Receive Transaction Ordering - 1.0 English

Versal Adaptive SoC Integrated Block for PCI Express LogiCORE IP Product Guide (PG343)

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The core contains logic on its receive side to ensure that TLPs received from the link and delivered on its completer request interface and requester completion interface do not violate the PCI Express® transaction ordering constraints. The ordering actions performed by the integrated block are based on the following key rules:

  • Posted requests must be able to pass Non-Posted requests on the Completer reQuest (CQ) interface. To enable this capability, the integrated block implements a flow control mechanism on the CQ interface through which user logic can control the flow of Non-Posted requests without affecting Posted requests. The user logic signals the availability of a buffer to receive a Non-Posted request by asserting the pcie_cq_np_req[0] signal.

    The integrated block delivers a Non-Posted request to the user application only when the available credit is non-zero. The integrated block continues to deliver Posted requests while the delivery of Non-Posted requests has been paused for lack of credit. When no back pressure is applied by the credit mechanism for the delivery of Non-Posted requests, the integrated block delivers Posted and Non-Posted requests in the same order as received from the link. For more information on controlling the flow of Non-Posted requests, see Selective Flow Control for Non-Posted Requests.

  • PCIe ordering requires that a completion TLP not be allowed to pass a Posted request, except in the following cases:
    • Completions with the Relaxed Ordering attribute bit set can pass Posted requests.
    • Completions with the ID-based ordering bit set can pass a Posted request if the Completer ID is different from the Posted Requester ID.

The integrated block does not start the transfer of a Completion TLP received from the link on the Requester Completion (RC) interface until it has completely transferred all Posted TLPs that arrived before it, unless one of the two rules applies.

After a TLP has been transferred completely to the user interface, it is the responsibility of the user application to enforce ordering constraints whenever needed.

Table 1. Receive Ordering Rules
Row Pass Posted Non-Posted Completion
Posted No Yes Yes
Non-Posted No No Yes

a) No

b) Yes (Relaxing Ordering)

Yes No