17.5.5.1.5 ID Collisions

Due to the dynamic nature of the QoS with respect to the IDs, two transactions of the same ID can potentially exist in two queues. The collisions can be classified as:

• Blue after Red (BAR): a command with ID x is presented to the blue queue where a command with the same ID x is outstanding in the red queue.
• Red after Blue (RAB): a command with ID x is presented to the red queue where a command with same ID x is outstanding in the blue queue.

In the XPI, for both collision types, the ordering consistency within a given ID is preserved by stalling the incoming transaction to be pushed into the address queue it is mapped to until the colliding transaction in the opposite queue exists. This is required for functional correctness.

In case of any ID collision, the UDDRC does not speed up the drain of the stored transactions in the address queue causing collision even if the incoming transaction is HPR. In other words, address queues request arbitration to the Port Arbiter with their normal mapped priorities.