|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.
|- gpio-tilt-polled: drivers/input/misc/gpio_tilt_polled.c provides tilt
| detection switches using GPIO, which is useful for your homebrewn pinball
| machine if for nothing else. It can detect different tilt angles of the
| monitored object.
|- 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
|- 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/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.
|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 using sysfs; 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.