Scenario: PCIe Link Degrade

Scenario

The same endpoint trains correctly on an eval board but degrades on the product board after a package change.

OBSERVED METRIC
device enumerates at x1 Gen3 instead of x4 Gen4

45-MINUTE INTERVIEW FLOW
0-5: clarify protocol, layer, and traffic setup
5-15: draw transaction flow and ownership
15-25: isolate first bad transaction or stalled progress
25-35: propose reduced experiment and fix options
35-45: state compliance, workload, and regression plan

Common pitfalls

• Blame software enumeration first

• Skip LTSSM history

• Ignore SI/package lane evidence

Scenario debrief

Score the answer: layer, transaction ID, metric, artifact, regression.

Reference stack

intent -> transaction -> channel -> physical

Metric pattern

baseline -> experiment -> target

Debrief prompts

1. Which protocol layer did you name first?

2. Which transaction identity did you carry through the story?

3. What reduced experiment did you propose?

4. What compliance and product regressions did you list?

Key takeaways

• Connect every protocol claim to a transaction identity and measurable metric.

• Store the artifact (waveform, log, counter) next to every signoff decision.

Common pitfalls

• Debugging timeouts without finding the first bad transaction.

• Quoting peak bus width without payload efficiency and retry overhead.

• Treating VIP compliance as a substitute for system integration replay.