1============================ 2Subsystem drivers using GPIO 3============================ 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 99- gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using 100 n GPIO lines such that you can mux in 2^n different devices by activating 101 different GPIO lines. Often the GPIOs are on a SoC and the devices are 102 some SoC-external entities, such as different components on a PCB that 103 can be selectively enabled. 104 105Apart from this there are special GPIO drivers in subsystems like MMC/SD to 106read card detect and write protect GPIO lines, and in the TTY serial subsystem 107to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The 108MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is 109usually connected directly to the flash. 110 111Use those instead of talking directly to the GPIOs from userspace; they 112integrate with kernel frameworks better than your userspace code could. 113Needless to say, just using the appropriate kernel drivers will simplify and 114speed up your embedded hacking in particular by providing ready-made components. 115