RGMII tuning is driven in phy framework using "rgmii-id", "rgmii-txid", "rgmii-rxid" properties Make sure to set phy-mode to any of these as per your board requirement. In addition to enabling tuning, some phys also give control of tuning values via devicetree. Please refer to the devicetree bindings documentation of the phy you use in order to tune these according to your board.