Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 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 It is legal for any IRQ consumer to request an IRQ from any irqchip no matter 77 if that is a combined GPIO+IRQ driver. The basic premise is that gpio_chip and 78 irq_chip are orthogonal, and offering their services independent of each 79 other. 80 81 gpiod_to_irq() is just a convenience function to figure out the IRQ for a 82 certain GPIO line and should not be relied upon to have been called before 83 the IRQ is used. 84 85 So always prepare the hardware and make it ready for action in respective 86 callbacks from the GPIO and irqchip APIs. Do not rely on gpiod_to_irq() having 87 been called first. 88 89 This orthogonality leads to ambiguities that we need to solve: if there is 90 competition inside the subsystem which side is using the resource (a certain 91 GPIO line and register for example) it needs to deny certain operations and 92 keep track of usage inside of the gpiolib subsystem. This is why the API 93 below exists. 94 95 96 Locking IRQ usage 97 ----------------- 98 Input GPIOs can be used as IRQ signals. When this happens, a driver is requested 99 to mark the GPIO as being used as an IRQ: 100 101 int gpiod_lock_as_irq(struct gpio_desc *desc) 102 103 This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock 104 is released: 105 106 void gpiod_unlock_as_irq(struct gpio_desc *desc) 107 108 When implementing an irqchip inside a GPIO driver, these two functions should 109 typically be called in the .startup() and .shutdown() callbacks from the 110 irqchip.