Emphasizing Functional Relevance Over State Restoration in Post-Silicon Signal Tracing

The state restoration ratio (SRR) has been a de facto standard for evaluating the quality of signals selected for post-silicon tracing and debug. In this paper, we establish that SRR is intrinsically unsuitable as a metric for evaluating trace signal quality, as it captures neither the higher-level functionality of the design nor the constraints and requirements on trace signals. We present an algorithm, based on PageRank [PageRank on Netlist (PRoN)], for post-silicon trace signal selection. PageRank is not designed to maximize SRR and is applied to the circuit netlist. We demonstrate that optimizing for SRR typically generates signals that are functionally irrelevant to the design and unusable for debug, for a comprehensive set of SRR-based techniques. We assess the scalability of different signal selection algorithms by applying them to an industrial scale OpenSPARC T2 design. Our results show that our PRoN algorithm consistently outperformed other techniques with respect to scalability and functional relevance of signals selected. It also has higher restorability than the other algorithms, despite not being optimized for that metric.