is possible. You should normally issue these calls from a task context. However, 79 for spinlock-safe GPIOs it is OK to use them before tasking is enabled, as part 80 of early board setup. 81 82 For output GPIOs, the value provided becomes the initial output value. This 83 helps avoid signal glitching during system startup. 84 85 A driver can also query the current direction of a GPIO: 86 87 int gpiod_get_direction(const struct gpio_desc *desc) 88 89 This function will return either GPIOF_DIR_IN or GPIOF_DIR_OUT. 90 91 Be aware that there is no default direction for GPIOs. Therefore, **using a GPIO 92 without setting its direction first is illegal and will result in undefined 93 behavior!** 94 95 96 Spinlock-Safe GPIO Access 97 ------------------------- 98 Most GPIO controllers can be accessed with memory read/write instructions. Those 99 don't need to sleep, and can safely be done from inside hard (non-threaded) IRQ 100 handlers and similar contexts. 101 102 Use the following calls to access GPIOs from an atomic context: 103 104 int gpiod_get_value(const struct gpio_desc *desc); 105 void gpiod_set_value(struct gpio_desc *desc, int value); 106 107 The values are boolean, zero for low, nonzero for high. When reading the value 108 of an output pin, the value returned should be what's seen on the pin. That 109 won't always match the specified output value, because of issues including 110 open-drain signaling and output latencies. 111 112 The get/set calls do not return errors because "invalid GPIO" should have been 113 reported earlier from gpiod_direction_*(). However, note that not all platforms 114 can read the value of output pins; those that can't should always return zero. 115 Also, using these calls for GPIOs that can't safely be accessed without sleeping 116 (see below) is an error. 117 118 119 GPIO Access That May Sleep 120 -------------------------- 121 Some GPIO controllers must be accessed using message based buses like I2C or 122 SPI. Commands to read or write those GPIO values require waiting to get to the 123 head of a queue to transmit a command and get its response. This requires 124 sleeping, which can't be done from inside IRQ handlers. 125 126 Platforms that support this type of GPIO distinguish them from other GPIOs by 127 returning nonzero from this call: 128 129 int gpiod_cansleep(const struct gpio_desc *desc) 130 131 To access such GPIOs, a different set of accessors is defined: 132 133 int gpiod_get_value_cansleep(const struct gpio_desc *desc) 134 void gpiod_set_value_cansleep(struct gpio_desc *desc, int value) 135 136 Accessing such GPIOs requires a context which may sleep, for example a threaded 137 IRQ handler, and those accessors must be used instead of spinlock-safe 138 accessors without the cansleep() name suffix. 139 140 Other than the fact that these accessors might sleep, and will work on GPIOs 141 that can't be accessed from hardIRQ handlers, these calls act the same as the 142 spinlock-safe calls. 143 144 145 Active-low State and Raw GPIO Values 146 ------------------------------------ 147 Device drivers like to manage the logical state of a GPIO, i.e. the value their 148 device will actually receive, no matter what lies between it and the GPIO line. 149 In some cases, it might make sense to control the actual GPIO line value. The 150 following set of calls ignore the active-low property of a GPIO and work on the 151 raw line value: 152 153 int gpiod_get_raw_value(const struct gpio_desc *desc) 154 void gpiod_set_raw_value(struct gpio_desc *desc, int value) 155 int gpiod_get_raw_value_cansleep(const struct gpio_desc *desc) 156 void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value) 157 int gpiod_direction_output_raw(struct gpio_desc *desc, int value) 158 159 The active-low state of a GPIO can also be queried using the following call: 160 161 int gpiod_is_active_low(const struct gpio_desc *desc) 162 163 Note that these functions should only be used with great moderation ; a driver 164 should not have to care about the physical line level. 165 166 GPIOs mapped to IRQs 167 -------------------- 168 GPIO lines can quite often be used as IRQs. You can get the IRQ number 169 corresponding to a given GPIO using the following call: 170 171 int gpiod_to_irq(const struct gpio_desc *desc) 172 173 It will return an IRQ number, or an negative errno code if the mapping can't be 174 done (most likely because that particular GPIO cannot be used as IRQ). It is an 175 unchecked error to use a GPIO that wasn't set up as an input using 176 gpiod_direction_input(), or to use an IRQ number that didn't originally come 177 from gpiod_to_irq(). gpiod_to_irq() is not allowed to sleep. 178 179 Non-error values returned from gpiod_to_irq() can be passed to request_irq() or 180 free_irq(). They will often be stored into IRQ resources for platform devices, 181 by the board-specific initialization code. Note that IRQ trigger options are 182 part of the IRQ interface, e.g. IRQF_TRIGGER_FALLING, as are system wakeup 183 capabilities. 184 185 186 Interacting With the Legacy GPIO Subsystem 187 ========================================== 188 Many kernel subsystems still handle GPIOs using the legacy integer-based 189 interface. Although it is strongly encouraged to upgrade them to the safer 190 descriptor-based API, the following two functions allow you to convert a GPIO 191 descriptor into the GPIO integer namespace and vice-versa: 192 193 int desc_to_gpio(const struct gpio_desc *desc) 194 struct gpio_desc *gpio_to_desc(unsigned gpio) 195 196 The GPIO number returned by desc_to_gpio() can be safely used as long as the 197 GPIO descriptor has not been freed. All the same, a GPIO number passed to 198 gpio_to_desc() must have been properly acquired, and usage of the returned GPIO 199 descriptor is only possible after the GPIO number has been released. 200 201 Freeing a GPIO obtained by one API with the other API is forbidden and an 202 unchecked error.