[PCIe] Downstream Port Acceptance Dependencies

[Eric89]

Hi~

I'm studying Transaction Ordering in PCIe specification and having trouble with the terms "Port Acceptance Dependency". Hope someone could help me figure it out.

Refer from the specification, "Port acceptance dependencies refer to the Ingress Port making the acceptance of a Posted Request or Completion dependent upon the Egress Port first being able to transmit one or more TLPs." According to the dependency rules, only Downstream Ports are allowed to have these dependencies. I'm surmising that the purpose of these "dependencies" is to avoid the congestion at the egress ports, since Downstream Ports are closer to Root Complex and have bigger chance to process more requests than Upstream Ports.

If my opinion of Port Acceptance Dependency is right, here are my questions.

1. How do Downstream Ports actually do if they have Port Acceptance Dependencies? Do they simply "reject" requests from the Upstream Ports? I cannot imagine how ports reject requests.

2. If Downstream Ports can actually "reject" requests, is there any rules for restricting  Downstream Ports refuse requests with no reason. According to the specification, "For Root Ports and Switch Downstream Ports, acceptance of a Posted Request or Completion must not depend upon the transmission of a Non-Posted Request within the same traffic class." Does this means Downstream Ports have the ability to NOT accept any TLPs when transmitting a Posted Request or Completion?

3. The chapter reference Appendix Request Dependencies when introducing the dependencies rules for Downstream Ports and Upstream Ports. What is the difference between Request Dependencies and Port Acceptance Dependencies? My apology for asking a dumb question, but I can't understand a word reading the Appendix Request Dependencies...

Much appreciation if someone can explain~

[Ethanity_su]

Downstream Ports and Port Acceptance Dependencies Downstream Ports don’t reject requests outright; instead, they delay accepting them. The request is held until the Egress Port can transmit the necessary data. This helps prevent congestion at the Egress Port. The Downstream Port only accepts the request when it can safely process and send the data. Rejecting Requests Downstream Ports cannot reject requests without a good reason. The specification ensures that Posted Requests or Completions should not be delayed because of Non-Posted Requests in the same traffic class. So, while requests can be delayed, they cannot be rejected arbitrarily. Port Acceptance Dependencies vs. Request Dependencies

Port Acceptance Dependencies manage when a request is accepted, based on the ability of the Egress Port to transmit data.

Request Dependencies define the order in which certain types of requests must be processed, such as a Completion needing a Posted Request to be processed first.

In short, Port Acceptance Dependencies control the timing of accepting requests, while Request Dependencies deal with the order of processing different types of requests.

EThan