A Burst Read transaction to OPB DDR when bank address boundaries are crossed with the refresh cycle occurring simultaneously causes the Ethernet SGDMA transaction to hang.
The DDR controller hangs while waiting for more RdAcks and RdReq, and it never negates.
This problem has been fixed in the latest "mch_opb_ddr_v1_00_b", which is included in the latest EDK 8.1i Service Pack available at:
http://www.xilinx.com/xlnx/xil_sw_updates_home.jsp
The first service pack containing the fix is EDK 8.1i Service Pack 1.