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
    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/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.