# Iteration 4 close (Phase 8) — Track A locked, three-iteration carryover resolved

Opened 2026-05-05 (just after iter3 close), closing 2026-05-05 same day. Locked candidate: **Track A solo** — fix the iter1+iter2+iter3 carryover frame-11 EINVAL. Substrate path 2 selected: diff our DPB+DECODE_PARAMS construction vs FFmpeg's `libavcodec/v4l2_request_h264.c::fill_dpb`.

## Verdict: GREEN

Track A's load-bearing defect is empirically resolved. mpv direct stress test on ohm via patched libva-v4l2-request-fourier:

```
$ LIBVA_DRIVER_NAME=v4l2_request mpv --hwdec=vaapi-copy --vo=null bbb_1080p30_h264.mp4
After 90s wall time:
  RequestBeginPicture:     2130     (was: bailed at 11 in iter3)
  RequestSyncSurface:      4254
  S_EXT_CTRLS EINVAL:      0
  "Unable to set control(s)": 0
  Generic EINVAL:          0
  ENETDOWN:                0
```

2130 frames at 24 fps in 90 seconds wall = real-time HW decode (>98% of theoretical 2160-frame max). The libva-side decode pipeline now sustains arbitrary BBB-class H.264 content without V4L2 errors.

## What landed

### libva-v4l2-request-fourier fork commits

The fix is split into three correctness commits + two debug-instrumentation commits, in apply order:

1. **`a12d299` iter4 DEBUG: Y2 v3 — retry with TRY_EXT_CTRLS** (instrumentation)
2. **`74d8dd1` iter4 partial fix: DPB fill matches FFmpeg semantics** (correctness)
   - `dpb[].fields = V4L2_H264_FRAME_REF` for every valid entry
   - Skip entries with `valid && !used`
3. **`4892656` iter4 DEBUG: pre-S_EXT_CTRLS DPB census + per-entry dump** (instrumentation)
4. **`385dee1` iter4 fix: fresh request_fd per frame (load-bearing)** (correctness)
   - In `RequestSyncSurface`, replace `media_request_reinit(request_fd)` with `close(request_fd); surface_object->request_fd = -1;`
   - Forces next `BeginPicture` to allocate a fresh fd via `media_request_alloc`
   - **This is THE fix that crossed the threshold.** All three of (74d8dd1, 385dee1, b81ce69) are correctness improvements; #2 (385dee1) is the one that flipped the outcome from "frame-11 EINVAL" to "2130 frames clean."
5. **`f21bdf0` iter4 DEBUG: per-control TRY isolation** (instrumentation — was the diagnostic that pivoted us from "bad control content" to "bad fd state")
6. **`b81ce69` iter4 fix: B-slice L1 reflist .fields copy-paste** (correctness; pre-existing iter1+ bug caught by Phase 5 review)

### libva-multiplanar campaign artifacts

- `phase0_findings_iter4.md` — substrate (7 candidates, locked A solo)
- `phase2_iter4_situation.md` — kernel V4L2 control validation analysis
- `phase4_iter4_plan.md` — diagnostic journey + fix authoring narrative
- `phase5_iter4_review.md` — sonnet review (initial YELLOW → GREEN after C1+C2 resolved)
- `phase8_iteration4_close.md` — this file

## Diagnostic lessons (for memory + future iterations)

### Kernel obfuscation extends to compound controls under TRY_EXT_CTRLS

The `v4l2-ctrls-api.c:222-224` comment promised that TRY_EXT_CTRLS would report `error_idx` for the specific failing control. Empirically, for our compound H.264 controls + request_fd path, TRY also returned `error_idx == count`. Either the comment is outdated or the cluster-commit failure path bypasses the per-control update for both S and TRY. Practical diagnostic implication: don't rely on TRY to pinpoint compound-control failures; use **per-control TRY isolation** instead — submit each control in a `count=1` `v4l2_ext_controls` and observe individual results.

### "All controls fail individually" → request_fd state, not content

The breakthrough diagnostic: when every individual control fails on its own with the same EINVAL, the request_fd is in a bad state — not the control values. Pivot from content-correctness to lifecycle-correctness investigation. Cheap to test: per-control TRY iso with `for i in 0..N { TRY([control_i]) }`.

### `media_request_alloc` is cheaper than chasing reinit-state semantics

The kernel's `MEDIA_REQUEST_IOC_REINIT` after queue+wait is supposed to be sufficient to clean a request for reuse, but for some surface-recycle pattern in our cap_pool it left the fd in a state that `S_EXT_CTRLS` rejected. We don't fully understand why. Allocating a fresh fd (`MEDIA_IOC_REQUEST_ALLOC` + `close` per frame) sidesteps the question. Cost is +1 ioctl pair per frame, well below noise on the V4L2 stack overhead.

### `dpb[].fields` is mandatory, not optional

For frame-coded streams, `V4L2_H264_FRAME_REF` (= `TOP_FIELD_REF | BOTTOM_FIELD_REF`) must be set on every valid DPB entry. The kernel's reflist builder skips entries with `fields == 0`. UAPI doc `Documentation/userspace-api/media/v4l/ext-ctrls-codec-stateless.rst` says so explicitly. Our pre-iter4 driver had `fields` zero-initialized and never written.

### FFmpeg's `libavcodec/v4l2_request_h264.c` is the empirical reference for V4L2-stateless H.264

Whenever our driver disagrees with FFmpeg semantically and we can't find documentation, FFmpeg is right. `references/ffmpeg-kwiboo/libavcodec/v4l2_request_h264.c::fill_dpb_entry` was the source-of-truth that surfaced two of the three correctness fixes this iteration.

## State that carries to iter5

- **Hardware**: ohm RK3568 hantro G1/G2, kernel 6.19.10 — unchanged.
- **Userspace**: firefox 150.0.1 stock + firefox-fourier 150.0.1-1.1 (PGO-instrumented, 3.6 GB libxul.so) at `/opt/firefox-fourier/` — unchanged.
- **Driver installed**: `/usr/lib/dri/v4l2_request_drv_video.so` sha256 (post-iter4 close): rebuild on iter5 start to confirm. iter4 ended with `46c6e2e078697d27...` (post-DPB fix) but b81ce69 needs to be rebuilt + redeployed before iter5 starts.
- **Test fixture**: bbb_1080p30_h264.mp4, sha256 `dcf8a7170fbd...` — unchanged.
- **Build container**: firefox-fourier LXD on boltzmann — unchanged, persistent.
- **Phase 7 evidence script**: `/home/mfritsche/iter3_phase7_evidence.sh` on ohm.vpn — unchanged.
- **mpv stress-test command** (iter4-introduced): documented above.

## State that does NOT carry

- The PGO-instrumented Firefox-fourier binary throttle. iter4 verified Track A via mpv direct because the PGO Firefox binary couldn't reach 720+ frames in 90s. iter5 may want a clean PGO-disabled Firefox rebuild for sustained Firefox-side stress testing.
- `/tmp/ff-fourier-stderr-v2.log` and `/tmp/mpv-iter4.log` are tmpfs-volatile.

## Documented limitations carried into iteration 5 substrate

- **DEBUG instrumentation density** (carried from iter1/iter2/iter3/iter4 backlog). Driver now carries iter1 ENTER/CAPTURE-dump traces + msync workaround, iter1+ POC sentinel strip, iter3 Y2 v1, iter4 Y2 v3 + per-control TRY iso + DPB census. The iter5 sweep is the natural next iteration.
- **mpv libplacebo `--vo=gpu` segfault** (carried from iter3 substrate, never iter3-or-iter4 scope). vaapi-copy + `--vo=null` works (iter4 verification), but the libplacebo Vulkan-fallback path still segfaults. iter5 candidate.
- **Multi-context libva safety** (Sonnet 9.6 from iter1) — still carried. iter4's mpv test was single-context; concurrent-libva not exercised.
- **PGO profile generation under sandbox** (iter3 Phase 6 finding) — `--enable-profile-generate=cross` PGO step still requires X11/Wayland that the LXC container can't provide. iter5 Firefox rebuild may want PGO disabled or a different rig.
- **Bootlin upstream prep** — with iter4's load-bearing fix landed, the fork is significantly closer to upstreamability. Per `feedback_no_upstream.md`, no PR/MR happens without explicit operator instruction. But iter5 DEBUG sweep + the Mozilla bug filing (iter3 candidate G) become natural prerequisites.

## Lessons distilled to memory

- **`feedback_kernel_obfuscation_compound.md`** (NEW) — V4L2 S_EXT_CTRLS deliberately hides which compound control failed (sets `error_idx = count` after `validate_ctrls` fails for set=true). The kernel comment in v4l2-ctrls-api.c claims TRY_EXT_CTRLS escapes the obfuscation, but empirically TRY also returns `error_idx == count` for compound H.264 controls. Use **per-control TRY isolation** (count=1 for each control individually) to pinpoint which one fails or, if all fail, conclude the request_fd state is the issue.

- **`feedback_request_fd_lifecycle.md`** (NEW) — when every individual control fails on the same fd with EINVAL, the fd's state is bad — not the control content. Allocating a fresh fd per frame (`MEDIA_IOC_REQUEST_ALLOC` + `close` per cycle) is cheaper to verify than chasing kernel `MEDIA_REQUEST_IOC_REINIT` lifecycle semantics. iter4's load-bearing fix uses this pattern. Cost: ~1 ioctl pair per frame, negligible on the V4L2 stack.

- **`reference_ffmpeg_v4l2_request_is_authority.md`** (NEW) — `libavcodec/v4l2_request_h264.c::fill_dpb_entry` is the working reference for V4L2-stateless H.264 control construction. iter4 surfaced two correctness fixes by direct comparison: `dpb[].fields = V4L2_H264_FRAME_REF` and "skip stale entries (= entries not in the consumer's current ReferenceFrames[])." When semantics disagree and no documentation resolves the disagreement, FFmpeg is the empirical authority. Cached locally at `references/ffmpeg-kwiboo/`.