Native PCI Express Ordering Rules
PCI Express transaction ordering for native devices can be summarized with four simple rules:
PCI Express requires strong ordering of transactions (i.e., performing transactions in the order issued by software) flowing through the fabric that have the same TC assignment (see item 4 for the exception to this rule). Because all transactions that have the same TC value assigned to them are mapped to a given VC, the same rules apply to transactions within each VC. No ordering relationship exists between transactions with different TC assignments. The ordering rules apply in the same way to all types of transactions: memory, IO, configuration, and messages. Under limited circumstances, transactions with the Relaxed Ordering attribute bit set can be ordered ahead of other transactions with the same TC.
These fundamental rules ensure that transactions always complete in the order intended by software. However, these rules are extremely conservative and do not necessarily result in optimum performance. For example, when transactions from many devices merge within switches, there may be no ordering relationship between transactions from these different devices. In such cases, more aggressive rules can be applied to improve performance as discussed in "Modified Ordering Rules Improve Performance" on page 322.
Producer/Consumer Model with Native Devices
Because the Producer/Consumer model depends on strong ordering, when the following conditions are met native PCI Express devices support this model without additional ordering rules:
All elements associated with the Producer/Consumer model reside within native PCI Express devices. All transactions associated with the operation of the Producer/Consumer model transverse only PCI Express links within the same fabric. All associated transactions have the same TC values. If different TC values are used, then the strong ordering relationship between the transactions is no longer guaranteed. The Relaxed Ordering (RO) attribute bit of the transactions must be cleared to avoid reordering the transactions that are part of the Producer/Consumer transaction series.
When PCI legacy devices reside within a PCI Express system, the ordering rules become more involved. Consequently, additional ordering rules apply because of PCI's delayed transaction protocol. Without ordering rules, this protocol could permit Producer/Consumer transactions to complete out of order and cause the programming model to break.
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