swayfx-mosaic/tooling/probes/step20c-core-recovery/gpu-field/README.md

STEP20C GPU DCC recovery — EXPERIMENTAL, offline only

This versioned validator copies the executed historical validate.py, analysis.py and test_fixture.py; exact parent paths/hashes are recorded in provenance.json, and parent-delta.patch records their complete source delta. The historical runner remains byte-identical and is deliberately not copied: this scope has no admissible runtime acquisition gate or launcher. The two runner-dependent controller tests are omitted from the copied test module; 31 inherited validator/model tests and five actual-G1 test methods run offline. Generated reports, logs and payload remain in the new private LAB.

Exact bounded contract

The local authority is /usr/include/libdrm/drm_fourcc.h, SHA256 992f33804acda60b0b46f1a272942c3592484ea3217d469239f1cb12249de2eb. Its AMD section assigns the main image to plane 0 and DCC to plane 1 when DCC is enabled without RETILE. The same meaning was independently checked against the primary Linux UAPI header. That moving upstream reference corroborates the semantics; the local SHA pins this fixture's actual source authority. The exact allowlisted modifier 020000001046bb04 has vendor AMD=2, tile-version GFX11=4, tile=27, DCC=1, RETILE=0 and PIPE_ALIGN=1. Other nonzero modifiers are rejected.

For the four known packed 32-bit RGB formats, the image plane must have a stride of at least width*4, divisible by four. The auxiliary DCC pitch is nonzero uint32; it is not an image stride. Offsets and arithmetic are bounded and the fixture restricts the metadata to image-before-auxiliary offsets. This latter restriction is a bounded sanity filter, not a universal modifier layout rule. Exported FD identities and allocation lengths are not observed. No DCC alignment, tiled physical size, auxiliary extent or allocation bounds are proved. An accepted plane record returns those explicit limitations. The linear one-plane case is a synthetic regression control only; no new non-DCC GPU run or qualification is claimed.

Actual G1 replay and second blocker

The replay reads only immutable epoch1 metadata/arm/payload and verifies their fixed hashes. Nine native reads, offsets/ROIs, hashes, attachment channels, context and all nine saved/restored GL/PACK state fields validate. The added G1 envelope checks all 11 native writes and two copies, source/destination, texture unit, sampler/matrix types, composition domains and native order. A successful envelope is only structural evidence: it never replaces the strict validate_epoch() result.

After the plane repair, the real epoch still rejects effective halfpixel. For stages 0–2, glGetUniformfv published bits [983818240,987398144], while float32 source expressions predict [983815946,987395086]. For stages 3–5, observed bits are [967041024,970620928] versus [967038730,970617870]. Both shaders declare precision mediump float; the private hook queries active uniforms with glGetUniformfv then preserves their bits. Quantization is a possible explanation, not a proven diagnosis of these readings. No post-observation tolerance or shader change is introduced, and the float64 ideal model is not upgraded into a GPU arithmetic model.

The hook itself explicitly says its alpha sentinel is not proof of GL read success or restoration of the GL error queue. GL_ERROR_QUEUE=UNOBSERVED and read_success=NOT_PROVEN remain immutable. Fixing those acquisition limits would require a separately authorized diagnostic contract/build; this ticket allows zero new compositor/GPU builds. Hence GPU_DCC_RUNTIME=NOT_RUN, no runtime reservation and no new GPU runner. GPU_MODIFIER_AWARE_OFFLINE is INCONCLUSIVE for the complete gate even though the isolated plane correction and adversarial tests pass. Independent review remains pending.

Only A exists: no B or A-return is acquired. The sampled 16×16 U0/cache RGBA bytes are exact; cache/presubmit differ. Neither observation establishes the cause of a whole-output inequality, physical DCC correctness, precision loss, or an A/B/A difference. GPU_WHOLE_FIELD_CAUSALITY=INCONCLUSIVE. The original G1 outcome, exit1, cleanup limits and INCONCLUSIVE remain unchanged.

Reproduce without starting anything

From repository root (historical private evidence must still exist):

PYTHONDONTWRITEBYTECODE=1 python -m unittest discover \
  -s tooling/probes/step20c-core-recovery/gpu-field -p 'test_*.py' -v
PYTHONDONTWRITEBYTECODE=1 python tooling/probes/step20c-core-recovery/gpu-field/replay.py

Mutation tests cover unknown formats/modifiers, wrong plane count, image/aux pitches, duplicated/overflowing offsets, missing fields, truncated/corrupt samples, read validity/sentinel, GL restoration/context/attachments, ROI/hash, missing copies/writes, source aliases, samplers and duplicate uniforms. The unchanged inherited suite additionally covers all nine restoration fields, clear/external write/lifetime events, blur order, region holes, ping-pong continuity, reconstruction and ideal-model negative predictions.