Debug playbook

Debug Playbook for Enumeration & Link Training focuses on link width, link speed, LTSSM failure state, enumeration time. The goal is to connect the observable symptom to protocol mechanism, ownership, and regression risk.

Protocol debug is a search for the FIRST deviation, not the loudest symptom. Timeouts and error flags are usually many cycles downstream of the real cause.

Root-cause tree

ROOT-CAUSE TREE — Enumeration & Link Training

link width, link speed, LTSSM failure state, enumeration time looks wrong
        |
   reproducible?
     /        \
   no          yes
   |            |
 flaky env   same first transaction every time?
 / seed         /            \
              yes             no
               |               |
        protocol rule     timing/reset/PVT
        or config bug     or load-dependent

1. Freeze the failing seed, firmware tag, and spec revision.

2. Find the first bad transaction, not the loudest downstream timeout.

3. Map the transaction to signals and VIP monitor events.

4. Classify the failure: protocol rule, integration config, timing/reset, or performance pressure.

5. Prove the mechanism with one reduced sequence.

6. Patch the smallest owner-controlled boundary and rerun compliance plus workload traffic.

Review memo template

STAFF PROTOCOL REVIEW MEMO — PCIe & CXL / Enumeration & Link Training

1. Symptom
   - Watched metric: link width, link speed, LTSSM failure state, enumeration time
   - Failing interface: <master/slave/endpoint/controller/PHY>
   - Transaction identity: <ID/tag/address/endpoint/lane>
   - Repro setup: <sim/emulation/FPGA/silicon + firmware tag>

2. Mechanism hypothesis
   - Primary mechanism: firmware enumeration and LTSSM link training establish topology, capabilities, resources, and negotiated speed.
   - Competing hypothesis: <timing, reset, bridge, ordering, firmware, or VIP issue>
   - Missing evidence: <waveform, analyzer trace, counter, spec clause, or log>

3. Proposed action
   - Minimal reversible change: <RTL, register setting, bridge config, scheduler, VIP check>
   - Expected metric movement: <delta and workload>
   - Regression risk: ordering, compatibility, performance, power, area, or timing

4. Signoff
   - Re-run artifact: LTSSM trace, config space dump, firmware enumeration log
   - Required owners: firmware owner, PHY owner, PCIe integration owner
   - Final decision: fix, waive, document limitation, or escalate to architecture

Protocol deep dive

PCIe is reliable packet delivery over unreliable links; debug flows PHY -> DLL -> TLP -> firmware.

Concept diagram

PCIe DEBUG TOP-DOWN

L0 link healthy?  -> credits OK?  -> TLP completes?  -> driver happy?

Skip a layer and you will mis-own the bug.

Metric graph

LINK DEGRADE EXAMPLE

target x4 Gen4  ---- ---- ---- ----
actual   x4 Gen4  ---- ---- ---- ----   (eval board)
actual   x1 Gen3  -                   (product board)

Package/SI often shows up as width downgrade, not hard fail.

Metrics and artifacts to collect

• link width/speed

• replay count

• completion timeout

• AER error log

• LTSSM history

Mini case study

Endpoint enumerated but DMA timed out: completion credits exhausted because a switch port was misconfigured in firmware, not because the endpoint was broken.

Debug branches

• If degrade at width/speed, PHY/SI before driver.

• If replay storm, link layer before transaction layer.

• If CXL coherency bug, separate .io vs .cache vs .mem traffic.

Senior review question

Ask: what is the first transaction that deviates, and which spec rule does it test?

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.

Principal review addendum

Re-read Enumeration & Link Training against one concrete product workload, not a synthetic directed test.

firmware enumeration and LTSSM link training establish topology, capabilities, resources, and negotiated speed.