[PATCH 1/3] dt-bindings: spi: Add spi-mux-gpio

[Chris Packham <chris.packham@alliedtelesis.co.nz>]

Add binding documentation for spi-mux-gpio which is a slightly more
complicated hardware implementation of using gpios to steer SPI chip
selects.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
---
 .../devicetree/bindings/spi/spi-mux-gpio.txt  | 45 +++++++++++++++++++
 1 file changed, 45 insertions(+)
 create mode 100644 Documentation/devicetree/bindings/spi/spi-mux-gpio.txt

diff --git a/Documentation/devicetree/bindings/spi/spi-mux-gpio.txt b/Documentation/devicetree/bindings/spi/spi-mux-gpio.txt
new file mode 100644
index 000000000000..a32f25321d37
--- /dev/null
+++ b/Documentation/devicetree/bindings/spi/spi-mux-gpio.txt
@@ -0,0 +1,45 @@
+SPI bus gpio multiplexer
+
+The SPI bus gpio multiplexer can be used to implement more complicated access
+logic than can be supported with the cs-gpios property of a SPI bus.
+
+In the example below we have a SoC with a single SPI CS that is gated by the
+state of a gpio to select the desired SPI device.
+
+     +----------+  CS    +-----+ CS0  +----+
+     |          |--------|     |------|    |
+     |          |        | \ / |      +----+
+     |   SoC    |        |  +  |
+     |          |  GPIO  | / \ | CS1  +----+
+     |          |--------|     |------|    |
+     +----------+        +-----+      +----+
+
+Required properties:
+- compatible	- must be "spi-mux-gpio"
+- gpios		- gpios used to implement the multiplexing logic
+- spi-parent-bus - parent spi bus to use
+
+Optional properties:
+- spi-parent-cs - chip select on parent bus to use. Defaults to 0 if not
+                  specified.
+
+Example for a multiplexer with a single gpio:
+
+	spi-mux {
+		compatible = "spi-mux-gpio";
+		#address-cells = <1>;
+		#size-cells = <0>;
+		gpios = <&gpio0 1 0>;
+		spi-parent-bus = <&spi0>;
+		spi-parent-cs = <0>;
+
+		spi-dev@0 {
+			compatible = "...";
+			reg = <0>;
+		}
+
+		spi-dev@1 {
+			compatible = "...";
+			reg = <1>;
+		}
+	};
-- 
2.21.0