Analysis of Worst Case DMA Response Time in a Fixed-Priority Bus Arbitration Protocol

We propose a technique for finding the worst case response time (WCRT) of a DMA request that is needed in the schedulability analysis of a whole real-time system. The technique consists of three steps. In the first step, we find the worst case bus usage pattern of each CPU task. Then in the second step, we combine the worst case bus usage patterns of CPU tasks to construct the worst case bus usage pattern of the CPU. This second step considers not only the bus requests made by CPU tasks individually but also those due to preemptions among the CPU tasks. Finally, in the third step, we use the worst case bus usage pattern of the CPU to derive the WCRT of DMA requests assuming the fixed-priority bus arbitration protocol. Experimental results show that overestimation of the DMA response time by the proposed technique is within 20% for most DMA request sizes and that the percentage overestimation decreases as the DMA request size increases.