overlays: Add the rp1-pio-uart overlay

An overlay to enable the PIO-based UART.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

Author: pelwell

arch/arm/boot/dts/overlays/Makefile

@@ -247,6 +247,7 @@ dtbo-$(CONFIG_ARCH_BCM2835) += \
 	ramoops-pi4.dtbo \
 	rootmaster.dtbo \
 	rotary-encoder.dtbo \
+	rp1-pio-uart.dtbo \
 	rpi-backlight.dtbo \
 	rpi-codeczero.dtbo \
 	rpi-dacplus.dtbo \

arch/arm/boot/dts/overlays/README

@@ -4527,6 +4527,17 @@ Params: pin_a                   GPIO connected to rotary encoder channel A
                                 common) and "binary".
 
 
+Name:   rp1-pio-uart
+Info:   Configures a software UART implemented using the RP1 PIO block, with
+        DMA moving data and a PIO interrupt used for break detection. Only
+        8N1 with no hardware flow control is supported. Only usable on
+        Raspberry Pi 5 and other platforms with an RP1 southbridge.
+Load:   dtoverlay=rp1-pio-uart,<param>=<val>
+Params: tx_pin                  GPIO used for TXD (any free - default 14)
+
+        rx_pin                  GPIO used for RXD (any free - default 15)
+
+
 Name:   rpi-backlight
 Info:   Raspberry Pi official display backlight driver
 Load:   dtoverlay=rpi-backlight

arch/arm/boot/dts/overlays/rp1-pio-uart-overlay.dts

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+// Device tree overlay for the RP1 PIO-based UART driver.
+/dts-v1/;
+/plugin/;
+
+/ {
+	compatible = "brcm,bcm2712";
+
+	fragment@0 {
+		target-path = "/";
+		__overlay__ {
+			rp1_pio_uart: rp1_pio_uart@e {
+				compatible = "raspberrypi,rp1-pio-uart";
+				tx-gpios = <&gpio 14 0>;
+				rx-gpios = <&gpio 15 0>;
+				status = "okay";
+			};
+		};
+	};
+
+	__overrides__ {
+		tx_pin = <&rp1_pio_uart>, "tx-gpios:4";
+		rx_pin = <&rp1_pio_uart>, "rx-gpios:4";
+	};
+};