IXGBE_TDT
IXGBE_WRITE_REG(hw, IXGBE_TDT(i), 0);
IXGBE_READ_REG(&sc->hw, IXGBE_TDT(txr->tx_idx)));
IXGBE_WRITE_REG(&sc->hw, IXGBE_TDT(txr->tx_idx), idx);
IXGBE_WRITE_REG(&sc->hw, IXGBE_TDT(txr->tx_idx), idx);