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Documentation / gpio / driver.txt

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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:03 EST.

1	GPIO Descriptor Driver Interface
2	================================
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.
8	Each GPIO controller driver needs to include the following header, which defines
9	the structures used to define a GPIO driver:
11		#include <linux/gpio/driver.h>
14	Internal Representation of GPIOs
15	================================
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.
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.
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.
37	Controller Drivers: gpio_chip
38	=============================
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:
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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:
101		int gpiod_lock_as_irq(struct gpio_desc *desc)
103	This will prevent the use of non-irq related GPIO APIs until the GPIO IRQ lock
104	is released:
106		void gpiod_unlock_as_irq(struct gpio_desc *desc)
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.
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