Non-Posted transactions with no payload (memory read requests, I/O read requests,
Configuration read requests) are transferred across the requester request interface in
the same manner as a memory write request, except that the AXI4-Stream packet contains
only the 16-byte descriptor. The following figure illustrates the transfer of a memory
read request across the requester request interface. The signal
s_axis_rq_tvalid must remain asserted over the duration of the
packet. The core are pull down s_axis_rq_tready to prolong the beat.
The signal s_axis_rq_tlast must be set in the last beat of the packet,
and the bits in s_axis_rq_tkeep[15:0] must be set in all Dword
positions where a descriptor is present.
The user application must indicate the valid bytes in the first and last Dwords of the
data block using the fields first_be[7:0] and
last_be[7:0], respectively, in the s_axis_rq_tuser
bus. For the special case of a zero-length memory read, the length of the request must
be set to one Dword, with both first_be[7:0] and
last_be[7:0] set to all 0s. The user application must also
communicate the offset of the first Dword of the payload of the resulting Completion,
when delivered over the requester completion interface, in the
addr_offset[3:0] field of the s_axis_rq_tuser bus.
In Straddled case, addr_offset[3:2], first_be[7:4],
and last_be[7:4] are used to indicate second TLP information while
addr_offset[1:0], first_be[3:0], and
last_be[3:0] are used to indicate the first TLP information on that
data beat.
