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 <[e-mail]>

Documentation/gpio/00-INDEX

···
 	- How to obtain and use GPIOs in a driver
 driver.txt
 	- How to write a GPIO driver
+drivers-on-gpio.txt:
+	- Drivers in other subsystems that can use GPIO to provide more
+	  complex functionality.
 board.txt
 	- How to assign GPIOs to a consumer device and a function
 sysfs.txt

Documentation/gpio/drivers-on-gpio.txt

···
+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.

Documentation/gpio/sysfs.txt

···
 standard kernels won't know about. And for some tasks, simple userspace
 GPIO drivers could be all that the system really needs.
 
-Note that standard kernel drivers exist for common "LEDs and Buttons"
-GPIO tasks:  "leds-gpio" and "gpio_keys", respectively. Use those
-instead of talking directly to the GPIOs; they integrate with kernel
-frameworks better than your userspace code could.
-
+DO NOT ABUSE SYFS TO CONTROL HARDWARE THAT HAS PROPER KERNEL DRIVERS.
+PLEASE READ THE DOCUMENT NAMED "drivers-on-gpio.txt" IN THIS DOCUMENTATION
+DIRECTORY TO AVOID REINVENTING KERNEL WHEELS IN USERSPACE. I MEAN IT.
+REALLY.
 
 Paths in Sysfs
 --------------