dm8168 dma read remote pcie memory return 0xFF

HI

    our topology is:

        DM8168_A      DM8168_B                                      DM8168_C    DM8168_D

           __|___________|__                                               __|___________|__

          |   PCIE SWITCH      |                                               |  PCIE SWITCH        |

          |_______________|                                              |________________|

                          |                                                                                  |

                PCI2PCIE bridge                                                     PCI2PCIE bridge

                         |___________________________________|

                         |___________ PCI bridge  ______________|  

                                                        |

                                                     RC                           

the base configure: DM8168's pcie width is X2, rate is 5Gbps;  PCI run on 66MHZ, 32 data line.

we use dma to read/write remote pcie memory region.

problem:                                                    

1.  when DM8168_A dma read DM8168_B's pcie memory region, the data is OK. (pls note that DM8168_A and B link to the same switch)

2.  when DM8168_A dma read DM8618_C's pcie memory region, then sometimes the data is full of F. (pls note that DM8168_A and C link to diff switch, and the path through pci bus).

3.  the test: DM8168_A cpu read DM8168_C's pcie memory region is TBD.

we know that the data is full of F, it mean there are timeout occur somewhere, maybe the pcie transcation initors, or maybe dma's. 

Is there any cue to fix the problem ? 

thanks.