drm/bridge: Add RGB to VGA bridge support

Some boards have an entirely passive RGB to VGA bridge, based on DACs
implemented by resistor ladders.

Those might or might not have an i2c bus routed to the VGA connector in
order to access the screen EDIDs.

Add a bridge that doesn't do anything but expose the modes available on the
screen, either based on the EDIDs if available, or based on the XGA
standards.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Tested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20160930143709.1388-3-maxime.ripard@free-electrons.com

1 files changed, 48 insertions, 0 deletions

diff --git a/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt b/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt
new file mode 100644
index 000000000000..003bc246a270
--- /dev/null
+++ b/Documentation/devicetree/bindings/display/bridge/dumb-vga-dac.txt
@@ -0,0 +1,48 @@
+Dumb RGB to VGA DAC bridge
+---------------------------
+
+This binding is aimed for dumb RGB to VGA DAC based bridges that do not require
+any configuration.
+
+Required properties:
+
+- compatible: Must be "dumb-vga-dac"
+
+Required nodes:
+
+This device has two video ports. Their connections are modelled using the OF
+graph bindings specified in Documentation/devicetree/bindings/graph.txt.
+
+- Video port 0 for RGB input
+- Video port 1 for VGA output
+
+
+Example
+-------
+
+bridge {
+	compatible = "dumb-vga-dac";
+	#address-cells = <1>;
+	#size-cells = <0>;
+
+	ports {
+		#address-cells = <1>;
+		#size-cells = <0>;
+
+		port@0 {
+			reg = <0>;
+
+			vga_bridge_in: endpoint {
+				remote-endpoint = <&tcon0_out_vga>;
+			};
+		};
+
+		port@1 {
+			reg = <1>;
+
+			vga_bridge_out: endpoint {
+				remote-endpoint = <&vga_con_in>;
+			};
+		};
+	};
+};