STUDY.md: checkpoint after first-day port work

Update with current state: library builds clean, vainfo enumerates
profiles, vaCreateContext succeeds on Brave (with STREAMON deferred as
WIP unblocker), next failure is frame pool initialization in
vaCreateSurfaces2. Documents the 12-step diff stack vs bootlin upstream
and what still needs to happen to actually decode a frame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

STUDY.md

@@ -23,26 +23,70 @@ a working libva-v4l2-request to bridge.
 
 ## State today
 
-Initial commits applied to bootlin tip `a3c2476`:
+Bootlin tip is `a3c2476`. Stack of WIP commits on top:
 
-1. `V4L2_PIX_FMT_H264_SLICE_RAW` → `V4L2_PIX_FMT_H264_SLICE` (kernel UAPI rename).
+### Build cleanly against current kernel UAPI
-2. `src/h264.c`: missing `#include "utils.h"` for `request_log()` (GCC 14 fatal).
-3. HEVC stripped — h265.c/h265.h excluded from `meson.build`, hevc-ctrls.h
-   replaced by passthrough to `<linux/v4l2-controls.h>`, four HEVC case blocks
-   removed from `picture.c` (kernel `V4L2_CID_MPEG_VIDEO_HEVC_*` was renamed
-   to `V4L2_CID_STATELESS_HEVC_*`; ohm has no HW HEVC anyway).
-4. `src/config.c`: profile probe falls back to `V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE`
-   when single-plane returns no formats. **`vainfo` now lists H.264 + MPEG-2
-   profiles on ohm**, and Brave's GPU process picks them up via VAAPI.
 
-Failure mode reached and stuck on:
+1. `V4L2_PIX_FMT_H264_SLICE_RAW` → `V4L2_PIX_FMT_H264_SLICE` rename.
+2. `src/h264.c`: missing `#include "utils.h"` for `request_log()`.
+3. HEVC stripped — `h265.c`/`h265.h` excluded from `meson.build`,
+   `hevc-ctrls.h` replaced by passthrough to `<linux/v4l2-controls.h>`,
+   four HEVC case blocks removed from `picture.c`.
+4. `include/h264-ctrls.h` made into a passthrough shim to
+   `<linux/v4l2-controls.h>` plus `V4L2_CID_MPEG_VIDEO_H264_* →
+   V4L2_CID_STATELESS_H264_*` aliases (kernel renamed during upstreaming).
+5. `src/h264.c` shape updates to track the upstreamed `struct
+   v4l2_ctrl_h264_slice_params`: drop `.size`, use `struct
+   v4l2_h264_reference {fields, index}` for `ref_pic_list*`, move
+   `pred_weight_table` out to its own `V4L2_CID_STATELESS_H264_PRED_WEIGHTS`
+   control, drop `decode_params.num_slices` (kernel infers from queued
+   controls).
+6. `src/tiled_yuv.S`: aarch64 stub of `tiled_to_planar` — the ARMv7 NEON
+   body is `#ifndef __aarch64__`-guarded; without a stub the `.so` had
+   an undefined symbol and dlopen failed.
 
-```
+Library now builds clean (~265 KB `.so`) and `vainfo` enumerates H.264 +
-ERROR:media/gpu/vaapi/vaapi_wrapper.cc:2407] vaCreateContext failed,
+MPEG-2 profiles.
-  VA error: operation failed
-```
 
-That's the next-phase boundary — the real port work below.
+### Probe + control flow fixes
+
+7. `src/video.c`: add NV12 multi-plane format entry; `video_format_find()`
+   takes `bool mplane` so single- and multi-plane NV12 entries don't
+   collide on pixelformat.
+8. `src/surface.c`: probe block tries `V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE`
+   as a fallback after single-plane probes fail.
+9. **Eager probe in `RequestInit`** — Chromium's vaapi_video_decoder calls
+   `vaCreateContext` *before* `vaCreateSurfaces2`, so the original lazy
+   "set `driver_data->video_format` in `RequestCreateSurfaces`" path was
+   too late. Promoted the probe into `video_format_probe()` in
+   `video.c` and call it at init time.
+10. `src/context.c`: same `V4L2_PIX_FMT_H264_SLICE_RAW → _SLICE` rename
+    that was missed in the first pass.
+11. **WIP**: defer `STREAMON` calls in `RequestCreateContext`. The V4L2
+    stateless protocol on hantro requires OUTPUT format → SPS controls →
+    first slice queued → THEN `STREAMON`. Deferring lets `vaCreateContext`
+    succeed; proper sequencing is the next phase.
+
+### Diagnostic logging (will revert before final)
+
+12. `src/utils.c`: `request_log()` tees to `/tmp/libva-fourier.log` so
+    sandboxed Chromium GPU processes don't swallow the trace output.
+
+### Failure mode reached now (2026-04-26)
+
+`vainfo` works fully. Brave on ohm with the patched `.so` reaches deeper:
+
+- Init: detects NV12 multi-plane CAPTURE format ✓
+- vaCreateContext: profile=H264Main 1920×1088, S_FMT + CREATE_BUFS pass ✓
+- vaCreateContext: STREAMON OUTPUT returned EINVAL ← deferred to unblock
+- Now stuck at: `failed Initialize()ing the frame pool` from
+  `vaapi_video_decoder.cc`
+
+That's the next-phase boundary. `vaCreateSurfaces2` (frame-pool init) is
+where Chromium discovers buffer geometry for output frames; the library
+needs to do CAPTURE-side `S_FMT` + `REQBUFS` + `EXPBUF` without single-
+plane assumptions on a freshly-created context, and the V4L2 stateless
+protocol's source-change-event dance probably needs implementing too.
 
 ## Port plan