diff --git a/arch/arm64/boot/dts/renesas/r8a77990-ebisu.dts b/arch/arm64/boot/dts/renesas/r8a77990-ebisu.dts
index 81db803f4389..b38f9d442fc0 100644
--- a/arch/arm64/boot/dts/renesas/r8a77990-ebisu.dts
+++ b/arch/arm64/boot/dts/renesas/r8a77990-ebisu.dts
@@ -271,6 +271,14 @@ phy0: ethernet-phy@0 {
 		interrupt-parent = <&gpio2>;
 		interrupts = <21 IRQ_TYPE_LEVEL_LOW>;
 		reset-gpios = <&gpio1 20 GPIO_ACTIVE_LOW>;
+		/*
+		 * TX clock internal delay mode is required for reliable
+		 * 1Gbps communication using the KSZ9031RNX phy present on
+		 * the Ebisu board, however, TX clock internal delay mode
+		 * isn't supported on r8a77990.  Thus, limit speed to
+		 * 100Mbps for reliable communication.
+		 */
+		max-speed = <100>;
 	};
 };
 
diff --git a/arch/arm64/boot/dts/renesas/r8a77995-draak.dts b/arch/arm64/boot/dts/renesas/r8a77995-draak.dts
index fe14792cedfb..0bbd4d701042 100644
--- a/arch/arm64/boot/dts/renesas/r8a77995-draak.dts
+++ b/arch/arm64/boot/dts/renesas/r8a77995-draak.dts
@@ -175,6 +175,14 @@ phy0: ethernet-phy@0 {
 		reg = <0>;
 		interrupt-parent = <&gpio5>;
 		interrupts = <19 IRQ_TYPE_LEVEL_LOW>;
+		/*
+		 * TX clock internal delay mode is required for reliable
+		 * 1Gbps communication using the KSZ9031RNX phy present on
+		 * the Draak board, however, TX clock internal delay mode
+		 * isn't supported on r8a77995.  Thus, limit speed to
+		 * 100Mbps for reliable communication.
+		 */
+		max-speed = <100>;
 	};
 };