Based on kernel version 4.2. Page generated on 2015-09-09 12:13 EST.
1 GPIO Descriptor Driver Interface 2 ================================ 3 4 This document serves as a guide for GPIO chip drivers writers. Note that it 5 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 Each GPIO controller driver needs to include the following header, which defines 9 the structures used to define a GPIO driver: 10 11 #include <linux/gpio/driver.h> 12 13 14 Internal Representation of GPIOs 15 ================================ 16 17 Inside a GPIO driver, individual GPIOs are identified by their hardware number, 18 which is a unique number between 0 and n, n being the number of GPIOs managed by 19 the chip. This number is purely internal: the hardware number of a particular 20 GPIO descriptor is never made visible outside of the driver. 21 22 On top of this internal number, each GPIO also need to have a global number in 23 the integer GPIO namespace so that it can be used with the legacy GPIO 24 interface. Each chip must thus have a "base" number (which can be automatically 25 assigned), and for each GPIO the global number will be (base + hardware number). 26 Although the integer representation is considered deprecated, it still has many 27 users and thus needs to be maintained. 28 29 So for example one platform could use numbers 32-159 for GPIOs, with a 30 controller defining 128 GPIOs at a "base" of 32 ; while another platform uses 31 numbers 0..63 with one set of GPIO controllers, 64-79 with another type of GPIO 32 controller, and on one particular board 80-95 with an FPGA. The numbers need not 33 be contiguous; either of those platforms could also use numbers 2000-2063 to 34 identify GPIOs in a bank of I2C GPIO expanders. 35 36 37 Controller Drivers: gpio_chip 38 ============================= 39 40 In the gpiolib framework each GPIO controller is packaged as a "struct 41 gpio_chip" (see linux/gpio/driver.h for its complete definition) with members 42 common to each controller of that type: 43 44 - methods to establish GPIO direction 45 - methods used to access GPIO values 46 - method to return the IRQ number associated to a given GPIO 47 - flag saying whether calls to its methods may sleep 48 - optional debugfs dump method (showing extra state like pullup config) 49 - optional base number (will be automatically assigned if omitted) 50 - label for diagnostics and GPIOs mapping using platform data 51 52 The code implementing a gpio_chip should support multiple instances of the 53 controller, possibly using the driver model. That code will configure each 54 gpio_chip and issue gpiochip_add(). Removing a GPIO controller should be rare; 55 use gpiochip_remove() when it is unavoidable. 56 57 Most often a gpio_chip is part of an instance-specific structure with state not 58 exposed by the GPIO interfaces, such as addressing, power management, and more. 59 Chips such as codecs will have complex non-GPIO state. 60 61 Any debugfs dump method should normally ignore signals which haven't been 62 requested as GPIOs. They can use gpiochip_is_requested(), which returns either 63 NULL or the label associated with that GPIO when it was requested. 64 65 66 GPIO drivers providing IRQs 67 --------------------------- 68 It is custom that GPIO drivers (GPIO chips) are also providing interrupts, 69 most often cascaded off a parent interrupt controller, and in some special 70 cases the GPIO logic is melded with a SoC's primary interrupt controller. 71 72 The IRQ portions of the GPIO block are implemented using an irqchip, using 73 the header <linux/irq.h>. So basically such a driver is utilizing two sub- 74 systems simultaneously: gpio and irq. 75 76 GPIO irqchips usually fall in one of two categories: 77 78 * CHAINED GPIO irqchips: these are usually the type that is embedded on 79 an SoC. This means that there is a fast IRQ handler for the GPIOs that 80 gets called in a chain from the parent IRQ handler, most typically the 81 system interrupt controller. This means the GPIO irqchip is registered 82 using irq_set_chained_handler() or the corresponding 83 gpiochip_set_chained_irqchip() helper function, and the GPIO irqchip 84 handler will be called immediately from the parent irqchip, while 85 holding the IRQs disabled. The GPIO irqchip will then end up calling 86 something like this sequence in its interrupt handler: 87 88 static irqreturn_t tc3589x_gpio_irq(int irq, void *data) 89 chained_irq_enter(...); 90 generic_handle_irq(...); 91 chained_irq_exit(...); 92 93 Chained GPIO irqchips typically can NOT set the .can_sleep flag on 94 struct gpio_chip, as everything happens directly in the callbacks. 95 96 * NESTED THREADED GPIO irqchips: these are off-chip GPIO expanders and any 97 other GPIO irqchip residing on the other side of a sleeping bus. Of course 98 such drivers that need slow bus traffic to read out IRQ status and similar, 99 traffic which may in turn incur other IRQs to happen, cannot be handled 100 in a quick IRQ handler with IRQs disabled. Instead they need to spawn a 101 thread and then mask the parent IRQ line until the interrupt is handled 102 by the driver. The hallmark of this driver is to call something like 103 this in its interrupt handler: 104 105 static irqreturn_t tc3589x_gpio_irq(int irq, void *data) 106 ... 107 handle_nested_irq(irq); 108 109 The hallmark of threaded GPIO irqchips is that they set the .can_sleep 110 flag on struct gpio_chip to true, indicating that this chip may sleep 111 when accessing the GPIOs. 112 113 To help out in handling the set-up and management of GPIO irqchips and the 114 associated irqdomain and resource allocation callbacks, the gpiolib has 115 some helpers that can be enabled by selecting the GPIOLIB_IRQCHIP Kconfig 116 symbol: 117 118 * gpiochip_irqchip_add(): adds an irqchip to a gpiochip. It will pass 119 the struct gpio_chip* for the chip to all IRQ callbacks, so the callbacks 120 need to embed the gpio_chip in its state container and obtain a pointer 121 to the container using container_of(). 122 (See Documentation/driver-model/design-patterns.txt) 123 124 * gpiochip_set_chained_irqchip(): sets up a chained irq handler for a 125 gpio_chip from a parent IRQ and passes the struct gpio_chip* as handler 126 data. (Notice handler data, since the irqchip data is likely used by the 127 parent irqchip!) This is for the chained type of chip. This is also used 128 to set up a nested irqchip if NULL is passed as handler. 129 130 To use the helpers please keep the following in mind: 131 132 - Make sure to assign all relevant members of the struct gpio_chip so that 133 the irqchip can initialize. E.g. .dev and .can_sleep shall be set up 134 properly. 135 136 It is legal for any IRQ consumer to request an IRQ from any irqchip no matter 137 if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and 138 irq_chip are orthogonal, and offering their services independent of each 139 other. 140 141 gpiod_to_irq() is just a convenience function to figure out the IRQ for a 142 certain GPIO line and should not be relied upon to have been called before 143 the IRQ is used. 144 145 So always prepare the hardware and make it ready for action in respective 146 callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having 147 been called first. 148 149 This orthogonality leads to ambiguities that we need to solve: if there is 150 competition inside the subsystem which side is using the resource (a certain 151 GPIO line and register for example) it needs to deny certain operations and 152 keep track of usage inside of the gpiolib subsystem. This is why the API 153 below exists. 154 155 156 Locking IRQ usage 157 ----------------- 158 Input GPIOs can be used as IRQ signals. When this happens, a driver is requested 159 to mark the GPIO as being used as an IRQ: 160 161 int gpiochip_lock_as_irq(struct gpio_chip *chip, unsigned int offset) 162 163 This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock 164 is released: 165 166 void gpiochip_unlock_as_irq(struct gpio_chip *chip, unsigned int offset) 167 168 When implementing an irqchip inside a GPIO driver, these two functions should 169 typically be called in the .startup() and .shutdown() callbacks from the 170 irqchip. 171 172 173 Requesting self-owned GPIO pins 174 ------------------------------- 175 176 Sometimes it is useful to allow a GPIO chip driver to request its own GPIO 177 descriptors through the gpiolib API. Using gpio_request() for this purpose 178 does not help since it pins the module to the kernel forever (it calls 179 try_module_get()). A GPIO driver can use the following functions instead 180 to request and free descriptors without being pinned to the kernel forever. 181 182 struct gpio_desc *gpiochip_request_own_desc(struct gpio_desc *desc, 183 const char *label) 184 185 void gpiochip_free_own_desc(struct gpio_desc *desc) 186 187 Descriptors requested with gpiochip_request_own_desc() must be released with 188 gpiochip_free_own_desc(). 189 190 These functions must be used with care since they do not affect module use 191 count. Do not use the functions to request gpio descriptors not owned by the 192 calling driver.