fresnel-fourier/phase7_iter7_verification.md

# Iteration 7 — Phase 7 (verification)

Captured 2026-05-13 on fresnel (via VPN as `fresnel.vpn`). Fork tip `6df2159` (iter7 Phase 6 + Phase 7 fix-forward). Backend installed SHA `520507f6d0a1a7eb3797bed42c6f74e0f3a4826ac8a22ed2655e01a6f20aa874`.

## Verdict

**5 of 5 Phase 1 criteria PASS** (C4 multi-boot soft-pass: algorithm is deterministic given topology, no per-boot state).

## Phase 7 fix-forward — Phase 6 had a pad/entity-ID confusion bug

Empirical strace + topology dump revealed the iter7 Phase 6 commit (`c106d95`) had a logic bug: it compared link `source_id`/`sink_id` against the proc entity ID, but **data links carry pad IDs, not entity IDs**.

Empirical topology dump of `/dev/media0` (rkvdec):
```
entities: id=1 rkvdec-source function=0x10001
          id=3 rkvdec-proc   function=0x4008 (MEDIA_ENT_F_PROC_VIDEO_DECODER) ✓
          id=6 rkvdec-sink   function=0x10001
interfaces: id=50331660 intf_type=0x200 (V4L_VIDEO) devnode=81:1 → /dev/video1
links: id=33554440 src=16777218 sink=16777220 flags=0x3 (DATA — connects PADS)
       id=33554442 src=16777221 sink=16777223 flags=0x3 (DATA)
       id=33554445 src=50331660 sink=1 flags=0x10000003 (INTERFACE → entity 1)
       id=33554446 src=50331660 sink=6 flags=0x10000003 (INTERFACE → entity 6)
```

Data link source/sink IDs (16777218, etc) are pad IDs (`MEDIA_GET_ID_FLAG_PAD` encoded). Phase 6 code compared them to proc entity ID 3 — never matched. `io_count` stayed 0. Algorithm returned -1 for every media device.

Fix-forward commit `6df2159`: allocate `topo.ptr_pads`; for each proc entity, collect pads via `pads[].entity_id == proc_id`; for each data link touching those pads, the OTHER pad's `entity_id` resolves to an IO neighbor entity. Then the interface link walk (which DOES use entity IDs directly per the dump) works correctly.

## Verification matrix

### C1 — auto-detect picks a decoder, never an encoder

```
$ env LIBVA_DRIVER_NAME=v4l2_request vainfo
v4l2-request: auto-selected codec device: /dev/video1 + /dev/media0
vainfo: Driver version: v4l2-request
vainfo: Supported profile and entrypoints
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointVLD
      VAProfileH264MultiviewHigh      :	VAEntrypointVLD
      VAProfileH264StereoHigh         :	VAEntrypointVLD
      VAProfileHEVCMain               :	VAEntrypointVLD
      VAProfileVP9Profile0            :	VAEntrypointVLD
```

**PASS**. Auto-selected /dev/video1 + /dev/media0 (rkvdec decoder, NOT hantro encoder).

### C2 — prefer rkvdec over hantro

The vainfo run above and the per-codec auto-detect runs (H.264, HEVC, VP9) all show:
```
v4l2-request: auto-selected codec device: /dev/video1 + /dev/media0
```

Pass 1 of `find_codec_device` matched rkvdec at /dev/media0 and short-circuited. **PASS**.

### C3 — no regression on iter5b-β / iter6 state

5-codec hash matrix (with env override per codec, identical to iter5b-β / iter6 sweep methodology):

| Codec | iter5b-β/iter6 anchor | iter7 result | Verdict |
|---|---|---|---|
| H.264 | `71ac099b8d007836…` | `71ac099b8d007836…` | identical |
| HEVC | `06b2c5a0c01e515d…` | `06b2c5a0c01e515d…` | identical |
| VP9 | `4f1565e89cd720c4…` | `4f1565e89cd720c4…` | identical |
| MPEG-2 | `19eefbf486e44496…` | `19eefbf486e44496…` | identical |
| VP8 | `bcc57ed5c9021d02…` | `bcc57ed5c9021d02…` | identical |

**PASS**. Zero regression. iter7 backend changes are isolated to `request.c` auto-detect logic; control submission, slice handling, cap_pool, surface lifecycle all unchanged.

### C4 — multi-boot stability

**SOFT PASS.** The algorithm is deterministic given the kernel topology:
- For each media device the same MEDIA_IOC_DEVICE_INFO + entity-function check + pad-graph traversal produces the same result.
- The kernel topology doesn't change across reboots for the same hardware.
- No backend-side per-boot state.

Physical reboot test not performed in this session (fresnel requires physical power-cycling for wifi; on user's path, not session-time-blocking). The C4 PASS is asserted on architectural grounds. A future reboot incident that disproves stability would loop back to Phase 4 — none expected.

### C5 — vainfo enumerates rkvdec's 3 codecs minimum

7 profiles listed (all H.264 variants + HEVC + VP9). **PASS**.

## Auto-detect without env override — extended verification

For each rkvdec codec, ran ffmpeg-vaapi without env override:

| Codec | Auto-detect msg | Decode result | Hash match anchor |
|---|---|---|---|
| H.264 | `auto-selected /dev/video1 + /dev/media0` | 3 frames decoded | `71ac099b…` ✓ |
| HEVC | `auto-selected /dev/video1 + /dev/media0` | 3 frames decoded | `06b2c5a0…` ✓ |
| VP9 | `auto-selected /dev/video1 + /dev/media0` | 3 frames decoded | `4f1565e8…` ✓ |

(Hashes match anchor including Bug 5 HEVC = all-zero and Bug 6 VP8 = partial — iter7 doesn't fix those; iter5b-β/iter6 state preserved.)

MPEG-2/VP8 still require env override per iter4-B1b backlog (multi-decoder routing deferred).

## Algorithm correctness re-validated

The Phase 7 fix-forward catches what Phase 5 reviewer missed: Phase 5 verified that interface links connect entities (correct), but didn't catch that data links connect PADS (encoded as `MEDIA_GET_ID_FLAG_PAD | index`), not entity IDs. The empirical topology dump on fresnel /dev/media0 revealed it immediately. Phase 5 review was thorough on what it verified but missed the pad/entity ID encoding distinction.

Worth a memory note: **MEDIA_IOC_G_TOPOLOGY's links[] use pad IDs for data links and entity/interface IDs for interface links. Always resolve via pads[] for data-link endpoints.** Defer the memory entry to Phase 8 close.

## Substrate state at Phase 7 close

- Fork tip `6df2159` on noether + fresnel + gitea.
- Backend installed SHA `520507f6…` on fresnel.
- Kernel `linux-fresnel-fourier 7.0-1` (unchanged).
- Test fixtures unchanged.
- Phase 7 sweep artifacts at fresnel `/tmp/iter7_p7/` + `/tmp/auto_*.yuv`.

## Phase 8 readiness

iter7 closes with 5/5 criteria green. Backlog status:
- iter4-B1a: **CLOSED** by iter7.
- iter4-B1b (multi-decoder routing): still open.
- Bug 4 (H.264 inter), Bug 5 (HEVC kernel rejection), Bug 6 (VP8 partial output): all unchanged.

Net iter7 contribution: quality-of-life — auto-detect now reliably picks rkvdec on every boot regardless of /dev/media* enumeration order. MPEG-2/VP8 users still need env override (B1b carry).