Hard Processor System Technical Reference Manual: Agilex™ 5 SoCs

ID 814346
Date 4/01/2024
Public
Document Table of Contents

4.1.5.9. Quality of Service

CCU implements simple priority arbitration-based QoS. The 4-bit QoS value for an incoming transaction is mapped to one of 8 QoS buckets (3-bit value). This bucket value is used for arbitration at all points in the system where requests accumulate in a buffer or transaction table (CAIU, DCE & DMI) or when they pass through a switch. Note that coherency serialization rules override this priority arbitration.

CCU implements a QoS starvation prevention mechanism that guarantees forward progress for low QoS transactions that might be stuck behind higher QoS transactions. This mechanism is implemented at all QoS priority arbitration points in the system.

The implementation of starvation prevention is based on the number of competing transactions making forward progress. If a transaction has not been able to make forward progress at an arbitration point, while a specified number of competing transactions made forward progress, this transaction is marked as starved and makes forward progress on the next arbitration cycle.