Outbound Ordering (Root Port)

Table 1. Root Port Outbound Ordering
Row Pass Column? Posted Request (Col 2) Non-Posted Request Completion
(Col 5)
Read Request
(Col 3)
 NPR with Data
(Col 4)
Posted request No Yes1 No2 Yes
Non-posted request Read request Order presented on AXI3 No Yes Yes/No
NPR with data No Yes/No No Yes/No
Completion Order presented on AXI4 Yes Yes No

If read and write transactions are serviceable (i.e., ready to be sent to the link) in the same clock cycle, the Enable AXI Bridge Write Priority bit in Debug Mux Control register dictates whether the write or the read is serviced.

  • A write transaction is deemed serviceable from the asynchronous FIFO when all data has reached the FIFO.
  • A read transaction is deemed serviceable when the read transaction sits in the asynchronous FIFO.

The completions from the AXI master interface are not ordered with respect to the read and write transactions on the AXI slave interface. All three transactions constitute TLPs flowing outbound.

1 When a non-posted request is blocked, a posted request can pass it.
2 The posted packets are blocked if non-posted packets are stuck in the pipeline.
3 If the write is blocked, the read can go ahead of the write. If there is an address overlap between the write and read, your application can wait for the write response before giving the read.
4 If the posted request is blocked, the completion can pass the posted request.