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. Marker responses to the user logic can also be
sent with the sbi bit if configured in the context. The
Marker responses are sent on Queue Status ports which 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 AXI
MM, Valid/Ready of ST), or is accepted by the PCIe Controller’s transaction layer for
transmission (C2H MM).