ChromeOS Embedded Controller

Google's ChromeOS EC is a Cortex-M device which talks to the AP and
implements various function such as keyboard and battery charging.

The EC can be connect through various means (I2C, SPI, LPC, RPMSG) and the
compatible string used depends on the interface. Each connection method has
its own driver which connects to the top level interface-agnostic EC driver.
Other Linux driver (such as cros-ec-keyb for the matrix keyboard) connect to
the top-level driver.

Required properties (I2C):
- compatible: "google,cros-ec-i2c"
- reg: I2C slave address

Required properties (SPI):
- compatible: "google,cros-ec-spi"
- reg: SPI chip select

Required properties (RPMSG):
- compatible: "google,cros-ec-rpmsg"

Optional properties (SPI):
- google,cros-ec-spi-pre-delay: Some implementations of the EC need a little
  time to wake up from sleep before they can receive SPI transfers at a high
  clock rate. This property specifies the delay, in usecs, between the
  assertion of the CS to the start of the first clock pulse.
- google,cros-ec-spi-msg-delay: Some implementations of the EC require some
  additional processing time in order to accept new transactions. If the delay
  between transactions is not long enough the EC may not be able to respond
  properly to subsequent transactions and cause them to hang. This property
  specifies the delay, in usecs, introduced between transactions to account
  for the time required by the EC to get back into a state in which new data
  can be accepted.

Required properties (LPC):
- compatible: "google,cros-ec-lpc"
- reg: List of (IO address, size) pairs defining the interface uses

Optional properties (all):
- google,has-vbc-nvram: Some implementations of the EC include a small
  nvram space used to store verified boot context data. This boolean flag
  is used to specify whether this nvram is present or not.

Example for I2C:

i2c@12ca0000 {
	cros-ec@1e {
		reg = <0x1e>;
		compatible = "google,cros-ec-i2c";
		interrupts = <14 0>;
		interrupt-parent = <&wakeup_eint>;
		wakeup-source;
	};


Example for SPI:

spi@131b0000 {
	ec@0 {
		compatible = "google,cros-ec-spi";
		reg = <0x0>;
		interrupts = <14 0>;
		interrupt-parent = <&wakeup_eint>;
		wakeup-source;
		spi-max-frequency = <5000000>;
		controller-data {
		cs-gpio = <&gpf0 3 4 3 0>;
		samsung,spi-cs;
		samsung,spi-feedback-delay = <2>;
		};
	};
};


Example for LPC is not supplied as it is not yet implemented.