Reading v4l2-core/v4l2-ctrls-api.c and v4l2-ctrls-core.c on the cloned
linux-pinetab2 v6.19.10-danctnix1 source: error_idx == count for
S_EXT_CTRLS is intentional kernel obfuscation, not under-reporting.
Line 629 deliberately overwrites error_idx with cs->count after
validate_ctrls failures in set mode, forcing the caller to bail rather
than partial-set.
The escape hatch is VIDIOC_TRY_EXT_CTRLS, which "never modifies controls
[so] error_idx is just set to whatever control has an invalid value"
(quoting v4l2-ctrls-api.c:222-224).
Path forward into Phase 4: amend Y2 instrumentation to retry with
TRY_EXT_CTRLS on S_EXT_CTRLS EINVAL, extract the actual failing control
index. From there, narrow the failing field by comparing frame-11
values against frames 1-10.
Phase 3 baseline anchored from iter3 Phase 7 — same rig, same EINVAL,
deterministic. No re-acquire needed.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
