Flow Control Mechanisms for Avoidance of Retries and/or Deadlocks in an Interconnect

Flow control mechanisms avoid or eliminate retries of transactions in a coherency interconnect. A class of transaction (CoT) framework is defined whereby individual transactions are associated with CoT labels consistent with chains of dependencies that exist between transactions initiated by any of the cooperating devices that participate in a given operation. In general, coherency protocols create dependencies that, when mapped to physical resources, can result in cycles in a graph of dependencies and deadlock. To support architectural mechanisms for deadlock avoidance, CoT labels are applied to individual transactions consistent with a precedence order of those transactions both (i) with respect to the operations of which such transactions are constituent parts and (ii) as amongst the set of such operations supported in the coherency interconnect. CoT labels applied to respective transactions constitute a CoT framework that may be used by coherency managers to efficiently support concurrent in-flight transactions without retry.