Legal and Illegal PST Transition Checks: Interview Drills

Interview drills

Interview Drills for Legal and Illegal PST Transition Checks is anchored on Illegal transition escape rate, transition-checker latency to first error, and percentage of legal arcs exercised with pass/fail evidence.. Convert observations into mechanism-backed and owner-bound actions.

PROMPT
You observe regression in Illegal transition escape rate, transition-checker latency to first error, and percentage of legal arcs exercised with pass/fail evidence. for Legal and Illegal PST Transition Checks. Explain root cause and release decision.

STRONG ANSWER
1. Defines failing phase and workload context.
2. Explains mechanism: Transition correctness is not only about start and end states; it depends on guards, temporal ordering, and confirmation events on each arc. Verification therefore encodes every legal PST arc with required preconditions (for example quiescent interconnect, save-ack observed, debug override cleared) and postconditions (such as supply good, isolation release, restore complete) while asserting that all non-enumerated arcs remain unreachable. Illegal transition checks must include both direct jumps and multi-step shortcuts created by overlapping requests, because concurrent software writes or interrupt-driven exits can collapse intended two-hop paths into electrically unsafe single-hop behavior. Advanced checkers track arc provenance, so when a violation occurs they identify which guard was bypassed, which handshake timed out, and whether recovery logic masked the violation by forcing a fallback state after corruption was already possible.
3. Requests proving artifact: Transition-arc checker specification with guard predicates, timeout rules, and illegal-arc diagnostics taxonomy.
4. Proposes bounded fix + owner + rollback-safe validation.

WEAK ANSWER
Gives generic low-power advice without mechanism proof, evidence, or ownership.

Low-power verification deep dive

Power-state correctness is a protocol contract: legal transitions, robust sequencing, and safe concurrent event handling.

Concept diagram

PST CONTROL LOOP

state request -> legality check -> handshake sequencing -> mode entry -> monitored exit

Metric graph

STATE RISK MIX

illegal transitions     ██████
sequence race bugs      █████
stable mode paths       ████████

Metrics and artifacts to collect

• PST legality matrix

• illegal transition histogram

• entry/exit handshake coverage

• mode sequencing anomaly log

Mini case study

A sporadic low-power failure closed only after proving a wake-versus-thermal race in PMU transition sequencing.

Debug branches

• Validate legal state graph first.

• Stress concurrent control events and asynchronous wakeups.

• Bind fixes to explicit transition and owner contracts.

Senior review question

Ask: what exact low-power transition boundary failed first, and which artifact proves the closure claim reproducibly?

Key takeaways

• Tie each LPV claim to a concrete transition boundary and one proving artifact.

• Prefer minimal reversible fixes with explicit owner and rollback criteria.

Common pitfalls

• Treating power-aware failures as random before boundary classification.

• Waiving X-prop failures before proving impact and root cause.

• Declaring closure without deterministic replay across key modes.

Principal LPV review addendum

Legal and Illegal PST Transition Checks should be reviewed as a transition integrity system, not just isolated checks.

Use Illegal transition escape rate, transition-checker latency to first error, and percentage of legal arcs exercised with pass/fail evidence. as alarm and Transition-arc checker specification with guard predicates, timeout rules, and illegal-arc diagnostics taxonomy. as proof.

Power-state verification is a protocol verification problem: legal transitions, ordering contracts, and corner-case concurrency. Closure quality comes from reproducible evidence and explicit owners.