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2Subsystem drivers using GPIO
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4
5Note that standard kernel drivers exist for common GPIO tasks and will provide
6the right in-kernel and userspace APIs/ABIs for the job, and that these
7drivers can quite easily interconnect with other kernel subsystems using
8hardware descriptions such as device tree or ACPI:
9
10- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to  GPIO
11  lines, giving you the LED sysfs interface
12
13- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
14  i.e. a LED will turn on/off in response to a GPIO line going high or low
15  (and that LED may in turn use the leds-gpio as per above).
16
17- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
18  can generate interrupts in response to a key press. Also supports debounce.
19
20- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
21  GPIO line cannot generate interrupts, so it needs to be periodically polled
22  by a timer.
23
24- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
25  up to three buttons by simply using GPIOs and no mouse port. You can cut the
26  mouse cable and connect the wires to GPIO lines or solder a mouse connector
27  to the lines for a more permanent solution of this type.
28
29- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
30  an external speaker connected to a GPIO line.
31
32- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
33  external connector status, such as a headset line for an audio driver or an
34  HDMI connector. It will provide a better userspace sysfs interface than GPIO.
35
36- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
37  the system by pulling a GPIO line and will register a restart handler so
38  userspace can issue the right system call to restart the system.
39
40- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
41  system down by pulling a GPIO line and will register a pm_power_off()
42  callback so that userspace can issue the right system call to power down the
43  system.
44
45- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
46  (off/on) that uses a GPIO, and integrated with the clock subsystem.
47
48- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
49  (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
50  appear as any other I2C bus to the system and makes it possible to connect
51  drivers for the I2C devices on the bus like any other I2C bus driver.
52
53- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
54  of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
55  GPIO hammering (bitbang). It will appear as any other SPI bus on the system
56  and makes it possible to connect drivers for SPI devices on the bus like
57  any other SPI bus driver. For example any MMC/SD card can then be connected
58  to this SPI by using the mmc_spi host from the MMC/SD card subsystem.
59
60- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
61  a GPIO line, integrating with the W1 subsystem and handling devices on
62  the bus like any other W1 device.
63
64- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
65  system, connected to a GPIO line (and optionally a GPIO alarm line),
66  presenting all the right in-kernel and sysfs interfaces to make your system
67  not overheat.
68
69- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
70  regulator providing a certain voltage by pulling a GPIO line, integrating
71  with the regulator subsystem and giving you all the right interfaces.
72
73- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
74  that will periodically "ping" a hardware connected to a GPIO line by toggling
75  it from 1-to-0-to-1. If that hardware does not receive its "ping"
76  periodically, it will reset the system.
77
78- gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip
79  to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
80  NAND flash MTD subsystem and provides chip access and partition parsing like
81  any other NAND driving hardware.
82
83- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
84  bus, data and clock line, by bit banging two GPIO lines. It will appear as
85  any other serio bus to the system and makes it possible to connect drivers
86  for e.g. keyboards and other PS/2 protocol based devices.
87
88- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
89  Consumer Electronics Control bus using only GPIO. It is used to communicate
90  with devices on the HDMI bus.
91
92- gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do
93  battery charging and all you have to go by to check the presence of the
94  AC charger or more complex tasks such as indicating charging status using
95  nothing but GPIO lines, this driver provides that and also a clearly defined
96  way to pass the charging parameters from hardware descriptions such as the
97  device tree.
98
99Apart from this there are special GPIO drivers in subsystems like MMC/SD to
100read card detect and write protect GPIO lines, and in the TTY serial subsystem
101to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
102MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
103usually connected directly to the flash.
104
105Use those instead of talking directly to the GPIOs from userspace; they
106integrate with kernel frameworks better than your userspace code could.
107Needless to say, just using the appropriate kernel drivers will simplify and
108speed up your embedded hacking in particular by providing ready-made components.
109