IIRC Mellanox was always doing Infiniband; they made the hardware and Voltaire wrote the drivers (before people understood open source and created OFED).
Mellanox made the HCAs (aka network cards) and Voltaire made the switches. Mellanox had their own drivers and their own MOFED (their incompatible fork of openfabrics upstream OFED with Mellanox specific enhancements).