====== GPIO framework for FreeBSD ======
===== Description =====

GPIO stands for General Purpose Input/Output. This interface may be
thought of as a set of pins which could serve as input or output
having logical 0 and logical 1 as their values (some other options
are applicable, but they're not crucial for this explanation).
Most common usage of GPIO interface is LED and button control for
various %%SoCs%%.

===== Architecture =====

All the HW-independent stuff resides in sys/dev/gpio
directory. It consists of gpioc (GPIO controller device),
gpiobus (bus that manages devices attached to GPIO controller)
and gpioled (implementation of LED driver that illustrates usage
pattern of gpiobus, utilizes led(4) interface).

HW-dependent part might be a part of !%%SoC%%, I2C extender, whatever.
It should implement interface defined in sys/dev/gpio/gpio_if.m:

<code>
gpio_pin_max           - get maximum pin available
gpio_pin_getcaps       - get capabilities for given pin
gpio_pin_getflags      - get flags for given pin
gpio_pin_setflags      - set flags for given pin
gpio_pin_getname       - get pin name (if any)
gpio_pin_set           - set pin value
gpio_pin_get           - get pin value
gpio_pin_toggle        - toggle (invert) pin value
</code>

And on attaching HW driver should add gpioc and gpiobus as its
children.

gpioc creates /dev/gpiocX entry that is used by gpioctl application
for managing GPIO pins using following ioctls:

<code>
GPIOMAXPIN            - get maximum pin available
GPIOGETCONFIG         - get flags/capabilities/name for given pin
GPIOSETCONFIG         - set flags for given pin
GPIOGET               - get pin value
GPIOSET               - set pin value
GPIOTOGGLE            - toggle (invert) pin value
</code>

gpioctl usage:

<code>
gpioctl -f ctldev -l [-v]
gpioctl -f ctldev -t pin
gpioctl -f ctldev -c pin flag ...
gpioctl -f ctldev pin [0|1]
</code>

gpiobus manages devices attached to gpio controller. Its interface
is a subset of of gpio_if.m interface and allows child devices talk
to GPIO controller. At the moment children could be set only by hints
file and pin space is limited to 32. Example:

<code>
# RF led
hint.gpioled.0.at="gpiobus0"
hint.gpioled.0.name="rf"
# pin 2
hint.gpioled.0.pins=0x0004
</code>

Interface functions:

<code>
gpiobus_pin_getcaps    - get capabilities for given pin
gpiobus_pin_getflags   - get flags for given pin
gpiobus_pin_setflags   - set flags for given pin
gpiobus_pin_set        - set pin value
gpiobus_pin_get        - get pin value
gpiobus_pin_toggle     - toggle (invert) pin value
</code>

===== Device implementations =====

  * **gpioled**. Reference implementation of child device. Provides on/off function for led(4) API
  * **gpioiic**. I2C bus over GPIO. (Author: Luiz Otavio O Souza %%<%%loos.br@gmail.com>)

===== Latest version =====
Code has been committed to HEAD

===== GPIO-hardware =====

Using GPIO pins, examples of use.
[[GPIO/Hardware]]

===== GPIO C examples =====

  * **libgpio**. Provides all the GPIO low level routines for a C program.

===== GPIO TODO =====

  * **interrupts**. Allow GPIO controllers to act as interrupt controllers
  * **runtime attach/detach**. Allow GPIO devices to be attached and detached at runtime
  * **pin groups**. Allow the read/write of a group of pins (8/16/32/64 pins at time ?)
  * **pin features**. Implement pin value inversion, in a generic way.