The 82574 under linux, failed to read its PHY registers through MDIC(0x00020 RW). The return value indicated that the bit30 of MDIC was set to 1. The datasheet describes : If the PHY does not generate a ZERO as the second bit of the turn-around cycle for reads, the MAC aborts the access, sets the E(error)bit, writes 0xFFFF to the data field to indicate an error condition, and sets the Ready bit.
What does "generate a ZERO as...." mean? And how does it happen? Please help me if anyone knows that.