[PATCH v2 1/2] riscv: dts: sophgo: add Milk-V Duo 256M ethernet support overlay

[Chen-Yu Yeh <chenyou910331@gmail.com>]

The SG2002 has an internal 10/100 Ethernet PHY, but on the Milk-V Duo
256M the Ethernet signals only reach the RJ45 connector on the optional
USB & Ethernet IO board. The MAC (gmac0) and the MDIO multiplexer used
to reach the internal PHY are already described in cv180x.dtsi but are
disabled, and they cannot be enabled in the board devicetree because
without the IO board the same pins may be used as GPIO.

Add an overlay that enables them.

Tested on Milk-V Duo 256M with the USB & Ethernet IO board: the link
comes up at 100Mbps/Full, the interface gets an address via DHCP and
passes traffic in both directions. Without the overlay, eth0 does not
appear.

Signed-off-by: Chen-Yu Yeh <chenyou910331@gmail.com>
---
 arch/riscv/boot/dts/sophgo/Makefile             |  3 +++
 .../dts/sophgo/sg2002-milkv-duo256m-eth.dtso    | 17 +++++++++++++++++
 2 files changed, 20 insertions(+)
 create mode 100644 arch/riscv/boot/dts/sophgo/sg2002-milkv-duo256m-eth.dtso

diff --git a/arch/riscv/boot/dts/sophgo/Makefile b/arch/riscv/boot/dts/sophgo/Makefile
index d5c80d4ff005..49639cb0888b 100644
--- a/arch/riscv/boot/dts/sophgo/Makefile
+++ b/arch/riscv/boot/dts/sophgo/Makefile
@@ -4,6 +4,9 @@ dtb-$(CONFIG_ARCH_SOPHGO) += cv1812h-huashan-pi.dtb
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2000-milkv-duo-s.dtb
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2002-licheerv-nano-b.dtb
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2002-milkv-duo256m.dtb
+sg2002-milkv-duo256m-eth-dtbs := sg2002-milkv-duo256m.dtb sg2002-milkv-duo256m-eth.dtbo
+dtb-$(CONFIG_ARCH_SOPHGO) += sg2002-milkv-duo256m-eth.dtb
+dtb-$(CONFIG_ARCH_SOPHGO) += sg2002-milkv-duo256m-eth.dtbo
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2042-milkv-pioneer.dtb
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2042-evb-v1.dtb
 dtb-$(CONFIG_ARCH_SOPHGO) += sg2042-evb-v2.dtb
diff --git a/arch/riscv/boot/dts/sophgo/sg2002-milkv-duo256m-eth.dtso b/arch/riscv/boot/dts/sophgo/sg2002-milkv-duo256m-eth.dtso
new file mode 100644
index 000000000000..cc93746d932c
--- /dev/null
+++ b/arch/riscv/boot/dts/sophgo/sg2002-milkv-duo256m-eth.dtso
@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: (GPL-2.0 OR MIT)
+/*
+ * Enable ethernet on Milk-V Duo 256M with the USB & Ethernet IO board.
+ *
+ * Copyright (c) 2026, Chen-Yu Yeh <chenyou910331@gmail.com>
+ */
+
+/dts-v1/;
+/plugin/;
+
+&gmac0 {
+	status = "okay";
+};
+
+&mdio {
+	status = "okay";
+};
-- 
2.43.0