UDSP feature introduced preferred peer nids list as opposed to keeping track of just a single preferred nid, but both methods of dealing with the peer preferred nids are kept around in the code (lnet_peer_ni struct, peer module). The list is used if there is more than one preferred nid and a single entry is used if there's just one preferred nid. The code would get simplified if just the "list" method is used, as it covers all requirements with negligible overhead.