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9 The documents in this directory give detailed instructions on how to access
10 GPIOs in drivers, and how to write a driver for a device that provides GPIOs
17 A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
18 digital signal. They are provided from many kinds of chips, and are familiar
19 to Linux developers working with embedded and custom hardware. Each GPIO
20 represents a bit connected to a particular pin, or "ball" on Ball Grid Array
21 (BGA) packages. Board schematics show which external hardware connects to
23 passes such pin configuration data to drivers.
25 System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
26 non-dedicated pin can be configured as a GPIO; and most chips have at least
29 often have a few such pins to help with pin scarcity on SOCs; and there are
31 Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
36   - Output values are writable (high=1, low=0). Some chips also have
37     options about how that value is driven, so that for example only one
38     value might be driven, supporting "wire-OR" and similar schemes for the
41   - Input values are likewise readable (1, 0). Some chips support readback
42     of pins configured as "output", which is very useful in such "wire-OR"
43     cases (to support bidirectional signaling). GPIO controllers may have
44     input de-glitch/debounce logic, sometimes with software controls.
46   - Inputs can often be used as IRQ signals, often edge triggered but
48     wakeup events, to wake the system from a low power state.
50   - Usually a GPIO will be configurable as either input or output, as needed
53   - Most GPIOs can be accessed while holding spinlocks, but those accessed
56 On a given board each GPIO is used for one specific purpose like monitoring
57 MMC/SD card insertion/removal, detecting card write-protect status, driving
58 a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware
66 and it is useful to understand them, especially if you need to define GPIO
69 Active-High and Active-Low
70 --------------------------
71 It is natural to assume that a GPIO is "active" when its output signal is 1
74 to have different conventions about what "active" means. Such decisions should
75 be transparent to device drivers, therefore it is possible to define a GPIO as
76 being either active-high ("1" means "active", the default) or active-low ("0"
77 means "active") so that drivers only need to worry about the logical signal and
81 --------------------------
82 Sometimes shared signals need to use "open drain" (where only the low signal
84 driven) signaling. That term applies to CMOS transistors; "open collector" is
86 This is sometimes called a "wire-AND"; or more practically, from the negative
87 logic (low=true) perspective this is a "wire-OR".
89 One common example of an open drain signal is a shared active-low IRQ line.
92 Some GPIO controllers directly support open drain and open source outputs; many
94 support it, there's a common idiom you can use to emulate it with any GPIO pin
103 The same logic can be applied to emulate open source signaling, by driving the
109 driving the shared signal low. That's not necessarily an error. As one common