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- 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.
- - 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.
- 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.
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