overlays: Add dpi18 overlay (#1634)

Add support for 18-bit DPI displays. Although the dpi24 overlay could
be used, this overlay leaves GPIOs 22-27 free for other uses.

Author: pelwell

arch/arm/boot/dts/overlays/Makefile

@@ -22,6 +22,7 @@ dtbo-$(RPI_DT_OVERLAYS) += audremap.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += bmp085_i2c-sensor.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += dht11.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += dionaudio-loco.dtbo
+dtbo-$(RPI_DT_OVERLAYS) += dpi18.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += dpi24.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += dwc-otg.dtbo
 dtbo-$(RPI_DT_OVERLAYS) += dwc2.dtbo

arch/arm/boot/dts/overlays/README

@@ -283,6 +283,14 @@ Load:   dtoverlay=dionaudio-loco
 Params: <None>
 
 
+Name:   dpi18
+Info:   Overlay for a generic 18-bit DPI display
+        This uses GPIOs 0-21 (so no I2C, uart etc.), and activates the output
+        2-3 seconds after the kernel has started.
+Load:   dtoverlay=dpi18
+Params: <None>
+
+
 Name:   dpi24
 Info:   Overlay for a generic 24-bit DPI display
         This uses GPIOs 0-27 (so no I2C, uart etc.), and activates the output

arch/arm/boot/dts/overlays/dpi18-overlay.dts

@@ -0,0 +1,31 @@
+/dts-v1/;
+/plugin/;
+
+/{
+	compatible = "brcm,bcm2708";
+
+	// There is no DPI driver module, but we need a platform device
+	// node (that doesn't already use pinctrl) to hang the pinctrl
+	// reference on - leds will do
+
+	fragment@0 {
+		target = <&leds>;
+		__overlay__ {
+			pinctrl-names = "default";
+			pinctrl-0 = <&dpi18_pins>;
+		};
+	};
+
+	fragment@1 {
+		target = <&gpio>;
+		__overlay__ {
+			dpi18_pins: dpi18_pins {
+				brcm,pins = <0 1 2 3 4 5 6 7 8 9 10 11
+					     12 13 14 15 16 17 18 19 20
+					     21>;
+				brcm,function = <6>; /* alt2 */
+				brcm,pull = <0>; /* no pull */
+			};
+		};
+	};
+};