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--- claude:dev_process [2026/04/30 19:29] – tighten Phase 3: real data not theatre + anti-fabrication markus_fritsche
+++ claude:dev_process [2026/05/02 15:57] (current) – Phase 6: add worked example for 'state the contract explicitly' (patch 0012, h264 SCALING_MATRIX) markus_fritsche
@@ Line 27: / Line 27: @@
   * **If baseline reveals the Phase 1 metric was tracking the wrong thing → loop back to Phase 1** with the corrected target.
   * Example: "max H.264 FPS" Phase 1 metric, but baseline shows DMA-setup + sync overhead dwarfs decode → real metric is bytes-copied-per-second / EGL surface-import time, not FPS.
+**Measurements describe what the system //does//, not what it //should do//.** Baseline data is evidence, not a specification. Do NOT derive API call sequences, struct layouts, or parameter values from observed behaviour (strace, perf, example output). Observable behaviour may reflect bugs, workarounds, or implementation accidents — anything you copy from it inherits those.
 ===== Phase 4 — Plan =====
@@ Line 38: / Line 40: @@
 ===== Phase 6 — Implementation =====
-Execute the plan. Scope strictly to what was planned — resist feature creep, refactor-creep, "while I'm here" cleanups, and chainsaw enthusiasm. If a plan revision is needed mid-implementation, surface it explicitly and re-enter Phase 4.
+Execute the plan. Scope strictly to what was planned — resist feature creep, refactor-creep, "while I'm here" cleanups, and over-eager scope expansion. If a plan revision is needed mid-implementation, surface it explicitly and re-enter Phase 4.
+**Contract before code.** Before writing or modifying any call site:
+  * Read the API contract — kernel docs, header comments, and upstream source for every call touched.
+  * State the contract explicitly before implementing against it (in the plan, the commit message, or a comment — somewhere reviewable).
+  * If the contract cannot be found: stop and surface the gap. Don't infer it from baseline behaviour or sibling code.
+**Copying from baseline measurements is not implementation. It is transcription of potentially broken behaviour.** A deliverable that matches baseline bytes but violates the API contract is not a deliverable — it is a deferred bug.
+==== What "state the contract explicitly" looks like ====
+Worked example: ''0012-h264-omit-scaling-matrix-frame-based.patch'' in ''~/src/ohm_gl_fix/phase6/step1/''. The commit message opens with the contract before any code:
+<code>
+VAAPI signals "explicit scaling lists are present in the bitstream"
+implicitly: the consumer (ffmpeg-vaapi, mpv, etc.) sends a
+VAIQMatrixBufferH264 alongside RenderPicture iff
+sps_scaling_matrix_present_flag || pps_scaling_matrix_present_flag.
+When the bitstream uses default (flat) scaling, no IQMatrixBuffer
+arrives [...]
+Earlier draft of this patch unconditionally omitted SCALING_MATRIX in
+FRAME_BASED. That's corpus-correct (bbb has no explicit scaling
+lists) but the wrong predicate: the kernel-side gating is by
+"matrix-supplied vs. not," not by decode mode. Streams that signal
+explicit scaling lists must submit SCALING_MATRIX in either mode.
+Contract verification (audit_0008_decode_params_2026-05-01.md +
+hantro_h264.c::assemble_scaling_list): the kernel uses the supplied
+matrix when SCALING_MATRIX is in the control batch and falls back
+to spec-defined defaults when absent. Mode-independent.
+</code>
+What this gets right:
+  * **Contract first** — per-control rules cited from kernel doc (''ext-ctrls-codec-stateless.rst:752''), kernel driver (''hantro_h264.c::assemble_scaling_list''), and sibling implementation (gst-plugins-bad commit 9e3e775) //before// any patch hunks.
+  * **Corpus-correct ≠ spec-correct, called out by name** — the rejected predicate ("omit SCALING_MATRIX in FRAME_BASED") //did// match the BBB baseline. It still got rejected, because the contract said the gate is "matrix-supplied vs. not," not "decode mode." This is exactly the Phase 3-derived-implementation trap.
+  * **Then** the diff implements one branch per contract clause: SPS/PPS/DECODE_PARAMS always, SCALING_MATRIX iff ''matrix_set'', SLICE_PARAMS iff SLICE_BASED, PRED_WEIGHTS iff SLICE_BASED + ''V4L2_H264_CTRL_PRED_WEIGHTS_REQUIRED''.
+Mirror format anywhere reviewable: PR description, commit message body, plan section, or a header comment block. The shape is //contract clauses with citations → code that maps 1:1 to those clauses//.
 ===== Phase 7 — Verification Measurements =====
@@ Line 60: / Line 102: @@
 Several recurring failures in prior work codify into individual rules — observer-first, simulate-before-flash, three-strikes-then-verify, "trust eyes not vibes," scope-strictly-to-plan, no-fake-dry-run. Those are all symptoms; this loop is the structural fix. Use it as the spine and let those rules show up as rejection patterns inside the appropriate phases.
-//"The seven-year-old with the chainsaw can do real work. Watch the blade, not the smile."//