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| 1 | +Subsystem drivers using GPIO |
| 2 | +============================ |
| 3 | + |
| 4 | +Note that standard kernel drivers exist for common GPIO tasks and will provide |
| 5 | +the right in-kernel and userspace APIs/ABIs for the job, and that these |
| 6 | +drivers can quite easily interconnect with other kernel subsystems using |
| 7 | +hardware descriptions such as device tree or ACPI: |
| 8 | + |
| 9 | +- leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO |
| 10 | + lines, giving you the LED sysfs interface |
| 11 | + |
| 12 | +- ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger, |
| 13 | + i.e. a LED will turn on/off in response to a GPIO line going high or low |
| 14 | + (and that LED may in turn use the leds-gpio as per above). |
| 15 | + |
| 16 | +- gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line |
| 17 | + can generate interrupts in response to a key press. Also supports debounce. |
| 18 | + |
| 19 | +- gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your |
| 20 | + GPIO line cannot generate interrupts, so it needs to be periodically polled |
| 21 | + by a timer. |
| 22 | + |
| 23 | +- gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with |
| 24 | + up to three buttons by simply using GPIOs and no mouse port. You can cut the |
| 25 | + mouse cable and connect the wires to GPIO lines or solder a mouse connector |
| 26 | + to the lines for a more permanent solution of this type. |
| 27 | + |
| 28 | +- gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from |
| 29 | + an external speaker connected to a GPIO line. |
| 30 | + |
| 31 | +- gpio-tilt-polled: drivers/input/misc/gpio_tilt_polled.c provides tilt |
| 32 | + detection switches using GPIO, which is useful for your homebrewn pinball |
| 33 | + machine if for nothing else. It can detect different tilt angles of the |
| 34 | + monitored object. |
| 35 | + |
| 36 | +- extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an |
| 37 | + external connector status, such as a headset line for an audio driver or an |
| 38 | + HDMI connector. It will provide a better userspace sysfs interface than GPIO. |
| 39 | + |
| 40 | +- restart-gpio: drivers/power/gpio-restart.c is used to restart/reboot the |
| 41 | + system by pulling a GPIO line and will register a restart handler so |
| 42 | + userspace can issue the right system call to restart the system. |
| 43 | + |
| 44 | +- poweroff-gpio: drivers/power/gpio-poweroff.c is used to power the system down |
| 45 | + by pulling a GPIO line and will register a pm_power_off() callback so that |
| 46 | + userspace can issue the right system call to power down the system. |
| 47 | + |
| 48 | +- gpio-gate-clock: drivers/clk/clk-gpio-gate.c is used to control a gated clock |
| 49 | + (off/on) that uses a GPIO, and integrated with the clock subsystem. |
| 50 | + |
| 51 | +- i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus |
| 52 | + (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will |
| 53 | + appear as any other I2C bus to the system and makes it possible to connect |
| 54 | + drivers for the I2C devices on the bus like any other I2C bus driver. |
| 55 | + |
| 56 | +- spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number |
| 57 | + of wires, atleast SCK and optionally MISO, MOSI and chip select lines) using |
| 58 | + GPIO hammering (bitbang). It will appear as any other SPI bus on the system |
| 59 | + and makes it possible to connect drivers for SPI devices on the bus like |
| 60 | + any other SPI bus driver. For example any MMC/SD card can then be connected |
| 61 | + to this SPI by using the mmc_spi host from the MMC/SD card subsystem. |
| 62 | + |
| 63 | +- w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using |
| 64 | + a GPIO line, integrating with the W1 subsystem and handling devices on |
| 65 | + the bus like any other W1 device. |
| 66 | + |
| 67 | +- gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the |
| 68 | + system, connected to a GPIO line (and optionally a GPIO alarm line), |
| 69 | + presenting all the right in-kernel and sysfs interfaces to make your system |
| 70 | + not overheat. |
| 71 | + |
| 72 | +- gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a |
| 73 | + regulator providing a certain voltage by pulling a GPIO line, integrating |
| 74 | + with the regulator subsystem and giving you all the right interfaces. |
| 75 | + |
| 76 | +- gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer |
| 77 | + that will periodically "ping" a hardware connected to a GPIO line by toggling |
| 78 | + it from 1-to-0-to-1. If that hardware does not recieve its "ping" |
| 79 | + periodically, it will reset the system. |
| 80 | + |
| 81 | +- gpio-nand: drivers/mtd/nand/gpio.c is used to connect a NAND flash chip to |
| 82 | + a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the |
| 83 | + NAND flash MTD subsystem and provides chip access and partition parsing like |
| 84 | + any other NAND driving hardware. |
| 85 | + |
| 86 | +Apart from this there are special GPIO drivers in subsystems like MMC/SD to |
| 87 | +read card detect and write protect GPIO lines, and in the TTY serial subsystem |
| 88 | +to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The |
| 89 | +MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is |
| 90 | +usually connected directly to the flash. |
| 91 | + |
| 92 | +Use those instead of talking directly to the GPIOs using sysfs; they integrate |
| 93 | +with kernel frameworks better than your userspace code could. Needless to say, |
| 94 | +just using the apropriate kernel drivers will simplify and speed up your |
| 95 | +embedded hacking in particular by providing ready-made components. |
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