Marker responses can be generated for any descriptor by setting the mrkr_req bit. Marker responses are generated after the
descriptor is completed. Similar to host writebacks, excessive marker response requests
can reduce descriptor engine performance. Along with mrkr_req signals,
sdi can also be set. In this case, the marker response is sent on queue status ports and
writeback is sent to the host. The marker responses are sent on queue status ports that
can be identified by the queue id.
Descriptor completion is defined as when the descriptor data transfer has
completed and its write data is acknowledged on AXI (H2C bresp for AXI4, Valid/Ready of ST), or is accepted by
the PCIe Controller’s transaction layer for transmission (C2H
MM).