The length field in a descriptor can be zero. In this case, the H2C Stream
Engine will issue a zero byte read request on PCIe. After the QDMA receives the
completion for the request, the H2C Stream Engine will send out one beat of data with
tlast on the QDMA H2C AXI4-Stream
interface. The zero byte packet will be indicated on the interface by setting the
zero_b_dma bit in the tuser. The user logic must set both the SOP and EOP for a zero byte
descriptor. If not done, an error will be flagged by the H2C Stream Engine.