gpio: gpio.txt

1       GPIO Interfaces
2       ===============
3       
4       The documents in this directory give detailed instructions on how to access
5       GPIOs in drivers, and how to write a driver for a device that provides GPIOs
6       itself.
7       
8       Due to the history of GPIO interfaces in the kernel, there are two different
9       ways to obtain and use GPIOs:
10      
11        - The descriptor-based interface is the preferred way to manipulate GPIOs,
12      and is described by all the files in this directory excepted gpio-legacy.txt.
13        - The legacy integer-based interface which is considered deprecated (but still
14      usable for compatibility reasons) is documented in gpio-legacy.txt.
15      
16      The remainder of this document applies to the new descriptor-based interface.
17      gpio-legacy.txt contains the same information applied to the legacy
18      integer-based interface.
19      
20      
21      What is a GPIO?
22      ===============
23      
24      A "General Purpose Input/Output" (GPIO) is a flexible software-controlled
25      digital signal. They are provided from many kinds of chip, and are familiar
26      to Linux developers working with embedded and custom hardware. Each GPIO
27      represents a bit connected to a particular pin, or "ball" on Ball Grid Array
28      (BGA) packages. Board schematics show which external hardware connects to
29      which GPIOs. Drivers can be written generically, so that board setup code
30      passes such pin configuration data to drivers.
31      
32      System-on-Chip (SOC) processors heavily rely on GPIOs. In some cases, every
33      non-dedicated pin can be configured as a GPIO; and most chips have at least
34      several dozen of them. Programmable logic devices (like FPGAs) can easily
35      provide GPIOs; multifunction chips like power managers, and audio codecs
36      often have a few such pins to help with pin scarcity on SOCs; and there are
37      also "GPIO Expander" chips that connect using the I2C or SPI serial buses.
38      Most PC southbridges have a few dozen GPIO-capable pins (with only the BIOS
39      firmware knowing how they're used).
40      
41      The exact capabilities of GPIOs vary between systems. Common options:
42      
43        - Output values are writable (high=1, low=0). Some chips also have
44          options about how that value is driven, so that for example only one
45          value might be driven, supporting "wire-OR" and similar schemes for the
46          other value (notably, "open drain" signaling).
47      
48        - Input values are likewise readable (1, 0). Some chips support readback
49          of pins configured as "output", which is very useful in such "wire-OR"
50          cases (to support bidirectional signaling). GPIO controllers may have
51          input de-glitch/debounce logic, sometimes with software controls.
52      
53        - Inputs can often be used as IRQ signals, often edge triggered but
54          sometimes level triggered. Such IRQs may be configurable as system
55          wakeup events, to wake the system from a low power state.
56      
57        - Usually a GPIO will be configurable as either input or output, as needed
58          by different product boards; single direction ones exist too.
59      
60        - Most GPIOs can be accessed while holding spinlocks, but those accessed
61          through a serial bus normally can't. Some systems support both types.
62      
63      On a given board each GPIO is used for one specific purpose like monitoring
64      MMC/SD card insertion/removal, detecting card write-protect status, driving
65      a LED, configuring a transceiver, bit-banging a serial bus, poking a hardware
66      watchdog, sensing a switch, and so on.
67      
68      
69      Common GPIO Properties
70      ======================
71      
72      These properties are met through all the other documents of the GPIO interface
73      and it is useful to understand them, especially if you need to define GPIO
74      mappings.
75      
76      Active-High and Active-Low
77      --------------------------
78      It is natural to assume that a GPIO is "active" when its output signal is 1
79      ("high"), and inactive when it is 0 ("low"). However in practice the signal of a
80      GPIO may be inverted before is reaches its destination, or a device could decide
81      to have different conventions about what "active" means. Such decisions should
82      be transparent to device drivers, therefore it is possible to define a GPIO as
83      being either active-high ("1" means "active", the default) or active-low ("0"
84      means "active") so that drivers only need to worry about the logical signal and
85      not about what happens at the line level.
86      
87      Open Drain and Open Source
88      --------------------------
89      Sometimes shared signals need to use "open drain" (where only the low signal
90      level is actually driven), or "open source" (where only the high signal level is
91      driven) signaling. That term applies to CMOS transistors; "open collector" is
92      used for TTL. A pullup or pulldown resistor causes the high or low signal level.
93      This is sometimes called a "wire-AND"; or more practically, from the negative
94      logic (low=true) perspective this is a "wire-OR".
95      
96      One common example of an open drain signal is a shared active-low IRQ line.
97      Also, bidirectional data bus signals sometimes use open drain signals.
98      
99      Some GPIO controllers directly support open drain and open source outputs; many
100     don't. When you need open drain signaling but your hardware doesn't directly
101     support it, there's a common idiom you can use to emulate it with any GPIO pin
102     that can be used as either an input or an output:
103     
104      LOW:   gpiod_direction_output(gpio, 0) ... this drives the signal and overrides
105             the pullup.
106     
107      HIGH:  gpiod_direction_input(gpio) ... this turns off the output, so the pullup
108             (or some other device) controls the signal.
109     
110     The same logic can be applied to emulate open source signaling, by driving the
111     high signal and configuring the GPIO as input for low. This open drain/open
112     source emulation can be handled transparently by the GPIO framework.
113     
114     If you are "driving" the signal high but gpiod_get_value(gpio) reports a low
115     value (after the appropriate rise time passes), you know some other component is
116     driving the shared signal low. That's not necessarily an error. As one common
117     example, that's how I2C clocks are stretched:  a slave that needs a slower clock
118     delays the rising edge of SCK, and the I2C master adjusts its signaling rate
119     accordingly.