## Iteration 21 — Phase 4 (plan)

Opens 2026-05-14. Continues iter20's localization: rkvdec sees all-zero ctx->ctrl_hdl SPS for libva, real bytes for kdirect. The break is between userspace S_EXT_CTRLS and rkvdec's read of `ctx->ctrl_hdl[SPS].p_cur.p`.

### Locked research question (iter21)

> *"At the v4l2_ctrl_request_setup() entry for libva's per-frame request_fd, is the V4L2 control-handler object (`obj`) found? For each control_ref in the request's hdl->ctrl_refs, is `p_req_valid == true`?"*

### What this narrows

`v4l2_ctrl_request_setup(req, main_hdl)` at IOC_QUEUE time iterates `hdl->ctrl_refs` and only applies controls where `ref->p_req_valid == true`. The bit gets set by the staging path in `try_set_ext_ctrls_common` (called from `try_set_ext_ctrls_request`) when which=V4L2_CTRL_WHICH_REQUEST_VAL.

If libva's S_EXT_CTRLS staged correctly, p_req_valid is true for each ctrl libva submitted. If staging failed silently, p_req_valid is false and `v4l2_ctrl_request_setup` skips them — ctx->ctrl_hdl stays at zero (matches iter20 evidence).

### Approach (α-24 inert → kernel path)

α-24 (libva G_EXT_CTRLS readback after S_EXT_CTRLS) was implemented in 1547a5d+a9c897f, returned `EACCES` for all 13 libva HEVC frames. Reverted in e109306. The kernel disallows G_EXT_CTRLS against a not-yet-completed request — userspace can't probe `req->p_new`. Mechanism distinguishing requires kernel printk.

iter21 patches `v4l2_ctrl_request_setup` with two printk lines:

```c
pr_info("iter21_setup: req=%p main_hdl=%p obj=%p\n",
        req, main_hdl, obj);

pr_info("iter21_setup_ref: ctrl_id=0x%x p_req_valid=%d have_new=%d\n",
        ctrl->id, ref->p_req_valid, have_new_data);
```

Build `linux-fresnel-fourier 7.0-5` (pkgrel 4→5), deploy, reboot, run libva-HEVC + kdirect-HEVC, capture dmesg.

### Outcome interpretation

| obj at setup entry | p_req_valid (libva run) | Diagnosis |
|---|---|---|
| NULL | n/a | req has no v4l2_ctrl_handler bound at queue time. libva's S_EXT_CTRLS never staged. Bug in libva's request lifecycle. |
| non-NULL | all false | obj found, but staging path never set `p_req_valid`. Bug in `try_set_ext_ctrls_common` for libva's invocation. |
| non-NULL | true for SPS | staging worked but `req_to_new` / `try_or_set_cluster` failed silently. Bug in apply path. Needs another printk after `req_to_new`. |

iter21 finishes when one of these is confirmed for libva. Compare to kdirect baseline (should always show p_req_valid=true for SPS).

### Substrate state at iter21 open

- Kernel `linux-fresnel-fourier 7.0-5` building on boltzmann (PID 1584834, log /tmp/iter21-kbuild.log).
- Backend SHA `c1d4bb53…` (iter15 stable) — backend unchanged from iter15.
- Fork tip `e109306` (α-24 reverted).
- 5-codec anchors: unchanged. Zero regression.

### Phase 5 review note

Diagnostic kernel patch (2 pr_info calls in well-known V4L2 framework function, no behavior change). Phase 5 review skipped per iter17 precedent for diagnostic-only kernel work.

### Phase 7 plan

After 7.0-5 deploys:
1. Reboot fresnel; sddm autologin reseats mfritsche.
2. `sudo dmesg -C`.
3. Run libva HEVC; capture rkvdec_iter20 + iter21_setup lines.
4. `sudo dmesg -C`.
5. Run kdirect HEVC; capture same.
6. Diff; localize bug to one of the three table-row diagnoses.