Interview Q&A: Sequence Debug Triage

[INT][SENIOR][UVM] MODEL ANSWER

Q: Hang triage — sequence vs driver vs objection?

A:
  MECHANISM:  Objection leak → run_phase never ends. finish_item hang → driver
              missing item_done. start_item hang → lock/arbitration block.
  MOTIVATION:  Three different root causes — same symptom, different first moves.
  WHEN:       Any UVM hang — classify before opening waves.
  STEPS:      1) phase.phase_done.display_objections(). 2) Last UVM_INFO before hang.
              3) If finish_item: driver get_next_item/item_done logs. 4) If start_item: lock audit.
  PITFALL:    Waveform-first debug on UVM hang — wastes 30+ minutes routinely.
  EXAMPLE:    display_objections shows vseq fork branch still holding — not driver bug.

[INT][SENIOR][UVM] MODEL ANSWER

Q: Blocked at finish_item?

A:
  MECHANISM:  finish_item waits for item_done from driver on current item.
  MOTIVATION:  Driver not running, not pulling, or stuck in drive task without completing.
  STEPS:      1) Confirm run_phase active (objections raised). 2) Driver: last log before hang.
              3) vif assigned? reset done? 4) Protocol ready wait infinite? 5) item_done on all paths?
  PITFALL:    Assuming sequencer bug — usually driver or vif/config issue.
  EXAMPLE:    Driver stuck on wait(vif.pready) — DUT never ready; finish_item forever.

[INT][SENIOR][UVM] MODEL ANSWER

Q: Blocked at start_item?

A:
  MECHANISM:  start_item waits for sequencer grant — blocked by lock, grab, or
              arbitration never selecting this sequence.
  MOTIVATION:  Different from finish_item — arbitration layer, not driver layer.
  STEPS:      1) UVM_FULL sequencer print — lock holder? 2) Audit unlock on all paths.
              3) Priority/arb mode starving seq? 4) Driver still processing prior item?
  PITFALL:    Debugging driver when prog_seq holds lock from 500 cycles ago.
  EXAMPLE:    Forgotten unlock in prog_seq break branch — bg_seq stuck at start_item.

[INT][SENIOR][UVM] hang decision tree

  Hang?
    ├─ run_phase never ends → display_objections (objection leak)
    ├─ finish_item block   → driver item_done / vif / reset
    └─ start_item block    → lock / arbitration / priority

[INT][SENIOR][UVM] MODEL ANSWER

Q: Minimal sequence repro?

A:
  MECHANISM:  Strip to one sequence, one item, same seed+plusargs, disable bg traffic.
  MOTIVATION:  Isolates handshake from arbitration and multi-agent noise.
  WHEN:       Nightly random fail — repro locally before filing bug.
  STEPS:      1) Same seed. 2) Disable seq_lib → single directed seq. 3) Kill bg seq.
              4) UVM_VERBOSITY=UVM_HIGH on driver+sequencer. 5) Shrink item count to 1.
  PITFALL:    Repro with different cfg (is_active, clock) — false negative.
  EXAMPLE:    axi_rand fail seed 4099 → axi_single_wr_seq only, seed 4099, reproduces hang.

[INT][SENIOR][UVM] MODEL ANSWER

Q: Verbosity for sequence debug?

A:
  MECHANISM:  UVM_HIGH on driver for get_next_item/item_done; UVM_FULL on sequencer
              for arbitration/lock; phase objection display API.
  MOTIVATION:  Structured logs localize stall to layer in minutes.
  WHEN:       Any hang triage; +uvm_set_verbosity for targeted components.
  PITFALL:    UVM_NONE everywhere in regression — zero triage data on failure.
  EXAMPLE:    +uvm_set_verbosity=*,uvm_sequencer,UVM,500,UVM_FULL shows lock holder name.

[INT][SENIOR][UVM] MODEL ANSWER

Q: Vseq fork hang?

A:
  MECHANISM:  fork/join waits for branches; join_none + early drop_objection kills
              branches; one branch hung at finish_item blocks join forever.
  MOTIVATION:  Multi-agent hangs need per-branch classification.
  STEPS:      1) Identify which branch last logged. 2) disable fork branches one-by-one.
              3) Check join vs join_none vs objection timing. 4) Per-branch finish_item probe.
  PITFALL:    drop_objection in test while vseq fork/join still running — partial scenario.
  EXAMPLE:    axi branch hung on finish_item; apb branch done — join waits on axi driver bug.

[INT][SENIOR][UVM] MODEL ANSWER

Q: Code review — sequence hang prevention?

A:
  MECHANISM:  Checklist: item_done all paths, unlock all paths, objection pairing,
              no seq.start before run_phase, fork/join matches objection scope.
  MOTIVATION:  Most hangs are review-catchable — not exotic simulator bugs.
  WHEN:       Every sequence PR; VIP integration reviews.
  PITFALL:    Reviewing randomization only — handshake/lifecycle ignored.
  EXAMPLE:    Reviewer flags: driver wait(pready) has no timeout — reject until bounded.