P2 arch design policy

Phase 2 Architecture Design Policy

P1 Algorithm Candidate HW Review Criteria

For each functional area's candidates from P1's domain-analysis.md, evaluate:

• Gate count estimate (from literature or analytical estimation)
• Critical path depth (combinational logic levels)
• SRAM requirements (capacity, banking, port count)
• External memory bandwidth (bytes/pixel, access patterns)
• Achievable throughput at target frequency (pixels/cycle, blocks/cycle)
• Implementation risk (tool support, verification complexity, proven vs novel)

Architecture Review Criteria (Dynamic Convergence Protocol)

Dynamic convergence review (min 2, max 5 rounds), coordinated by rtl-architect:

• 3 mandatory parallel reviewers each round: (a) rtl-architect: spec compliance (Feature Coverage Checklist) + structural review (b) vcodec-architecture-expert: memory access patterns, performance analysis (c) ref-model-dev: architecture ↔ C model consistency (block mapping, data flow, interfaces)
• +1 conditional reviewer: (d) ref-model-reviewer: independent C model quality gate (algorithm fidelity, numerical precision, undefined behavior/build warning risk) when ref model is newly created or substantially revised
• ref-model-reviewer activation threshold: Invoke when ref model is newly created OR

  30% of source lines changed from prior version. Both sequential and team orchestrators use this same threshold. Skip review tasks when ref model is unchanged.

• Each round: review → rebuttal (designer accepts/rejects each finding with rationale) → tree exploration for issues → targeted revision (rejections recorded in per-round artifact)
• Last round (converged or max reached): cross-block interface audit + memory conflict analysis
• Convergence check after round >= min_rounds: finding_delta < 0.1, all critical resolved, wonder stable
• After max_rounds if not converged → escalate to user via AskUserQuestion

Architecture Naming Conventions

• Block names: snake_case (become RTL module names in Phase 4)
• Interface descriptions: data width, protocol type, direction — NOT RTL port naming
• RTL naming conventions (i_/o_ prefix, clock/reset naming) applied in Phase 4, not here

HW Candidate Review Format

docs/phase-2-architecture/hw-candidate-review.md must contain:

1. Per functional area: candidate list with algorithm description
2. Comparison matrix columns: gate count, critical path depth, SRAM requirements, external memory BW, throughput at target freq, memory latency impact, implementation risk
3. Selected candidate per area with REQ-NNN justification
4. User decision record (AskUserQuestion response summary)

Bandwidth Analysis Workflow

• ref-model produces bandwidth_report.json during Step 3
• arch-designer consumes bandwidth_report.json during Step 4 (after synchronization)
• If bandwidth exceeds limits → adjust block partitioning or PARALLEL_LANES before review

Memory Architecture Classification

Each block's storage elements must be classified at architecture level:

• Local storage: SRAM or register file (Phase 3 specifies exact type: SP/TP/DP wrapper)
• Cross-domain storage: flag for dual-clock SRAM (DP) when read and write are in different clock domains
• External memory: DDR/HBM accessed via bus interface

Architecture does NOT select wrapper types (that is Phase 3 μArch responsibility), but MUST identify:

• Estimated capacity per block (bits = depth × width)
• Port count requirement (1 R/W, simultaneous R+W, multi-port)
• Clock domain: same domain (→ SP or TP in Phase 3) vs cross-domain (→ DP in Phase 3)

Memory Access Latency Awareness

Architecture review must verify:

• Per-block latency budget distinguishes compute cycles vs memory access cycles
• Default assumptions: internal memory = 1 cycle, external memory = 500 cycles
• MEM_LATENCY_EXTERNAL is parameterizable (design-specific, technology-dependent)
• Blocks with heavy external memory access have documented latency hiding strategy (prefetch, double buffering, pipelining, or accepted latency penalty with justification)
• bandwidth_report.json estimated cycle counts validated against timing_constraints.json budgets
• Comparison of estimated latency WITH and WITHOUT latency hiding, to quantify strategy benefit

Wonder Log (Required)

Each review round MUST produce a wonder-log entry:

• File: docs/phase-2-architecture/wonder-log.md
• Format: Markdown table with columns: Round, Assumption, Domain, Risk(H/M/L), Resolution
• Purpose: Track unvalidated assumptions across rounds
• Exit gate: All High-risk assumptions must be resolved or explicitly accepted before phase completion

Review Convergence Criteria

Review rounds use dynamic convergence instead of fixed 3 rounds:

Parameter	Value	Rationale
min_rounds	2	Minimum for meaningful review
max_rounds	5	Prevent infinite loops
finding_delta_threshold	0.1	< 10% new findings = stable
critical_resolution	ALL	All Critical/High must be resolved
wonder_stability	true	No new High-risk assumptions

Early exit (round 2): When findings converge quickly (simple designs) Extended review (rounds 4-5): For complex designs with emergent issues

This is inspired by Ouroboros's ConvergenceCriteria:

• Stability signal: finding_delta < threshold
• Stagnation detection: same findings repeated across rounds
• Oscillation detection: finding toggling resolved↔reopened

Escalation & Stop Conditions

• max_rounds reached but issues remain → escalate to user via AskUserQuestion
• Domain constraint conflict → document conflict, ask user for priority
• Memory access infeasible (bandwidth exceeds technology) → escalate, propose alternative
• Architecture ↔ ref model fundamental mismatch → escalate, may require ref model rewrite

Architecture Feature Completeness Gate

Phase 2 architecture review MUST verify that ALL spec features flow through to the design:

1. Spec-to-architecture mapping: Every REQ-F-* from P1 must map to an architecture block

   • Missing mapping → CRITICAL finding (spec feature not architected)

2. Ref model-to-architecture alignment: When C reference model exists:

   • Compare ref model function coverage against spec feature list
   • "Enum declared but not called in any function" → COVERAGE_GAP
   • Escalate gaps to user: "Spec feature X exists in enum but ref model doesn't implement it"

3. Omission justification: Spec features omitted from architecture require:

   • User-approved ADR from Phase 1 (with documented impact)
   • ADR-less omissions → Phase 2 gate FAIL

4. Impact quantification (domain-specific):

   • Codec: BD-rate impact of omitted modes/tools
   • Processor: missing instruction coverage
   • Generic: functional coverage percentage vs spec

Open Resolution Protocol

Phase 2 receives docs/phase-1-research/open-requirements.json containing OPEN-1-* research topics. For each OPEN-1-* item, the architecture team must:

1. Conduct architecture research using the item's candidates and evaluation_criteria
2. Select a winner with quantitative justification (gate count, throughput, area, etc.)
3. Record the decision in docs/phase-2-architecture/iron-requirements.json (REQ-A-*) with:
   • resolved_from: the OPEN-1-* ID that was resolved
   • resolution_rationale: why this candidate was selected
   • rejected_alternatives: all non-selected candidates with rejection reasons
   • upstream_compliance: verification that new REQ-A-* does not violate P1 iron
   • violation_policy: "agent_retry" (authority=2)
   • acceptance_criteria: measurable criteria for the architectural decision
4. Verify: ALL OPEN-1-* items must be resolved before Phase 2 exit

Phase 2 may also produce docs/phase-2-architecture/open-requirements.json (OPEN-2-*) for research topics to be resolved by Phase 3 (e.g., micro-architecture choices).

Compliance Check Procedure

After docs/phase-2-architecture/iron-requirements.json (REQ-A-*) is finalized:

1. Invoke compliance-checker agent with:
   • upstream_iron: ["docs/phase-1-research/iron-requirements.json"]
   • target_artifacts: Phase 2 output artifacts
2. Gate: compliance-report.json verdict must be PASS
3. On VIOLATION: enter authority-differentiated escalation ladder
   • Authority 1 (P1 functional): Custom budget: Primary 3 + Fallback 2 + Last-chance 1
   • Authority 2 (P2 architecture): Custom budget: Primary 4 + Fallback 3 + Last-chance 1

Upstream Challenge Protocol

If compliance VIOLATION persists after Primary stage and is technically infeasible:

1. Orchestrator produces infeasibility assessment with quantitative evidence
2. Compliance-checker validates the infeasibility claim
3. If validated: produce upstream challenge report with PPA estimates
   • Required fields: frequency_mhz, area_gate_count, pixel_rate_mpps, achievable_fps
4. Present challenge to user via AskUserQuestion with comparison table

Ambiguity Gate (Phase 2)

Apply ambiguity scoring to all new REQ-A-* decisions:

• "Would re-evaluating this architecture produce the same conclusion?"
• Score ≤ 0.5 required before REQ-A-* becomes iron
• Ambiguous decisions cannot be iron

Final Checklist

• docs/phase-2-architecture/hw-candidate-review.md exists with per-block selection + HW rationale
• P1 algorithm candidates reviewed from HW perspective
• architecture.md exists with all blocks and data paths
• architecture.md includes D2 block diagram
• Every REQ (REQ-F-, REQ-P-, REQ-A-*) mapped to at least one architecture block
• Dynamic convergence review completed (min 2 rounds, or gaps escalated and approved)
• Memory access patterns reviewed for all large blocks
• Architecture ↔ ref model consistency verified
• Ref model code reviewed for quality and bitexact correctness
• reviews/phase-2-architecture/ref-model-review.md saved when ref model changed
• bandwidth_report.json reviewed, external memory bandwidth validated
• Internal vs external memory classified per block
• C model executed and verified — architecture produces correct results
• Tree exploration used for issue resolution in each review round
• Per-round review artifacts saved (r1.md, r2.md, and additional rounds if needed)
• reviews/phase-2-architecture/feature-coverage.md saved
• reviews/phase-2-architecture/architecture-review.md saved (consolidated)
• reviews/phase-2-architecture/architecture-diagram.md saved (D2)
• docs/decisions/ADR-*.md generated (3-5 key architectural decisions)
• docs/phase-2-architecture/wonder-log.md exists with per-round assumption tracking
• All High-risk assumptions in wonder-log resolved or explicitly accepted
• Per-round review artifacts saved (r1.md through rN.md, minimum 2 rounds)
• All OPEN-1-* items from P1 resolved with rationale and rejected_alternatives
• docs/phase-2-architecture/iron-requirements.json exists with REQ-A-* entries and resolved_from tracking
• Compliance check against P1 iron: verdict = PASS
• Ambiguity score ≤ 0.5 for all new REQ-A-* requirements
• docs/phase-2-architecture/open-requirements.json (OPEN-2-*) created for Phase 3 homework (if any)