GTS AXI Multichannel DMA IP for PCI Express* User Guide
A.1.5. Configuration Slave (CS)
This interface is applicable only in Root Port mode. The Configuration Slave (CS) is an AXI-MM non-bursting interface and essentially converts single-cycle, AXI-MM read and write transactions into AXI-Stream reads and writes of PCIe configuration TLPs to be sent to the GTS AXI Streaming IP (to be sent over the PCIe link). This module also processes the completion TLPs (Cpl and CplD) it receives in return.
The CS module converts the AXI-MM request into a configuration TLP with a fixed TAG value (decimal 255) assigned to it and sends it to scheduler. One unique TAG is sufficient as it does not support more than one outstanding transaction. This unique TAG helps in rerouting the completions to the CS module. On the receiving side, the TLP RX scheduler parses the completion field to decode the completion TLP on a fixed TAG and route the transaction over to CS.