Three different traffic types are defined:
- Low latency: typically CPU to DDR memory transactions
- Isochronous: real-time deadlines
- Best effort: bulk transfers and not time critical
Low Latency Transactions
For high-priority transactions, low latency is the key for meeting performance requirements. The APU and RPU MPCores can specify low latency transactions so their memory access requests are serviced in a timely manner to avoid or minimize CPU pipeline stalls.
Low latency transactions are usually associated with cache fill and replacement.
Video Isochronous Transactions
Another category of transactions that can live with longer latency in typical conditions is isochronous (or video class) transactions. However, there is critical moment (maximum latency) in which data must be available without causing a system degradation. The key requirement is a guaranteed maximum latency. Isochronous transactions are used for video encoder/decoder, camera sensors, display controllers, and other real-time sensitive controllers.
Best Effort Transactions
High-throughput and best effort transactions are allowed with long latency but need high throughput to achieve architectural performance goals. These types of transactions come from DMAs, the CPM, and PL functional units.