linux/Documentation/devicetree/bindings/gpio/spear_spics.txt

=== ST Microelectronics SPEAr SPI CS Driver ===

SPEAr platform provides a provision to control chipselects of ARM PL022 Prime
Cell spi controller through its system registers, which otherwise remains under
PL022 control. If chipselect remain under PL022 control then they would be
released as soon as transfer is over and TxFIFO becomes empty. This is not
desired by some of the device protocols above spi which expect (multiple)
transfers without releasing their chipselects.

Chipselects can be controlled by software by turning them as GPIOs. SPEAr
provides another interface through system registers through which software can
directly control each PL022 chipselect. Hence, it is natural for SPEAr to export
the control of this interface as gpio.

Required properties:

  * compatible: should be defined as "st,spear-spics-gpio"
  * reg: mentioning address range of spics controller
  * st-spics,peripcfg-reg: peripheral configuration register offset
  * st-spics,sw-enable-bit: bit offset to enable sw control
  * st-spics,cs-value-bit: bit offset to drive chipselect low or high
  * st-spics,cs-enable-mask: chip select number bit mask
  * st-spics,cs-enable-shift: chip select number program offset
  * gpio-controller: Marks the device node as gpio controller
  * #gpio-cells: should be 1 and will mention chip select number

All the above bit offsets are within peripcfg register.

Example:
-------
spics: spics@e0700000{
        compatible = "st,spear-spics-gpio";
        reg = <0xe0700000 0x1000>;
        st-spics,peripcfg-reg = <0x3b0>;
        st-spics,sw-enable-bit = <12>;
        st-spics,cs-value-bit = <11>;
        st-spics,cs-enable-mask = <3>;
        st-spics,cs-enable-shift = <8>;
        gpio-controller;
        #gpio-cells = <2>;
};


spi0: spi@e0100000 {
        status = "okay";
        num-cs = <3>;
        cs-gpios = <&gpio1 7 0>, <&spics 0>,
                   <&spics 1>;
	...
}