Recently, massive multiple-input multiple-output (MIMO) system has attracted tremendous research interests in wireless communications. Among various performance aspects and design problems regarding massive MIMO, precoding at the base station (BS) is one of the most important function components to ensure reliable downlink transmission. This paper provides a survey of linear precoding techniques for downlink transmission under both single-cell (SC) and multicell (MC) scenarios. In an SC scenario, a series of classical precoding techniques are reviewed and compared. In an MC scenario, massive MIMO suffers from pilot contamination problems, where orthogonal pilot sequences have to be reused among cells, leading to estimation errors of channel state information at the transmitter. Precoding techniques with and without the cooperation among BSs are reviewed and the cooperative precoding techniques are further divided into partial and full cooperations. To the best of our knowledge, this is the first work that comprehensively surveys all the existing linear precoding techniques. The comprehensive review reveals current challenges in designing the precoders, where technical merits are usually bounded by practical implementation issues. We show that a viable precoding technique for massive MIMO systems is still unknown to date, and suggest several research potentials that are worthy of future research efforts.