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. (If the beep is controlled by 31 off/on, for an actual PWM waveform, see pwm-gpio below.) 32 33- pwm-gpio: drivers/pwm/pwm-gpio.c is used to toggle a GPIO with a high 34 resolution timer producing a PWM waveform on the GPIO line, as well as 35 Linux high resolution timers can do. 36 37- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an 38 external connector status, such as a headset line for an audio driver or an 39 HDMI connector. It will provide a better userspace sysfs interface than GPIO. 40 41- restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot 42 the system by pulling a GPIO line and will register a restart handler so 43 userspace can issue the right system call to restart the system. 44 45- poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the 46 system down by pulling a GPIO line and will register a pm_power_off() 47 callback so that userspace can issue the right system call to power down the 48 system. 49 50- gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock 51 (off/on) that uses a GPIO, and integrated with the clock subsystem. 52 53- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus 54 (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will 55 appear as any other I2C bus to the system and makes it possible to connect 56 drivers for the I2C devices on the bus like any other I2C bus driver. 57 58- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number 59 of wires, at least SCK and optionally MISO, MOSI and chip select lines) using 60 GPIO hammering (bitbang). It will appear as any other SPI bus on the system 61 and makes it possible to connect drivers for SPI devices on the bus like 62 any other SPI bus driver. For example any MMC/SD card can then be connected 63 to this SPI by using the mmc_spi host from the MMC/SD card subsystem. 64 65- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using 66 a GPIO line, integrating with the W1 subsystem and handling devices on 67 the bus like any other W1 device. 68 69- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the 70 system, connected to a GPIO line (and optionally a GPIO alarm line), 71 presenting all the right in-kernel and sysfs interfaces to make your system 72 not overheat. 73 74- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a 75 regulator providing a certain voltage by pulling a GPIO line, integrating 76 with the regulator subsystem and giving you all the right interfaces. 77 78- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer 79 that will periodically "ping" a hardware connected to a GPIO line by toggling 80 it from 1-to-0-to-1. If that hardware does not receive its "ping" 81 periodically, it will reset the system. 82 83- gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip 84 to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the 85 NAND flash MTD subsystem and provides chip access and partition parsing like 86 any other NAND driving hardware. 87 88- ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio 89 bus, data and clock line, by bit banging two GPIO lines. It will appear as 90 any other serio bus to the system and makes it possible to connect drivers 91 for e.g. keyboards and other PS/2 protocol based devices. 92 93- cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC 94 Consumer Electronics Control bus using only GPIO. It is used to communicate 95 with devices on the HDMI bus. 96 97- gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do 98 battery charging and all you have to go by to check the presence of the 99 AC charger or more complex tasks such as indicating charging status using 100 nothing but GPIO lines, this driver provides that and also a clearly defined 101 way to pass the charging parameters from hardware descriptions such as the 102 device tree. 103 104- gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using 105 n GPIO lines such that you can mux in 2^n different devices by activating 106 different GPIO lines. Often the GPIOs are on a SoC and the devices are 107 some SoC-external entities, such as different components on a PCB that 108 can be selectively enabled. 109 110Apart from this there are special GPIO drivers in subsystems like MMC/SD to 111read card detect and write protect GPIO lines, and in the TTY serial subsystem 112to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The 113MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is 114usually connected directly to the flash. 115 116Use those instead of talking directly to the GPIOs from userspace; they 117integrate with kernel frameworks better than your userspace code could. 118Needless to say, just using the appropriate kernel drivers will simplify and 119speed up your embedded hacking in particular by providing ready-made components. 120