[RFC 1/2] dt-bindings: gpio: Document shared GPIO line usage

[Peter Ujfalusi]

Boards might use the same GPIO line to control several external devices.
Add section to document on how a shared GPIO pin can be described.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@xxxxxx>
---
 .../devicetree/bindings/gpio/gpio.txt         | 66 +++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/Documentation/devicetree/bindings/gpio/gpio.txt b/Documentation/devicetree/bindings/gpio/gpio.txt
index a8895d339bfe..644e6513607c 100644
--- a/Documentation/devicetree/bindings/gpio/gpio.txt
+++ b/Documentation/devicetree/bindings/gpio/gpio.txt
@@ -228,6 +228,72 @@ Example of two SOC GPIO banks defined as gpio-controller nodes:
 		#gpio-cells = <2>;
 	};
 
+On boards one GPIO line might be connected to multiple devices as reset, enable
+or other control pins. In order to make it safer for this usage of a GPIO line
+one can describe the shared GPIO pin.
+
+Each shared GPIO pin definition is represented as a child node of the GPIO
+controller.
+
+Required properties:
+- gpio-shared:	A property specifying that this child node represents a shared
+		GPIO pin.
+- gpios:	Store the GPIO information (id, flags, ...) for each GPIO to
+		affect. Shall contain an integer multiple of the number of cells
+		specified in its parent node (GPIO controller node).
+Only one of the following properties scanned in the order shown below.
+This means that when multiple properties are present they will be searched
+in the order presented below and the first match is taken as the intended
+configuration.
+- output-low:	A property specifying to set the GPIO direction as output with
+		the value low initially.
+- output-high:	A property specifying to set the GPIO direction as output with
+		the value high initially.
+The shared GPIO line management strategy can be selected with either of the
+following properties:
+- refcounted-low: The line must be kept low as long as there is at least one
+		request asking it to be low.
+- refcounted-high: The line must be kept high as long as there is at least one
+		request asking it to be high.
+If neither of the refcounting strategy was selected then the shared GPIO is
+handled as pass through. In this mode all user requests will be forwarded to the
+shared GPIO pin without refcounting.
+
+Optional properties:
+- line-name:  The GPIO label name. If not present the node name is used.
+
+Example of shared GPIO use:
+
+	qe_pio_a: gpio-controller@1400 {
+		compatible = "fsl,qe-pario-bank-a", "fsl,qe-pario-bank";
+		reg = <0x1400 0x18>;
+		gpio-controller;
+		#gpio-cells = <2>;
+
+		line_a {
+			gpio-shared;
+			gpios = <5 0>;
+			output-low;
+			refcounted-high;
+			line-name = "enable-for-devices";
+		};
+	};
+
+	device@0 {
+		compatible = "some,device";
+		enable-gpios = <&qe_pio_a 5 0>;
+	};
+
+	device@1 {
+		compatible = "some,device";
+		enable-gpios = <&qe_pio_a 5 0>;
+	};
+
+	component@0 {
+		compatible = "some,component";
+		select-gpios = <&qe_pio_a 5 0>;
+	};
+
 2.1) gpio- and pin-controller interaction
 -----------------------------------------
 
-- 
Peter

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