[PATCH] riscv: dts: spacemit: k3-pico-itx: Fix non-functional ethernet TX timing

[Andrew Rembrandt]

The "rgmii-id" PHY mode applies a symmetric 2ns internal delay in the
PHY. On the Pico-ITX board this leaves insufficient TX timing margin and
no ethernet traffic passes.

Add a 400ps MAC-side TX internal delay on top of the PHY's delay to
correct the TX clock/data skew. The value follows the vendor fork's
asymmetric tuning (tx-phase 73 x 33.3ps - 2000ps ID ~= 400ps); the
driver rounds tx-internal-delay-ps to the nearest supported step.

This builds on commit 3ea695eb111f ("dts: riscv: spacemit: k3: Fix I/O
power settings"), which fixed the GMAC0 1.8V/3.3V I/O switch

Tested on Pico-ITX hardware with iperf3: 60s TCP each direction and 60s
UDP, all at 1Gbps with no TX/RX errors on the interface.

Fixes: 74657a376960 ("riscv: dts: spacemit: Add ethernet device for K3")
Signed-off-by: Andrew Rembrandt <kernel at rembrandt.dev>
---
 arch/riscv/boot/dts/spacemit/k3-pico-itx.dts | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/riscv/boot/dts/spacemit/k3-pico-itx.dts b/arch/riscv/boot/dts/spacemit/k3-pico-itx.dts
index b89c1521e664..59aecaa2d32a 100644
--- a/arch/riscv/boot/dts/spacemit/k3-pico-itx.dts
+++ b/arch/riscv/boot/dts/spacemit/k3-pico-itx.dts
@@ -186,6 +186,7 @@ &eth0 {
 	pinctrl-names = "default";
 	pinctrl-0 = <&gmac0_rgmii_0_cfg>, <&gmac0_phy_0_cfg>;
 	phy-mode = "rgmii-id";
+	tx-internal-delay-ps = <400>;
 	phy-handle = <&phy0>;
 	status = "okay";
 
-- 
2.54.0