18.10.2 Arbitration Priority Scheme
The MATRIX arbitration scheme is organized in priority pools, each corresponding to an access criticality class as shown in the “Latency Quality of Service” column in the following table.
| Priority Pool | Latency Quality of Service |
|---|---|
| 3 | Latency Critical |
| 2 | Latency Sensitive |
| 1 | Bandwidth Sensitive |
| 0 | Background Transfers |
Round-robin priority is used in the highest and lowest priority pools 3 and 0, whereas fixed level priority is used between priority pools and in the intermediate priority pools 2 and 1. See Round-robin Arbitration.
For each client, each host is assigned to one of the client priority pools through the priority registers for clients (MxPR fields of MATRIX_PRAS and MATRIX_PRBS). When evaluating host requests, this priority pool level always takes precedence.
After reset, most of the hosts belong to the lowest priority pool (MxPR = 0, Background Transfer) and are therefore granted bus access in a true round-robin order.
The highest priority pool must be specifically reserved for hosts requiring very low access latency. If more than one host belongs to this pool, they will be granted bus access in a biased round-robin manner which allows tight and deterministic maximum access latency from system bus requests. In the worst case, any currently occurring high-priority host request will be granted after the current bus host access has ended and other high priority pool host requests, if any, have been granted once each.
The lowest priority pool shares the remaining bus bandwidth between hosts.
Intermediate priority pools allow fine priority tuning. Typically, a latency-sensitive host or a bandwidth-sensitive host will use such a priority level. The higher the priority level (MxPR value), the higher the host priority.
To optimize processor performance, it is recommended configure CPU priority with the default reset value 2 (Latency Sensitive).
All combinations of MxPR values are allowed for all hosts and clients. For example, some hosts might be assigned the highest priority pool (round-robin), and remaining hosts the lowest priority pool (round-robin), with no host for intermediate fixed priority levels.