25.8.2 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 table below. When the Latency Quality of Service is enabled for a host-client pair through the MATRIX, the priority pool number to use for arbitration at the client port is determined from the host. When the Latency Quality of Service is disabled, it is determined through the MATRIX user interface. See MATRIX_PRASx.

After reset, the Latency Quality of Service is enabled by default on all of the host ports that are connected to a host driving the Latency Quality of Service signals, as shown in the bit LQOSEN of MATRIX_PRASx and MATRIX_PRBSx.

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.

For each client, each host is assigned to one of the client priority pools based on the Latency Quality of Service inputs or to 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, those hosts are granted bus access in a biased round-robin manner which enables tight and deterministic maximum access latency from system bus requests. In the worst case, any currently occurring high-priority host request is granted after the current bus host access has ended and any other high priority pool host requests have been granted once each.

The lowest priority pool shares the remaining bus bandwidth between hosts.

Intermediate priority pools enable fine priority tuning. Typically, a latency-sensitive host or a bandwidth-sensitive host use such a priority level. The higher the priority level (MxPR value), the higher the host priority.

For good CPU performance, it is recommended to let the CPU priority configured 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.