Iteration 2 opens. Hardening, not new feature work.
Phase 0 (phase0_findings_iter2.md): locked research question — make
DMA-BUF (vaapi no -copy) path render without artifacts on direct-
render consumers. Plus WSI alignment for 864-wide videos in Firefox
and multi-resolution kernel-state recovery.
Phase 2 (phase2_iter2_analysis.md): the bug origin is picture.c:375
re-QBUFing surface_object->destination_index every decode cycle
while the kernel CAPTURE buffer is still being read by the consumer
via an EXPBUF'd dma_buf fd. V4L2 doesn't enforce the constraint;
userspace must coordinate.
Three architecture options analyzed:
A. More buffers + LRU recycling (cheapest, statistical mitigation)
B. Per-buffer dma_buf-refcount-aware recycling (correct, requires
kernel changes or userspace V4L2_MEMORY_DMABUF rewrite)
C. Kernel patch to enforce QBUF rejection (out of campaign scope)
Picking Option A for iteration 2.
Phase 3 (in-session baseline anchor): mpv vaapi --vo=gpu shows
91 drops in 14s at 1080p, 9 CAPTURE buffer indices used (2-10),
~10-19 re-queues per buffer in 14s window. Per-buffer re-queue
interval ~875ms vs typical compositor hold ~50ms — race window
opens episodically.
Phase 4 (phase4_iter2_plan.md): three independent fixes ordered
cheapest-first.
Fix 1: invalidate LAST_OUTPUT_WIDTH cache on session teardown.
Fix 2: try DRM_FORMAT_MOD_INVALID for WSI compatibility.
Fix 3: decoupled CAPTURE buffer pool with LRU recycling
(~150-200 lines, the load-bearing fix).
Phase 5 sonnet review BEFORE Fix 3 implementation.
Out-of-scope for iteration 2 (carry to iter 3): EACCES probe,
multi-slice num_ref_idx, HACK block MPEG-2 cleanup, seek-to-non-IDR,
DEBUG instrumentation cleanup, V4L2_MEMORY_DMABUF rewrite, perf
metrics.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
