Extends bench_flush_frame with an argv[5] substrate selector
(auto/cpu/qpu). Same enum as test_idct_bitexact's argv[4] — keeps
both binaries' CLI in sync.
The whole point of plumbing the selector through is to put a number
on the "QPU is default substrate" decree (2026-05-23,
feedback_qpu_is_default_substrate.md) for the IDCT layer
specifically. The decree said: "What can be done, will be done in
QPU. Dispatch overhead is fixable defect." This measurement
quantifies the unfixed defect.
Bench config: 1920x1088, 100 iters, 5 warmup, half 4x4 / half 8x8
luma MBs + chroma always 4x4. Pi 5 / V3D 7.1 / daedalus-fourier
0.1.0 (with cycle 6/7/9 H.264 IDCT shaders). Hertz, idle system.
Results:
substrate min median mean p99 fps (median)
─────────────────────────────────────────────────────────────
CPU NEON 8.75 9.27 11.10 33.06 107.8
QPU V3D7 31.92 37.77 37.67 47.27 26.5
AUTO 31.99 33.19 36.04 92.23 30.1
Targets: 30 fps @ 1080p (project_30fps_floor_is_fine.md).
Stages NOT yet measured: intra prediction, MC, deblock.
Interpretation:
- For the IDCT-only workload at frame batch granularity, CPU NEON
is 4.1x faster than QPU V3D7.
- AUTO → recipe table → QPU per the decree → BELOW the 30 fps
target with no headroom for the remaining decoder stages.
- The earlier "101 fps median at 1080p" measurement reported in
PR #8's commit was actually the CPU NEON path — the daedalus-
fourier install on hertz at the time predated the cycle 6 H.264
QPU shader, so recipe AUTO silently fell back to CPU NEON.
PR #8's "Path C is viable" conclusion stands, but the substrate
label was wrong. Apologies for the misleading number.
What this means for the campaign:
- The decree's "fixable defect" claim is still aspirational for
the H.264 IDCT shaders. The current QPU shader dispatch costs
~3.6 ms per IDCT round-trip (luma 4x4 + luma 8x8 + chroma 4x4 =
~10 ms total cf. CPU's 2.3 ms), which dominates over the compute.
- daedalus-decoder doesn't need to take a position on this — the
AUTO path follows the recipe table and respects the decree.
The substrate selector is the escape hatch when consumers want
to override.
- For the libavcodec intercept patch when it lands, the right
move is probably to start with CPU NEON for IDCT and switch to
QPU once the dispatch overhead drops (issue #162 dmabuf import
+ further pool work on the daedalus-fourier side).
No source change to flush_frame itself; this is purely a measurement
add. The bench is opt-in (not a ctest) — these numbers belong in
commit messages and the campaign log, not in CI gating.
claude-noether
0b6482bc8f