diff --git a/target/linux/ar71xx/files/arch/mips/ath79/dev-eth.c b/target/linux/ar71xx/files/arch/mips/ath79/dev-eth.c
index e6a50760193..9cd6e37efce 100644
--- a/target/linux/ar71xx/files/arch/mips/ath79/dev-eth.c
+++ b/target/linux/ar71xx/files/arch/mips/ath79/dev-eth.c
@@ -616,7 +616,14 @@ static int __init ath79_setup_phy_if_mode(unsigned int id,
 			break;
 
 		case ATH79_SOC_AR7242:
-			/* FIXME */
+			switch (pdata->phy_if_mode) {
+			case PHY_INTERFACE_MODE_RGMII:
+				break;
+			default:
+				/* Other PHY modes are not tested yet. */
+				return -EINVAL;
+			}
+			break;
 
 		case ATH79_SOC_AR9341:
 		case ATH79_SOC_AR9342: