Documentation/driver-api/gpio/drivers-on-gpio.rst - linux - Git at Google

 ============================
 Subsystem drivers using GPIO
 ============================

 Note that standard kernel drivers exist for common GPIO tasks and will provide
 the right in-kernel and userspace APIs/ABIs for the job, and that these
 drivers can quite easily interconnect with other kernel subsystems using
 hardware descriptions such as device tree or ACPI:

 - leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to  GPIO
   lines, giving you the LED sysfs interface

 - ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
   i.e. a LED will turn on/off in response to a GPIO line going high or low
   (and that LED may in turn use the leds-gpio as per above).

 - gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
   can generate interrupts in response to a key press. Also supports debounce.

 - gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
   GPIO line cannot generate interrupts, so it needs to be periodically polled
   by a timer.

 - gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
   up to three buttons by simply using GPIOs and no mouse port. You can cut the
   mouse cable and connect the wires to GPIO lines or solder a mouse connector
   to the lines for a more permanent solution of this type.

 - gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
   an external speaker connected to a GPIO line. (If the beep is controlled by
   off/on, for an actual PWM waveform, see pwm-gpio below.)

 - pwm-gpio: drivers/pwm/pwm-gpio.c is used to toggle a GPIO with a high
   resolution timer producing a PWM waveform on the GPIO line, as well as
   Linux high resolution timers can do.

 - extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
   external connector status, such as a headset line for an audio driver or an
   HDMI connector. It will provide a better userspace sysfs interface than GPIO.

 - restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
   the system by pulling a GPIO line and will register a restart handler so
   userspace can issue the right system call to restart the system.

 - poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
   system down by pulling a GPIO line and will register a pm_power_off()
   callback so that userspace can issue the right system call to power down the
   system.

 - gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
   (off/on) that uses a GPIO, and integrated with the clock subsystem.

 - i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
   (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
   appear as any other I2C bus to the system and makes it possible to connect
   drivers for the I2C devices on the bus like any other I2C bus driver.

 - spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
   of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
   GPIO hammering (bitbang). It will appear as any other SPI bus on the system
   and makes it possible to connect drivers for SPI devices on the bus like
   any other SPI bus driver. For example any MMC/SD card can then be connected
   to this SPI by using the mmc_spi host from the MMC/SD card subsystem.

 - w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
   a GPIO line, integrating with the W1 subsystem and handling devices on
   the bus like any other W1 device.

 - gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
   system, connected to a GPIO line (and optionally a GPIO alarm line),
   presenting all the right in-kernel and sysfs interfaces to make your system
   not overheat.

 - gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
   regulator providing a certain voltage by pulling a GPIO line, integrating
   with the regulator subsystem and giving you all the right interfaces.

 - gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
   that will periodically "ping" a hardware connected to a GPIO line by toggling
   it from 1-to-0-to-1. If that hardware does not receive its "ping"
   periodically, it will reset the system.

 - gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip
   to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
   NAND flash MTD subsystem and provides chip access and partition parsing like
   any other NAND driving hardware.

 - ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
   bus, data and clock line, by bit banging two GPIO lines. It will appear as
   any other serio bus to the system and makes it possible to connect drivers
   for e.g. keyboards and other PS/2 protocol based devices.

 - cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
   Consumer Electronics Control bus using only GPIO. It is used to communicate
   with devices on the HDMI bus.

 - gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do
   battery charging and all you have to go by to check the presence of the
   AC charger or more complex tasks such as indicating charging status using
   nothing but GPIO lines, this driver provides that and also a clearly defined
   way to pass the charging parameters from hardware descriptions such as the
   device tree.

 - gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using
   n GPIO lines such that you can mux in 2^n different devices by activating
   different GPIO lines. Often the GPIOs are on a SoC and the devices are
   some SoC-external entities, such as different components on a PCB that
   can be selectively enabled.

 Apart from this there are special GPIO drivers in subsystems like MMC/SD to
 read card detect and write protect GPIO lines, and in the TTY serial subsystem
 to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
 MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
 usually connected directly to the flash.

 Use those instead of talking directly to the GPIOs from userspace; they
 integrate with kernel frameworks better than your userspace code could.
 Needless to say, just using the appropriate kernel drivers will simplify and
 speed up your embedded hacking in particular by providing ready-made components.
	============================
	Subsystem drivers using GPIO
	============================

	Note that standard kernel drivers exist for common GPIO tasks and will provide
	the right in-kernel and userspace APIs/ABIs for the job, and that these
	drivers can quite easily interconnect with other kernel subsystems using
	hardware descriptions such as device tree or ACPI:

	- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO
	lines, giving you the LED sysfs interface

	- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
	i.e. a LED will turn on/off in response to a GPIO line going high or low
	(and that LED may in turn use the leds-gpio as per above).

	- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
	can generate interrupts in response to a key press. Also supports debounce.

	- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
	GPIO line cannot generate interrupts, so it needs to be periodically polled
	by a timer.

	- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
	up to three buttons by simply using GPIOs and no mouse port. You can cut the
	mouse cable and connect the wires to GPIO lines or solder a mouse connector
	to the lines for a more permanent solution of this type.

	- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
	an external speaker connected to a GPIO line. (If the beep is controlled by
	off/on, for an actual PWM waveform, see pwm-gpio below.)

	- pwm-gpio: drivers/pwm/pwm-gpio.c is used to toggle a GPIO with a high
	resolution timer producing a PWM waveform on the GPIO line, as well as
	Linux high resolution timers can do.

	- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
	external connector status, such as a headset line for an audio driver or an
	HDMI connector. It will provide a better userspace sysfs interface than GPIO.

	- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
	the system by pulling a GPIO line and will register a restart handler so
	userspace can issue the right system call to restart the system.

	- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
	system down by pulling a GPIO line and will register a pm_power_off()
	callback so that userspace can issue the right system call to power down the
	system.

	- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
	(off/on) that uses a GPIO, and integrated with the clock subsystem.

	- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
	(two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
	appear as any other I2C bus to the system and makes it possible to connect
	drivers for the I2C devices on the bus like any other I2C bus driver.

	- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
	of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
	GPIO hammering (bitbang). It will appear as any other SPI bus on the system
	and makes it possible to connect drivers for SPI devices on the bus like
	any other SPI bus driver. For example any MMC/SD card can then be connected
	to this SPI by using the mmc_spi host from the MMC/SD card subsystem.

	- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
	a GPIO line, integrating with the W1 subsystem and handling devices on
	the bus like any other W1 device.

	- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
	system, connected to a GPIO line (and optionally a GPIO alarm line),
	presenting all the right in-kernel and sysfs interfaces to make your system
	not overheat.

	- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
	regulator providing a certain voltage by pulling a GPIO line, integrating
	with the regulator subsystem and giving you all the right interfaces.

	- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
	that will periodically "ping" a hardware connected to a GPIO line by toggling
	it from 1-to-0-to-1. If that hardware does not receive its "ping"
	periodically, it will reset the system.

	- gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip
	to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
	NAND flash MTD subsystem and provides chip access and partition parsing like
	any other NAND driving hardware.

	- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
	bus, data and clock line, by bit banging two GPIO lines. It will appear as
	any other serio bus to the system and makes it possible to connect drivers
	for e.g. keyboards and other PS/2 protocol based devices.

	- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
	Consumer Electronics Control bus using only GPIO. It is used to communicate
	with devices on the HDMI bus.

	- gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do
	battery charging and all you have to go by to check the presence of the
	AC charger or more complex tasks such as indicating charging status using
	nothing but GPIO lines, this driver provides that and also a clearly defined
	way to pass the charging parameters from hardware descriptions such as the
	device tree.

	- gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using
	n GPIO lines such that you can mux in 2^n different devices by activating
	different GPIO lines. Often the GPIOs are on a SoC and the devices are
	some SoC-external entities, such as different components on a PCB that
	can be selectively enabled.

	Apart from this there are special GPIO drivers in subsystems like MMC/SD to
	read card detect and write protect GPIO lines, and in the TTY serial subsystem
	to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
	MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
	usually connected directly to the flash.

	Use those instead of talking directly to the GPIOs from userspace; they
	integrate with kernel frameworks better than your userspace code could.
	Needless to say, just using the appropriate kernel drivers will simplify and
	speed up your embedded hacking in particular by providing ready-made components.