Reconvergence Debug: Mechanism

Mechanism to understand

Mechanism for Reconvergence Debug focuses on hazard reproduction success, debug cycle time, recurrence rate. The goal is to convert issue observations into mechanism-backed closure decisions.

Reconvergence failures are intermittent and mode-dependent; debugging requires path correlation, event alignment, and protocol state reconstruction. Treat each warning as a behavior contract violation candidate, then prove whether it is real risk or tool noise.

• Classify crossing type and data criticality.

• State source/destination clock or reset relationship.

• Identify when protocol semantics dominate topology choice.

System flow

CDC/RDC SIGNOFF FLOW — Reconvergence Debug

crossing inventory + reset map
          |
          v
crossing classification (level/pulse/bus/reset)
          |
          v
structure + protocol + reset checks
          |
          v
critical issues + waiver review
          |
          v
fix / validate / regress / signoff

Intermittent hazard debug

1) isolate mode + seed
2) align async event traces
3) map reconvergent fanin
4) prove legal protocol state
5) patch and stress regress

Layer responsibilities

CDC/RDC OWNERSHIP LAYERS — Reconvergence Debug

layer                 owns                            common failure
------------------    -----------------------------   -----------------------------
design intent         crossing architecture           wrong topology selected
protocol semantics    req/ack, fifo, ordering        liveness/deadlock bugs
reset behavior        assert/deassert sequencing      boot instability
analysis setup        tool rules + waivers            false confidence
signoff governance    risk acceptance + dashboard     stale critical waivers

CDC/RDC deep dive

Reconvergence failures are timing + logic coupling hazards.

Concept diagram

RECONVERGENCE

branch A sync --                 -> merge logic -> illegal combination window
branch B sync --/

Metric graph

HAZARD REPRODUCTION RATE

before fix: 1/2000 runs
after fix: 0/100000 runs

Reports and artifacts

• reconvergence warnings

• gray transition checks

• hazard replay traces

• multi-bit strategy map

Mini case study

Gray pointer was legal, but downstream merge decoded mixed-era values during burst stress.

Debug branches

• Trace fanin cones

• align event windows

• verify decode assumptions

Senior review question

Ask: what evidence proves this risk is closed for silicon, not just tool-clean?

Key takeaways

• State crossing class, assumptions, and owner with every issue.

• Run structural and dynamic regressions after each fix.

Common pitfalls

• Treating all warnings as equivalent risk.

• Waiving issues without containment evidence.

• Skipping reset and reconvergence stress after CDC fixes.

Mechanism deep dive

Reconvergence failures are intermittent and mode-dependent; debugging requires path correlation, event alignment, and protocol state reconstruction.