fresnel-fourier/phase3_iter1_baseline.md

# Iteration 1 — Phase 3 (baseline measurements)

Phase 3 baselines for the iter1 MPEG-2 boolean-correctness work, captured 2026-05-08 00:39 CEST after fresnel reboot. All four baseline runs and the verbatim output anchors are in [`phase0_evidence/2026-05-07/iter1_phase3/`](phase0_evidence/2026-05-07/iter1_phase3/).

Phase 3 discipline (per `feedback_dev_process.md`): raw data leads, derived numbers are anchored to the visible tool invocation that produced them. Every "this matches Phase 2 prediction" claim cites a verbatim line from the captured trace, not a paraphrase.

## Reset context

```
$ ssh fresnel 'hostname; uname -r; uptime'
fresnel
6.19.9-99-eos-arm
 00:31:35 up 4 min,  2 users,  load average: 1.01, 0.70, 0.34

$ loginctl list-sessions --no-pager | head
SESSION  UID USER      SEAT  LEADER CLASS   TTY  IDLE SINCE
      2 1000 mfritsche seat0 2244   user    tty1 no   -
```

fresnel rebooted between Phase 2 and Phase 3 (uptime 4 min explains the `No route to host` Phase 2 noted). SDDM watchpoint condition not triggered: `journalctl -u sddm -b | grep -E "(qFatal|crash|Error|fatal|SIGABRT)"` returns empty for this boot. Plasma Wayland session active on tty1, mfritsche logged in.

## Baseline A — pre-patch failure mode

**Goal**: capture the exact path that produces `vaCreateConfig: 12 (UNSUPPORTED_PROFILE)` on master tip `65969da`, before any iter1 work touches the code.

**Invocation**:

```bash
strace -ff -tt -y -e trace=ioctl,openat,close \
  -o /tmp/iter1_phase3/baseline_A_ffmpeg/ffmpeg.strace \
  env LIBVA_DRIVER_NAME=v4l2_request \
      LIBVA_V4L2_REQUEST_VIDEO_PATH=/dev/video5 \
      LIBVA_V4L2_REQUEST_MEDIA_PATH=/dev/media2 \
      LIBVA_TRACE=/tmp/iter1_phase3/baseline_A_ffmpeg/libva.trace \
  ffmpeg -hide_banner -loglevel debug -hwaccel vaapi \
    -i ~/fourier-test/bbb_720p10s_mpeg2.ts -frames:v 2 -f null -
```

**Verbatim libva trace** ([`baseline_A_ffmpeg/libva.trace.*`](phase0_evidence/2026-05-07/iter1_phase3/baseline_A_ffmpeg/)):

```
[5276.234210][ctx none] VA-API vendor string: v4l2-request
[5276.234352][ctx none] vaInitialize ret = VA_STATUS_SUCCESS, success (no error)
[5276.264142][ctx none] vaQueryConfigProfiles ret = VA_STATUS_SUCCESS, success (no error)
[5276.264347][ctx none] va_TraceCreateConfig
[5276.264351][ctx none]   profile = 1, VAProfileMPEG2Main
[5276.264354][ctx none]   entrypoint = 1, VAEntrypointVLD
[5276.264357][ctx none]   num_attribs = 0
[5276.264606][ctx none] vaCreateConfig ret = VA_STATUS_ERROR_UNSUPPORTED_PROFILE, the requested VAProfile is not supported
[5276.425879][ctx none] va_TraceTerminate
[5276.426283][ctx none] vaTerminate ret = VA_STATUS_SUCCESS, success (no error)
```

**Verbatim ffmpeg log** ([`baseline_A_ffmpeg/ffmpeg.stdout`](phase0_evidence/2026-05-07/iter1_phase3/baseline_A_ffmpeg/ffmpeg.stdout)):

```
[VAAPI @ 0xaaaaef94e010] libva: User environment variable requested driver 'v4l2_request'
[VAAPI @ 0xaaaaef94e010] libva: Trying to open /usr/lib/dri/v4l2_request_drv_video.so
[VAAPI @ 0xaaaaef94e010] libva: Found init function __vaDriverInit_1_23
[VAAPI @ 0xaaaaef94e010] Initialised VAAPI connection: version 1.23
[VAAPI @ 0xaaaaef94e010] VAAPI driver: v4l2-request.
[mpeg2video @ 0xaaaaef99ee10] Format vaapi chosen by get_format().
[mpeg2video @ 0xaaaaef99ee10] Format vaapi requires hwaccel mpeg2_vaapi initialisation.
[mpeg2video @ 0xaaaaef99ee10] Failed to create decode configuration: 12 (the requested VAProfile is not supported).
[mpeg2video @ 0xaaaaef99ee10] Failed setup for format vaapi: hwaccel initialisation returned error.
[mpeg2video @ 0xaaaaef99ee10] Format vaapi not usable, retrying get_format() without it.
```

**Verbatim ioctl summary**:

```
     10 ioctl(6</dev/video5>, VIDIOC_ENUM_FMT
      4 ioctl(4</dev/dri/renderD128>, DRM_IOCTL_VERSION
      1 ioctl(6</dev/video5>, VIDIOC_QUERYCAP
```

12 ftrace v4l2 lines (the ENUM_FMT probe events). Zero VIDIOC_S_FMT, zero REQBUFS, zero S_EXT_CTRLS, zero MEDIA_IOC_REQUEST_ALLOC. Decode never starts.

**Confirms Phase 2 Bug 1 directly**: `vaCreateConfig` returns `VA_STATUS_ERROR_UNSUPPORTED_PROFILE` for `VAProfileMPEG2Main` — the missing `break;` at `src/config.c:65-66` makes execution fall through to `default:` (line 67-68) which returns the error code we observe (`12`).

**mpv-as-driver does not reach this path**: a separate baseline run with `mpv --hwdec=vaapi-copy` instead of `ffmpeg -hwaccel vaapi` showed mpv silently fall back to SW decode without ever loading libva for MPEG-2 (no `[vaapi]` log lines, zero V4L2 ioctls, zero libva trace files). mpv's hwdec policy filters MPEG-2 out of vaapi-copy candidates before libva is touched. **Phase 1 success criterion #3 (mpv MPEG-2 engages backend) requires either an mpv config override or a different invocation path** — to be addressed in Phase 4 plan. ffmpeg-direct invocations exercise the full path.

## Baseline B — post Bug 1 patch failure mode

**Goal**: with the missing `break;` added as a Phase 3 scratch test, see what fails downstream. Predicted by Phase 2: `VIDIOC_S_EXT_CTRLS` returns `EINVAL` because the staging-era CID `V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS` (= `0x9909fa`) doesn't exist on this kernel.

**Patch applied** (scratch only, on throwaway branch `iter1-phase3-scratch`, reverted at end of Phase 3):

```diff
@@ -63,6 +63,8 @@ VAStatus RequestCreateConfig(VADriverContextP context, VAProfile profile,
 		break;
 	case VAProfileMPEG2Simple:
 	case VAProfileMPEG2Main:
+		// Phase 3 scratch — would break here in real fix
+		break;
 	case VAProfileHEVCMain:
 	default:
 		return VA_STATUS_ERROR_UNSUPPORTED_PROFILE;
```

Rebuilt + installed (`ninja -C build && sudo ninja -C build install`).

**Verbatim libva trace** showing config-create now succeeds:

```
[5329.689037][ctx none] vaQueryConfigProfiles ret = VA_STATUS_SUCCESS
[5329.689228][ctx none] va_TraceCreateConfig
[5329.689233][ctx none]   profile = 1, VAProfileMPEG2Main
[5329.689237][ctx none]   entrypoint = 1, VAEntrypointVLD
[5329.689239][ctx none]   num_attribs = 0
[5329.689471][ctx none] vaCreateConfig ret = VA_STATUS_SUCCESS, success (no error)
[5329.689703][ctx none] vaQuerySurfaceAttributes ret = VA_STATUS_SUCCESS
... (9 more vaQuerySurfaceAttributes calls; surface format negotiation works)
```

**Verbatim failing ioctl** (extracted from `baseline_B_postbug1/ffmpeg.strace.*`, 6 occurrences across 5 frames):

```
ioctl(6</dev/video5>, VIDIOC_S_EXT_CTRLS,
    {ctrl_class=0xf010000 /* V4L2_CTRL_CLASS_??? */,
     count=1,
     controls=[
       {id=V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS,
        size=56,
        string="h\1\17\0\0\0\0\0H\307Z\226\254i\255\30..."}
     ]}
    => {controls=[...], error_idx=1}
    ) = -1 EINVAL (Invalid argument)
```

- **CID** `V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS` = `V4L2_CID_MPEG_BASE+250` = `0x9909fa` — the fork's `include/mpeg2-ctrls.h:13` symbol. **The kernel doesn't recognize this CID** (mainline removed it; replaced by the split `V4L2_CID_STATELESS_MPEG2_{SEQUENCE,PICTURE,QUANTISATION}` at `0xa409dc/dd/de`).
- **size=56** matches `sizeof(struct v4l2_ctrl_mpeg2_slice_params)` from the fork's local header (combined sequence + picture + slice fields).
- **`error_idx=1`** = first (and only) control in the array failed validation.
- 6 calls visible across the 5-frame attempt — one per frame, plus one buffer-pool bring-up artefact.

**Confirms Phase 2 Bug 2 directly**: the kernel rejects the staging-era CID. The fork's UAPI is stale; rewriting `mpeg2.c` against the new split API is non-optional.

**Auxiliary EINVAL — H.264 device-init runs unconditionally on hantro**:

```
ioctl(6</dev/video5>, VIDIOC_S_EXT_CTRLS,
    {ctrl_class=0,
     count=2,
     controls=[
       {id=0xa40900 /* V4L2_CID_STATELESS_H264_DECODE_MODE */, size=0, value=1, value64=1},
       {id=0xa40901 /* V4L2_CID_STATELESS_H264_START_CODE */,  size=0, value=1, value64=1}
     ]}
    => {error_idx=2}
    ) = -1 EINVAL (Invalid argument)
```

Source: `src/context.c:142-155` unconditionally sets the H.264 device-wide controls (`V4L2_CID_STATELESS_H264_DECODE_MODE = FRAME_BASED`, `V4L2_CID_STATELESS_H264_START_CODE = ANNEX_B`) on every CreateContext, regardless of profile. **This is intentional** per the existing comment at `src/context.c:138-141`:

> Errors here are not fatal: not every backing driver supports both controls (e.g. cedrus may default to SLICE_BASED without exposing DECODE_MODE).

The error is silently swallowed via `(void)v4l2_set_controls(...)` (line 153 — explicit cast discards the return). For MPEG-2 on hantro-vpu-dec the call's a no-op; the EINVAL is noise in the trace, not a bug. **Out of scope for iter1**; documented as auxiliary finding.

**Verbatim ioctl summary** (post-Bug-1 path engages full V4L2 lifecycle):

```
     40 ioctl(6</dev/video5>, VIDIOC_QUERYBUF
     22 ioctl(6</dev/video5>, VIDIOC_ENUM_FMT
     16 ioctl(7</dev/media2>, MEDIA_IOC_REQUEST_ALLOC
     10 ioctl(6</dev/video5>, VIDIOC_QBUF
     10 ioctl(6</dev/video5>, VIDIOC_G_FMT
     10 ioctl(6</dev/video5>, VIDIOC_DQBUF
      6 ioctl(6</dev/video5>, VIDIOC_S_EXT_CTRLS    <-- 6 fail with EINVAL
      4 ioctl(4</dev/dri/renderD128>, DRM_IOCTL_VERSION
      2 ioctl(6</dev/video5>, VIDIOC_STREAMON
      2 ioctl(6</dev/video5>, VIDIOC_STREAMOFF
      2 ioctl(6</dev/video5>, VIDIOC_REQBUFS
      2 ioctl(6</dev/video5>, VIDIOC_CREATE_BUFS
      1 ioctl(9<anon_inode:request>, MEDIA_REQUEST_IOC_REINIT
      1 ioctl(9<anon_inode:request>, MEDIA_REQUEST_IOC_QUEUE
      1 ioctl(6</dev/video5>, VIDIOC_S_FMT
      1 ioctl(6</dev/video5>, VIDIOC_QUERYCAP
```

The buffer-pool bring-up + per-frame plumbing (CREATE_BUFS, QUERYBUF, REQBUFS, STREAMON, REQUEST_ALLOC, QBUF, DQBUF, REQUEST_IOC_QUEUE) all succeeds. **Only the codec-specific control submission is broken**. This bounds the iter1 fix scope sharply.

## Baseline C — cross-validator verbatim contract anchor

**Goal**: extract the exact byte-level `VIDIOC_S_EXT_CTRLS` payload that the working independent V4L2 client (ffmpeg's `-hwaccel v4l2request`) submits per MPEG-2 frame. This is the contract Phase 4 implementation must match.

**Invocation** (verbose strace decoding compound `v4l2_ext_control` payloads):

```bash
strace -ff -tt -y -v -e trace=ioctl \
  -o /tmp/iter1_phase3/baseline_C_xvalidator/ffmpeg.strace \
  ffmpeg -hide_banner -loglevel error -hwaccel v4l2request \
    -i ~/fourier-test/bbb_720p10s_mpeg2.ts -frames:v 2 -f null -
```

**Verbatim per-frame submission** (5 frames captured, frame 1 shown verbatim):

```
ioctl(5</dev/video5>, VIDIOC_S_EXT_CTRLS,
  {ctrl_class=0xf010000 /* V4L2_CTRL_CLASS_CODEC_STATELESS */,
   count=3,
   controls=[
     {id=0xa409dc /* V4L2_CID_STATELESS_MPEG2_SEQUENCE */,
      size=12,
      string="\0\5\320\2\0\30\25\0\0\0\1\1"},
     {id=0xa409dd /* V4L2_CID_STATELESS_MPEG2_PICTURE */,
      size=32,
      string="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\202\0\0\0\17\17\17\17\1\3\0\0\0\0\0\0"},
     {id=0xa409de /* V4L2_CID_STATELESS_MPEG2_QUANTISATION */,
      size=256,
      string="\10\20\20\23\20\23\26\26\26\26\26\26\32\30\32\33\33\33\32\32\32\32\33\33\33\35\35\35\"\"\"\35..."}
  ]}) = 0
```

**Field-by-field decode** (frame 1, BBB +30s seek into MPEG-2 fixture):

`SEQUENCE` (12 bytes, `struct v4l2_ctrl_mpeg2_sequence` per `linux/v4l2-controls.h:2009`):

| Bytes (LE) | Value | Field |
|---|---|---|
| `\0\5` | `0x0500` = 1280 | `horizontal_size` |
| `\320\2` | `0x02d0` = 720 | `vertical_size` |
| `\0\30\25\0` | `0x00151800` | `vbv_buffer_size` |
| `\0\0` | `0x0000` | `profile_and_level_indication` |
| `\1` | 1 | `chroma_format` (4:2:0) |
| `\1` | `V4L2_MPEG2_SEQ_FLAG_PROGRESSIVE` (0x01) | `flags` |

`PICTURE` (32 bytes, `struct v4l2_ctrl_mpeg2_picture` per `linux/v4l2-controls.h:2056`):

| Bytes (LE) | Value | Field |
|---|---|---|
| `\0\0\0\0\0\0\0\0` | 0 | `backward_ref_ts` (I-frame, no ref) |
| `\0\0\0\0\0\0\0\0` | 0 | `forward_ref_ts` (I-frame, no ref) |
| `\202\0\0\0` | `0x82` = `FRAME_PRED_DCT(0x02) \| PROGRESSIVE(0x80)` | `flags` |
| `\17\17\17\17` | `f_code = {{15,15},{15,15}}` (default for I) | `f_code[2][2]` |
| `\1` | 1 | `picture_coding_type` (I) |
| `\3` | 3 | `picture_structure` (FRAME) |
| `\0` | 0 | `intra_dc_precision` (8-bit) |
| `\0\0\0\0\0` | 0 | `reserved[5]` |

Frame 2 (P-picture per the MPEG-2 GOP) shows different values, verbatim from the next strace line:

```
PICTURE: "\0\0\0\0\0\0\0\0p\27\0\0\0\0\0\0\202\0\0\0\1\1\17\17\2\3\0\0\0\0\0\0"
```

| Bytes (LE) | Value | Field |
|---|---|---|
| `\0\0\0\0\0\0\0\0` | 0 | `backward_ref_ts` (P, no backward) |
| `p\27\0\0\0\0\0\0` | `0x1770` ns = 6000 (matches the I-frame's timestamp) | `forward_ref_ts` (refers to frame 1) |
| `\202\0\0\0` | 0x82 | `flags` (same FRAME_PRED_DCT \| PROGRESSIVE) |
| `\1\1\17\17` | `f_code = {{1,1},{15,15}}` (real motion vectors for P) | `f_code` |
| `\2` | 2 | `picture_coding_type` (P) |
| `\3` | 3 | `picture_structure` (FRAME) |
| `\0` | 0 | `intra_dc_precision` |
| `\0\0\0\0\0` | 0 | `reserved[5]` |

`QUANTISATION` (256 bytes = 4 × 64 bytes):

```
intra_quantiser_matrix[64] = [8, 16, 16, 19, 16, 19, 22, 22, 22, 22, 22, 22, 26, 24, 26, 27, 27, 27, 26, 26, 26, 26, 27, 27, 27, 29, 29, 29, 34, 34, 34, 29, ...]
```

This is the canonical MPEG-2 default intra matrix in zigzag scanning order — `8` is at zigzag-position 0 (DC coefficient), `16,16` at positions 1-2 (first AC pair). Confirms the kernel-doc claim that matrices arrive **in zigzag scanning order** (`linux/v4l2-controls.h:2076-2084`).

The remaining 192 bytes (non-intra, chroma_intra, chroma_non_intra) all start with `\20\20...` = 16,16,... — MPEG-2 default non-intra is a flat 16 matrix; chroma matrices duplicate luma when chroma_format=4:2:0.

**This is the iter1 contract anchor**: every Phase 4 implementation diff and Phase 7 verification must produce a `VIDIOC_S_EXT_CTRLS` call that's structurally indistinguishable from this — `count=3`, `ctrl_class=V4L2_CTRL_CLASS_CODEC_STATELESS=0xf010000`, the three CIDs in this order, with sizes 12 / 32 / 256 and the per-frame field values matching what VAAPI's `VAPictureParameterBufferMPEG2` and `VAIQMatrixBufferMPEG2` carry for the same fixture.

## Baseline D — H.264 regression check (Phase 1 success criterion #5)

**Goal**: prove Bug 1 fix in isolation doesn't break H.264. Re-runs T4's reference incantation against `bbb_1080p30_h264.mp4` with the scratch backend installed; expects byte-identical SHA-256 hashes against T4's reference values (`f623d5f7…` for frame 1, `7d7bc6f2…` for frame 2 at +30s seek).

**Verbatim hash output**:

```
f623d5f7a41697f67dd227275c6f1b21ffc257f65626d32fde8229357f8764c9  /tmp/iter1_phase3/baseline_D_h264_regr/png_seek_hw/00000001.jpg
7d7bc6f2146dda8b2d223bba622c4b9fbe9674181ff1e02afe286b620342e0a8  /tmp/iter1_phase3/baseline_D_h264_regr/png_seek_hw/00000002.jpg
f623d5f7a41697f67dd227275c6f1b21ffc257f65626d32fde8229357f8764c9  /tmp/iter1_phase3/baseline_D_h264_regr/png_seek_sw/00000001.jpg
7d7bc6f2146dda8b2d223bba622c4b9fbe9674181ff1e02afe286b620342e0a8  /tmp/iter1_phase3/baseline_D_h264_regr/png_seek_sw/00000002.jpg
```

Pass: HW frame 1 == SW frame 1 == T4 reference; HW frame 2 == SW frame 2 == T4 reference. Bug 1 fix in isolation does not regress H.264.

## Scratch state cleanup

After Baseline D, the scratch patch was reverted to keep master clean for Phase 4 work:

```
$ git checkout -- src/config.c
$ ninja -C build && sudo ninja -C build install
$ git branch -D iter1-phase3-scratch       # branch was empty — patch was uncommitted
```

**Sanity check** (master backend reinstalled, MPEG-2 should fail again):

```
$ ffmpeg -hwaccel vaapi -i bbb_720p10s_mpeg2.ts -frames:v 2 -f null -
[mpeg2video] Failed to create decode configuration: 12 (the requested VAProfile is not supported).
[mpeg2video] Failed setup for format vaapi: hwaccel initialisation returned error.
```

Back to Baseline A state — scratch state successfully cleaned.

## What Phase 3 confirms / refutes from Phase 2

| Phase 2 claim | Phase 3 evidence | Status |
|---|---|---|
| Bug 1: `RequestCreateConfig` rejects MPEG-2 via fall-through to `default:` returning `VA_STATUS_ERROR_UNSUPPORTED_PROFILE` | Baseline A libva trace shows `vaCreateConfig ret = VA_STATUS_ERROR_UNSUPPORTED_PROFILE` for `profile=VAProfileMPEG2Main` | ✅ confirmed |
| Bug 1 fix is sufficient to advance past `vaCreateConfig` | Baseline B libva trace shows `vaCreateConfig ret = VA_STATUS_SUCCESS` post-patch | ✅ confirmed |
| Bug 2: `mpeg2.c` uses staging-era CID `V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS` (`0x9909fa`) which mainline kernel removed | Baseline B strace shows `VIDIOC_S_EXT_CTRLS id=V4L2_CID_MPEG_VIDEO_MPEG2_SLICE_PARAMS size=56 = -1 EINVAL` | ✅ confirmed |
| Bug 2: kernel exposes the new split API (`0xa409dc/dd/de`) | Baseline C strace shows ffmpeg-v4l2request submits these three CIDs successfully | ✅ confirmed |
| Cross-validator submits 1 batched `VIDIOC_S_EXT_CTRLS, count=3` per frame | Baseline C verbatim payload shows exactly that | ✅ confirmed |
| Quantisation matrices arrive in zigzag order | Baseline C QUANTISATION first 4 bytes `[8, 16, 16, 19]` matches MPEG-2 default intra matrix in zigzag | ✅ confirmed |
| H.264 device-init in context.c is best-effort and silently swallows EINVAL on hantro | Baseline B strace shows the `0xa40900/0xa40901` EINVAL (silently); `src/context.c:153` has `(void)v4l2_set_controls(...)` cast | ✅ confirmed (auxiliary, not iter1 scope) |
| Phase 1 criterion #5 (H.264 regression check) holds with scratch Bug 1 fix in isolation | Baseline D hashes match T4 reference exactly | ✅ confirmed |
| mpv-as-driver engages the libva backend for MPEG-2 | Baseline A's mpv variant: zero V4L2 ioctls, zero libva trace, silent SW fallback | ❌ **refuted** — mpv silently bypasses vaapi for MPEG-2; Phase 1 criterion #3 needs adjustment |

## Phase 1 criterion #3 needs adjustment (Phase 3 → Phase 1 loopback)

Per `feedback_dev_process.md`'s Phase 3 → Phase 1 loopback edge:

> If baseline reveals the Phase 1 metric was tracking the wrong thing → loop back to Phase 1 with the corrected target.

The Phase 1 lock specified:

> 3. End-to-end decode engages the backend. `mpv --hwdec=vaapi-copy --frames=2 --vo=null --no-audio …` logs the `[vaapi] libva: Trying to open …` chain, the `Using hardware decoding (vaapi-copy)` confirmation, and exits 0 with no `Failed to create decode configuration` lines.

Phase 3 baseline shows mpv-vaapi-copy **never engages libva for MPEG-2** in the current configuration — neither pre-patch nor post-Bug-1-patch produces `[vaapi]` log lines or VA-API ioctls when invoked via mpv on the MPEG-2 fixture. The mpv hwdec policy (or ffmpeg-mpv glue layer) filters MPEG-2 out before libva is touched. So the Phase 1 criterion #3 wording can never be satisfied via the original incantation, regardless of whether iter1 fixes the libva backend.

**Adjusted Phase 1 criterion #3** (proposed; locks alongside Phase 4 plan):

> 3. End-to-end decode engages the backend. `ffmpeg -hwaccel vaapi -i bbb_720p10s_mpeg2.ts -frames:v 2 -f null - 2>&1` (with the v4l2_request env vars set) shows the `[VAAPI] libva: Trying to open /usr/lib/dri/v4l2_request_drv_video.so` chain, `vaCreateConfig ret = VA_STATUS_SUCCESS`, no `Failed to create decode configuration` lines, no `EINVAL` from `VIDIOC_S_EXT_CTRLS`, and exits 0 cleanly.

The `mpv` invocation moves from criterion to optional follow-up (potentially needs an `--vd` override or a fix to mpv's hwdec-codecs filter; a separate, smaller iteration after iter1 lands).

The other four Phase 1 criteria (vainfo enumeration regression, vaCreateConfig success, DMA-BUF GL pixel verify HW=SW, T4 H.264 regression) hold as locked — no adjustment needed.

## Phase 4 plan inputs

Phase 4 plan should specify:

1. **Diff scope** (per Phase 2 `phase2_iter1_situation.md`):
   - `src/config.c:55-69` — add `break;` for `VAProfileMPEG2Simple/Main` cases (3 lines).
   - `src/mpeg2.c` — full rewrite against the new split API.
   - `include/mpeg2-ctrls.h` — delete or empty (drop the staging-era header that masks the kernel's modern definitions).
   - `src/picture.c` — no changes (verified wired correctly in Phase 2).
   - `src/context.c` — no changes (the H.264 device-init EINVAL is auxiliary, by-design swallowed).

2. **Contract anchor** (per Phase 6 contract-before-code per `feedback_dev_process.md`):
   - Cite verbatim from `linux/v4l2-controls.h:1985-2105` — the three control struct definitions and flag constants.
   - Cite verbatim from FFmpeg `libavcodec/v4l2_request_mpeg2.c:130-155` — the batched submission shape (3 controls, `ctrl_class=V4L2_CTRL_CLASS_CODEC_STATELESS`).
   - Cite verbatim from kernel `drivers/media/platform/verisilicon/hantro_mpeg2.c::hantro_mpeg2_dec_copy_qtable` — the kernel-side zigzag-to-raster permutation (so the libva backend doesn't double-permute).
   - Cite Baseline C verbatim payload from this document (`SEQUENCE` 12 bytes, `PICTURE` 32 bytes, `QUANTISATION` 256 bytes per frame).

3. **Field mapping table** (per Phase 2 `phase2_iter1_situation.md` Bug 2 detail):
   - 6 structural changes from old to new API documented.
   - Source-data extraction in current `src/mpeg2.c` is sound; only destination-control-struct shape and CIDs change.

4. **Phase 7 verification harness**:
   - Re-run all 5 Phase 1 boolean checks (with criterion #3 adjusted as above).
   - Compare the post-fix `VIDIOC_S_EXT_CTRLS` payload byte-by-byte against Baseline C's verbatim payload — exact match expected for SEQUENCE field values; PICTURE will differ on `forward_ref_ts/backward_ref_ts` (different timestamp source) and `f_code` (real values not defaults); QUANTISATION should match exactly for the same fixture.

5. **Mpv-criterion-3 follow-up** (deferred):
   - File a follow-up task to investigate why mpv-vaapi-copy filters MPEG-2 out and how to override. Out of iter1 scope.

## Phase 3 → Phase 4 close

Phase 3 baselines all four green. Phase 2 Bug 1 + Bug 2 + Bug 3 confirmed empirically. Phase 1 criterion #3 needs a minor wording adjustment (mpv → ffmpeg-direct anchor) — proposed text above; locks with Phase 4 plan if approved. Phase 4 can proceed with the contract-anchored diff scope.