The GT Quad interface is clocked by rx_alt_serdes_clk[0] through
rx_alt_serdes_clk[5] in the DCMAC Subsystem RX
direction and clocked by tx_alt_serdes_clk[0] through
tx_alt_serdes_clk[5] in the DCMAC Subsystem TX
direction. The rx_serdes_clk[N] connects to a GT Quad transceiver
generated clock whose frequency is twice the frequency of
rx_alt_serdes_clk[N]. Similarly, the
tx_serdes_clk[N] connects to a GT Quad transceiver generated clock
whose frequency is twice the frequency of tx_alt_serdes_clk[N].