0001 ============================
0002 Subsystem drivers using GPIO
0003 ============================
0004
0005 Note that standard kernel drivers exist for common GPIO tasks and will provide
0006 the right in-kernel and userspace APIs/ABIs for the job, and that these
0007 drivers can quite easily interconnect with other kernel subsystems using
0008 hardware descriptions such as device tree or ACPI:
0009
0010 - leds-gpio: drivers/leds/leds-gpio.c will handle LEDs connected to GPIO
0011 lines, giving you the LED sysfs interface
0012
0013 - ledtrig-gpio: drivers/leds/trigger/ledtrig-gpio.c will provide a LED trigger,
0014 i.e. a LED will turn on/off in response to a GPIO line going high or low
0015 (and that LED may in turn use the leds-gpio as per above).
0016
0017 - gpio-keys: drivers/input/keyboard/gpio_keys.c is used when your GPIO line
0018 can generate interrupts in response to a key press. Also supports debounce.
0019
0020 - gpio-keys-polled: drivers/input/keyboard/gpio_keys_polled.c is used when your
0021 GPIO line cannot generate interrupts, so it needs to be periodically polled
0022 by a timer.
0023
0024 - gpio_mouse: drivers/input/mouse/gpio_mouse.c is used to provide a mouse with
0025 up to three buttons by simply using GPIOs and no mouse port. You can cut the
0026 mouse cable and connect the wires to GPIO lines or solder a mouse connector
0027 to the lines for a more permanent solution of this type.
0028
0029 - gpio-beeper: drivers/input/misc/gpio-beeper.c is used to provide a beep from
0030 an external speaker connected to a GPIO line.
0031
0032 - extcon-gpio: drivers/extcon/extcon-gpio.c is used when you need to read an
0033 external connector status, such as a headset line for an audio driver or an
0034 HDMI connector. It will provide a better userspace sysfs interface than GPIO.
0035
0036 - restart-gpio: drivers/power/reset/gpio-restart.c is used to restart/reboot
0037 the system by pulling a GPIO line and will register a restart handler so
0038 userspace can issue the right system call to restart the system.
0039
0040 - poweroff-gpio: drivers/power/reset/gpio-poweroff.c is used to power the
0041 system down by pulling a GPIO line and will register a pm_power_off()
0042 callback so that userspace can issue the right system call to power down the
0043 system.
0044
0045 - gpio-gate-clock: drivers/clk/clk-gpio.c is used to control a gated clock
0046 (off/on) that uses a GPIO, and integrated with the clock subsystem.
0047
0048 - i2c-gpio: drivers/i2c/busses/i2c-gpio.c is used to drive an I2C bus
0049 (two wires, SDA and SCL lines) by hammering (bitbang) two GPIO lines. It will
0050 appear as any other I2C bus to the system and makes it possible to connect
0051 drivers for the I2C devices on the bus like any other I2C bus driver.
0052
0053 - spi_gpio: drivers/spi/spi-gpio.c is used to drive an SPI bus (variable number
0054 of wires, at least SCK and optionally MISO, MOSI and chip select lines) using
0055 GPIO hammering (bitbang). It will appear as any other SPI bus on the system
0056 and makes it possible to connect drivers for SPI devices on the bus like
0057 any other SPI bus driver. For example any MMC/SD card can then be connected
0058 to this SPI by using the mmc_spi host from the MMC/SD card subsystem.
0059
0060 - w1-gpio: drivers/w1/masters/w1-gpio.c is used to drive a one-wire bus using
0061 a GPIO line, integrating with the W1 subsystem and handling devices on
0062 the bus like any other W1 device.
0063
0064 - gpio-fan: drivers/hwmon/gpio-fan.c is used to control a fan for cooling the
0065 system, connected to a GPIO line (and optionally a GPIO alarm line),
0066 presenting all the right in-kernel and sysfs interfaces to make your system
0067 not overheat.
0068
0069 - gpio-regulator: drivers/regulator/gpio-regulator.c is used to control a
0070 regulator providing a certain voltage by pulling a GPIO line, integrating
0071 with the regulator subsystem and giving you all the right interfaces.
0072
0073 - gpio-wdt: drivers/watchdog/gpio_wdt.c is used to provide a watchdog timer
0074 that will periodically "ping" a hardware connected to a GPIO line by toggling
0075 it from 1-to-0-to-1. If that hardware does not receive its "ping"
0076 periodically, it will reset the system.
0077
0078 - gpio-nand: drivers/mtd/nand/raw/gpio.c is used to connect a NAND flash chip
0079 to a set of simple GPIO lines: RDY, NCE, ALE, CLE, NWP. It interacts with the
0080 NAND flash MTD subsystem and provides chip access and partition parsing like
0081 any other NAND driving hardware.
0082
0083 - ps2-gpio: drivers/input/serio/ps2-gpio.c is used to drive a PS/2 (IBM) serio
0084 bus, data and clock line, by bit banging two GPIO lines. It will appear as
0085 any other serio bus to the system and makes it possible to connect drivers
0086 for e.g. keyboards and other PS/2 protocol based devices.
0087
0088 - cec-gpio: drivers/media/platform/cec-gpio/ is used to interact with a CEC
0089 Consumer Electronics Control bus using only GPIO. It is used to communicate
0090 with devices on the HDMI bus.
0091
0092 - gpio-charger: drivers/power/supply/gpio-charger.c is used if you need to do
0093 battery charging and all you have to go by to check the presence of the
0094 AC charger or more complex tasks such as indicating charging status using
0095 nothing but GPIO lines, this driver provides that and also a clearly defined
0096 way to pass the charging parameters from hardware descriptions such as the
0097 device tree.
0098
0099 - gpio-mux: drivers/mux/gpio.c is used for controlling a multiplexer using
0100 n GPIO lines such that you can mux in 2^n different devices by activating
0101 different GPIO lines. Often the GPIOs are on a SoC and the devices are
0102 some SoC-external entities, such as different components on a PCB that
0103 can be selectively enabled.
0104
0105 Apart from this there are special GPIO drivers in subsystems like MMC/SD to
0106 read card detect and write protect GPIO lines, and in the TTY serial subsystem
0107 to emulate MCTRL (modem control) signals CTS/RTS by using two GPIO lines. The
0108 MTD NOR flash has add-ons for extra GPIO lines too, though the address bus is
0109 usually connected directly to the flash.
0110
0111 Use those instead of talking directly to the GPIOs from userspace; they
0112 integrate with kernel frameworks better than your userspace code could.
0113 Needless to say, just using the appropriate kernel drivers will simplify and
0114 speed up your embedded hacking in particular by providing ready-made components.
