gpio: document interaction with other subsystems
Now I am very fed up with people reinventing kernel wheels in
userspace "just because they can" (read, sysfs). Put in a angry
blurb in sysfs doc and put in a new file with pointers to other
subsystem drivers utilizing GPIOs.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/Documentation/gpio/drivers-on-gpio.txt b/Documentation/gpio/drivers-on-gpio.txt
new file mode 100644
index 0000000..f612132
--- /dev/null
+++ b/Documentation/gpio/drivers-on-gpio.txt
@@ -0,0 +1,95 @@
+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.
+
+- gpio-tilt-polled: drivers/input/misc/gpio_tilt_polled.c provides tilt
+ detection switches using GPIO, which is useful for your homebrewn pinball
+ machine if for nothing else. It can detect different tilt angles of the
+ monitored object.
+
+- 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/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/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-gate.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, atleast 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 recieve its "ping"
+ periodically, it will reset the system.
+
+- gpio-nand: drivers/mtd/nand/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.
+
+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 using sysfs; they integrate
+with kernel frameworks better than your userspace code could. Needless to say,
+just using the apropriate kernel drivers will simplify and speed up your
+embedded hacking in particular by providing ready-made components.