In both Dword-aligned and 128-bit address aligned modes, the transfer starts with the
sixteen descriptor bytes, followed by the payload bytes. The user application must keep
the signal s_axis_rq_tvalid asserted over the duration of the packet.
The core treats the deassertion of s_axis_rq_tvalid during the packet
transfer as an error, and nullifies the corresponding request TLP transmitted on the
link to avoid data corruption.
The user application must also assert the signal
s_axis_rq_tlast in the last beat of the packet. The core are by
pull down s_axis_rq_tready in any cycle if it is not ready to accept
data. The user application must not change the values on s_axis_rq_tdata and s_axis_rq_tlast during
the transfer when the core has deasserted s_axis_rq_tready. The AXI4-Stream
interface signals m_axis_rq_tkeep (one per Dword
position) must be set to indicate the valid Dwords in the packet including the
descriptor and any null bytes inserted between the descriptor and the payload. That is,
the m_axis_rq_tkeep bits must be set to 1 contiguously
from the first Dword of the descriptor until the last Dword of the payload. During the
transfer of a packet, the m_axis_rq_tkeep bits can be 0
only in the last beat of the packet, when the packet does not fill the entire width of
the interface.
The requester request interface also includes the First Byte Enable and the Last Enable
bits in the s_axis_rq_tuser bus. These must be set in the first beat of
the packet, and provides information of the valid bytes in the first and last Dwords of
the payload.
The user application must limit the size of the payload transferred in a single
request to the maximum payload size configured in the core, and must ensure that the
payload does not cross a 4 Kbyte boundary. For memory writes of two Dwords or less, the
1s in first_be[15:0] and last_be[15:0] are not be contiguous. For the special case of a zero-length
memory write request, the user application must provide a dummy one_dword payload with first_be[15:0] and
last_be[15:0] both set to all 0s. In all other
cases, the 1 bits in first_be[15:0] and last_be[15:0] must be contiguous. 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 and so on.
The following figure illustrates the Dword-aligned transfer of a memory write request from the user logic across the requester request interface. For the purpose of illustration, the size of the data block being written into user memory is assumed to be n Dwords, for some n = k*16 - 1, where k > 1 .
The following figure illustrates the 128-bit address aligned transfer of a
memory write request from the user application across the requester request interface.
For the purpose of illustration, the starting Dword offset of the data block is assumed
to be (m*16 +3), for some integer m > 0. Its size is assumed to be n Dwords, for some
n = k*16 -1, k > 1. In the
128-bit address-aligned mode, the delivery of the payload always starts in the second
128-bit quarter of the 1024-bit word, following the descriptor in the first quarter. The
user application must communicate the offset of the first Dword of the payload in the
addr_offset[15:0] field of the s_axis_rq_tuser bus. The user application must also set the
bits in first_be[15:0] to indicate the valid bytes in
the first Dword and the bits in last_be[15:0] to
indicate the valid bytes in the last Dword of the payload.
