dtoverlay: Add an overlay for the Raspberry Pi firmware UART

Add a device-tree overlay to configure the GPIOs for the
Raspberry Pi firmware UART.

Example config.txt
dtoverlay=rpi-fw-uart,txd0_pin=20,rxd0_pin=21

Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Tim Gover <tim.gover@raspberrypi.com>

Author: timg236

arch/arm/boot/dts/overlays/Makefile

@@ -234,6 +234,7 @@ dtbo-$(CONFIG_ARCH_BCM2835) += \
 	rpi-dacpro.dtbo \
 	rpi-digiampplus.dtbo \
 	rpi-ft5406.dtbo \
+	rpi-fw-uart.dtbo \
 	rpi-poe.dtbo \
 	rpi-poe-plus.dtbo \
 	rpi-sense.dtbo \

arch/arm/boot/dts/overlays/README

@@ -4175,6 +4175,18 @@ Params: touchscreen-size-x      Touchscreen X resolution (default 800)
         touchscreen-swapped-x-y Swap X and Y cordinates (default 0);
 
 
+Name:   rpi-fw-uart
+Info:   Configures the firmware software UART driver.
+        This driver requires exclusive usage of the second VPU core. The
+        following config.txt entries should be set when this driver is used.
+        dtparam=audio=off
+        isp_use_vpu0=1
+Load:   dtoverlay=rpi-fw-uart,<param>[=<val>]
+Params: txd0_pin                GPIO pin for TXD0 (any free - default 20)
+
+        rxd0_pin                GPIO pin for RXD0 (any free - default 21)
+
+
 Name:   rpi-poe
 Info:   Raspberry Pi PoE HAT fan
 Load:   dtoverlay=rpi-poe,<param>[=<val>]

arch/arm/boot/dts/overlays/rpi-fw-uart-overlay.dts

@@ -0,0 +1,41 @@
+// SPDX-License-Identifier: GPL-2.0
+// Overlay for the Raspberry Pi Firmware UART driver
+/dts-v1/;
+/plugin/;
+
+/{
+	compatible = "brcm,bcm2835";
+
+	fragment@0 {
+		target = <&gpio>;
+		__overlay__ {
+			rpi_fw_uart_pins: rpi_fw_uart_pins@4 {
+				brcm,pins = <20 21>;
+				brcm,function = <1 0>; /* output input */
+				brcm,pull = <0 2>; /* none pull-up */
+			};
+		};
+	};
+
+	fragment@1 {
+		target = <&soc>;
+		__overlay__ {
+			rpi_fw_uart: rpi_fw_uart@7e000000 {
+			compatible = "raspberrypi,firmware-uart";
+			reg = <0x7e000000 0x100>; /* VideoCore MS sync regs */
+			firmware = <&firmware>;
+			pinctrl-names = "default";
+			pinctrl-0 = <&rpi_fw_uart_pins>;
+			tx-gpios = <&gpio 20 0>;
+			rx-gpios = <&gpio 21 0>;
+			};
+		};
+	};
+
+	__overrides__ {
+		txd0_pin = <&rpi_fw_uart>,"tx-gpios:4",
+			 <&rpi_fw_uart_pins>, "brcm,pins:0";
+		rxd0_pin = <&rpi_fw_uart>,"rx-gpios:4",
+			 <&rpi_fw_uart_pins>, "brcm,pins:4";
+	};
+};