linux/Documentation/devicetree/bindings/gpio/spear_spics.txt
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   1=== ST Microelectronics SPEAr SPI CS Driver ===
   2
   3SPEAr platform provides a provision to control chipselects of ARM PL022 Prime
   4Cell spi controller through its system registers, which otherwise remains under
   5PL022 control. If chipselect remain under PL022 control then they would be
   6released as soon as transfer is over and TxFIFO becomes empty. This is not
   7desired by some of the device protocols above spi which expect (multiple)
   8transfers without releasing their chipselects.
   9
  10Chipselects can be controlled by software by turning them as GPIOs. SPEAr
  11provides another interface through system registers through which software can
  12directly control each PL022 chipselect. Hence, it is natural for SPEAr to export
  13the control of this interface as gpio.
  14
  15Required properties:
  16
  17  * compatible: should be defined as "st,spear-spics-gpio"
  18  * reg: mentioning address range of spics controller
  19  * st-spics,peripcfg-reg: peripheral configuration register offset
  20  * st-spics,sw-enable-bit: bit offset to enable sw control
  21  * st-spics,cs-value-bit: bit offset to drive chipselect low or high
  22  * st-spics,cs-enable-mask: chip select number bit mask
  23  * st-spics,cs-enable-shift: chip select number program offset
  24  * gpio-controller: Marks the device node as gpio controller
  25  * #gpio-cells: should be 1 and will mention chip select number
  26
  27All the above bit offsets are within peripcfg register.
  28
  29Example:
  30-------
  31spics: spics@e0700000{
  32        compatible = "st,spear-spics-gpio";
  33        reg = <0xe0700000 0x1000>;
  34        st-spics,peripcfg-reg = <0x3b0>;
  35        st-spics,sw-enable-bit = <12>;
  36        st-spics,cs-value-bit = <11>;
  37        st-spics,cs-enable-mask = <3>;
  38        st-spics,cs-enable-shift = <8>;
  39        gpio-controller;
  40        #gpio-cells = <2>;
  41};
  42
  43
  44spi0: spi@e0100000 {
  45        status = "okay";
  46        num-cs = <3>;
  47        cs-gpios = <&gpio1 7 0>, <&spics 0>,
  48                   <&spics 1>;
  49        ...
  50}
  51