Using the PCIe-XVC-VSEC approach, the Debug Bridge IP uses a PCIe Vendor Specific Extended Capability (VSEC) to implement the connection from PCIe to the Debug Bridge IP. The PCIe extended configuration space is set up as a linked list of extended capabilities that are discoverable from a Host PC. This is specifically valuable for platforms where one version of the design implements the PCIe-XVC-VSEC and another design implementation does not. The linked list can be used to detect the existence or absence of the PCIe-XVC-VSEC and respond accordingly.
The PCIe Extended Configuration Interface uses PCIe configuration transactions rather than PCIe memory BAR transactions. While PCIe configuration transactions are much slower, they do not interfere with PCIe memory BAR transactions at the PCIe IP boundary. This allows for separate data and debug communication paths within the FPGA. This is ideal if you expect to debug the datapath. Even if the datapath becomes corrupt or halted, the PCIe Extended Configuration Interface can remain operational to perform debug. The following figure describes the connectivity between the PCIe IP and the Debug Bridge IP to implement the PCIe-XVC-VSEC.
