# Iteration 7 — Phase 4 (plan)

Phase 1 criteria scoped to B1a (encoder/decoder discrimination) only — B1b (multi-decoder routing) is bigger and stays backlog. Phase 4 specifies a minimum-LOC fix that satisfies all 5 amended criteria.

## Empirical baseline (Phase 3 evidence)

On the 2026-05-12 boot:

| /dev/media | driver | Entities (relevant) | Resulting /dev/video |
|---|---|---|---|
| `/dev/media0` | `rkvdec` | rkvdec-source, rkvdec-proc, rkvdec-sink | /dev/video1 (decoder; the only V4L node) |
| `/dev/media1` | `hantro-vpu` | `rockchip,rk3399-vpu-enc-{source,proc,sink}` (entities 1, 3, 6) → /dev/video2 ; AND `rockchip,rk3399-vpu-dec-{source,proc,sink}` (entities 15, 17, 20) → /dev/video3 | TWO V4L nodes inside one media device |
| `/dev/media2` | `uvcvideo` | (USB camera) | not relevant |

The current `find_video_node_via_topology` walks `interfaces[]` (`MEDIA_INTF_T_V4L_VIDEO`) and picks the FIRST one. For /dev/media0 (rkvdec) that's /dev/video1 = correct. For /dev/media1 (hantro-vpu) that COULD be /dev/video2 (encoder) — wrong if the interface ordering puts encoder first.

The fix needs to:
- Recognize hantro-vpu's encoder entity vs decoder entity inside the same media device.
- Prefer rkvdec over hantro-vpu when both are present (rkvdec serves 3 of 5 codecs; hantro serves 2 of 5).

## Contract clauses

### C1 — Add `find_decoder_video_node_via_topology` to request.c

Replaces the current `find_video_node_via_topology` (which picks the first V4L interface). New logic:

1. `MEDIA_IOC_G_TOPOLOGY` fetches both `entities[]` and `interfaces[]` and `links[]`.
2. Iterate `entities[]` looking for `function == MEDIA_ENT_F_PROC_VIDEO_DECODER`. Collect the matching entity IDs (typically 1, but multiple is fine).
3. For each decoder-proc entity:
   - Find linked V4L "source" or "sink" entities (entity ↔ entity links).
   - Find the interface (`MEDIA_INTF_T_V4L_VIDEO`) linked to either of those V4L entities (interface ↔ entity links).
   - Resolve the interface's `devnode.major/minor` to /dev/videoN.
   - Return that.
4. If no decoder entity found, return -1 (this media device has no decoder; caller skips).

The links graph: in v2 topology, each `media_v2_link` has `source_id` and `sink_id` referring to either entity IDs or interface IDs. The `flags` field distinguishes data links from interface links.

Predicted size: ~80-120 LOC for the new function, replacing ~40 LOC of the old one. Net +40-80 LOC.

### C2 — Prefer rkvdec in `known_decoder_drivers`

The existing array already lists `"rkvdec"` first, then `"hantro-vpu"`. Walking `/dev/media*` in numeric order means whichever media device hits the rkvdec match first wins. On RK3399, rkvdec is usually at /dev/media0 or /dev/media1 — already-first-discovered most of the time.

But if rkvdec enumerates at /dev/media2 and hantro at /dev/media0, the walk picks hantro first. To prefer rkvdec ALWAYS: do TWO passes — first pass accepts only rkvdec; second pass accepts any decoder. This is mechanical; ~20 LOC.

Alternative: do ONE pass but break early ONLY on rkvdec; keep hantro match as "best so far" and continue scanning. ~10 LOC.

Predicted approach: simpler 2-pass.

### C3 — `LIBVA_V4L2_REQUEST_NO_AUTODETECT` escape hatch preserved

The current code at request.c:279+ honors `LIBVA_V4L2_REQUEST_NO_AUTODETECT=1` to revert to legacy hardcoded paths. iter7 doesn't touch that branch.

### C4 — Inline comment update

Replace the existing comment block at request.c:60-89 explaining the walk. Document the entity-function check, the two-pass preference, and remove the iter4-B1 backlog reference (since iter7 closes the encoder/decoder discrimination part).

### C5 — Verification matrix

Phase 7 sweep:
1. `vainfo` (no env override) — must list ≥3 codecs (rkvdec's H.264/HEVC/VP9 at minimum).
2. ffmpeg-vaapi H.264 fixture — must engage decoder; verify via strace `/dev/video<rkvdec>` opened.
3. Optional: explicit env-override route for MPEG-2/VP8 still works (B1b carry).
4. 5-codec sweep equivalent to iter6 baseline — verify VP9 still PASS, MPEG-2 still PASS (with env override), no regression.
5. Reboot fresnel; re-verify steps 1-4.

### C6 — Phase 8 close

Document: B1a closed; B1b still open (multi-decoder dispatch needs ~200-400 LOC architectural work). Update backlog accordingly.

## Risk register

| Risk | Probability | Impact | Mitigation |
|---|---|---|---|
| MEDIA_IOC_G_TOPOLOGY's links array doesn't link interface→entity the way I expect | LOW | MEDIUM | Phase 5 reviewer empirically checks link structure on fresnel; Phase 6 instruments before-implementation to dump topology |
| Existing iter4 Commit Z code paths break under refactor | LOW | LOW | Keep escape hatch; revert is easy |
| `cedrus` / `sun4i_csi` (Allwinner drivers) may not set MEDIA_ENT_F_PROC_VIDEO_DECODER | UNKNOWN | LOW | Falls back via the two-pass: pass-1 rkvdec specifically, pass-2 any decoder including cedrus if it has the entity function; if cedrus doesn't, the fix is too strict for that platform. Not a fresnel-fourier blocker. |
| H.264 / HEVC / VP9 paths regress (somehow) | LOW | HIGH | Phase 7 verifies; revert single iter7 commit if so |

## Predicted iter7 cadence

- Phase 5: ~30 min (small surface).
- Phase 6: ~45 min (single commit, ~100-150 LOC).
- Phase 7: ~30 min (sweep + reboot).
- Phase 8: ~15 min (close doc).

Total: ~2 hours wallclock.

## Phase 5 review concerns to invite

1. **Entity function semantics**: does `MEDIA_ENT_F_PROC_VIDEO_DECODER` actually appear on hantro-dec-proc and rkvdec-proc entities? Empirically check on fresnel.
2. **Link discovery**: does the links graph use `MEDIA_LNK_FL_INTERFACE_LINK` flag to distinguish entity↔entity (data) from interface↔entity (control)? Verify.
3. **Multi-decoder-in-one-media-device case**: my fix handles 2 decoders in one media device by returning the FIRST one. For hantro-vpu media device on RK3399 there's only ONE decoder entity (the dec-proc). For SoCs with multiple decoder cores in one media device — undefined for now; first-match acceptable.
4. **Phase 7 reboot test**: how do we exercise the case where hantro encoder enumerates first? On the current boot rkvdec=/dev/media0, hantro=/dev/media1. The fix should handle BOTH orders. Forcing a re-enumeration may need a USB-cam unplug-replug or similar. Phase 7 attempts but accepts "single boot order verified" as sufficient.
