Last change's docs concluded "AUTO never splits, retire the arm" from the
sub-frame-ON measurement alone. Measured the missing leg before implementing it,
and the conclusion was wrong.
On .21 at 4K, plain AUTO (env unset, the resolver's fallthrough):
sub-frame ON -> 5023/5157 us/frame ~= DISABLE 4979/5000 (does NOT split)
sub-frame OFF -> 2401/2352 us/frame ~= TWO_FORCED 2319/2378 (DOES split)
So AUTO is CONDITIONAL, not dead. Retiring it would have silently cost every
sub-frame-off session its second engine -- a regression introduced while
"cleaning up" an arm that looked inert. Split and sub-frame are mutually
unsupported for HEVC, so the driver resolves AUTO to no-split only in that
combination.
Fix is disclosure, not removal:
- resolve_split_subframe debug-logs the inert HEVC + AUTO + sub-frame case, which
is the fleet default shape: "split_mode=AUTO" has meant "no split" for every
default session and nothing said so. Deliberately NOT rewritten to DISABLE --
the mode we pass is what the driver was actually given, and the ceiling-cache
key must keep describing that.
- New unit test `auto_survives_the_arbitration_in_both_subframe_states` pins the
contract so the arm cannot be simplified away later.
- The resolver doc now records both measured legs instead of "AUTO is dead".
Also in this change:
- WP1.6: `resolve_subframe`'s doc said "Windows passes `false`". Stale since the
2026-07-31 .173 A/B flipped Windows to caps-gated default-on. It mattered:
it made the AUTO-plus-sub-frame dead combination look Linux-only when it is
fleet-wide.
- Windows session-ready log parity: split_mode + engines + subframe. The Windows
line had no split_mode at all, so a Windows field report could not answer "did
this session actually split?" -- the question that started this whole thread.
Verified: fmt clean; .21 clippy -p pf-encode --features nvenc --all-targets
-D warnings clean, 58 unit tests (1 new), 22/22 NVENC on-hardware tests green;
.133 Windows clippy --features nvenc --all-targets -D warnings clean (15m cold,
zero errors or warnings) -- the Windows backend is cfg'd out on both macOS and
the Linux box, so that leg needed a real Windows host.
enricobuehler
9a1d8be4cc