First cycle of the libavcodec.so substitution arc (reauktion/daedalus-v4l2#11
step 2). H264DSPContext.idct_add — called per 4×4 block from the
intra-4×4 decode path in libavcodec/h264_mb.c — now dispatches through
daedalus_recipe_dispatch_h264_idct4 instead of ff_h264_idct_add_neon.
## What
- Add 0003-h264-idct4-daedalus-fourier.patch (in both arch/ and
debian/ ffmpeg-v4l2-request-fourier/). Creates
libavcodec/aarch64/h264_idct_daedalus.c (ff_h264_idct_add_daedalus
shim + lazy pthread_once context init via
daedalus_ctx_create_no_qpu), patches
libavcodec/aarch64/h264dsp_init_aarch64.c to wire c->idct_add to
the shim, adds the new .o to libavcodec/aarch64/Makefile.
- arch/PKGBUILD + debian/build-deb.sh: fetch + build
daedalus-fourier (pinned at d87239d — lockstep with the
daedalus-v4l2 daemon's inline build) with
-DCMAKE_POSITION_INDEPENDENT_CODE=ON into a per-build temp prefix,
then pass --extra-cflags=-I.../include --extra-ldflags=-L.../lib
--extra-libs="-ldaedalus_core -lvulkan -lpthread" to FFmpeg
configure. daedalus_core.a is static-linked into libavcodec.so.62.
- debian/control Depends gains libvulkan1 (daedalus_core PUBLIC-links
Vulkan::Vulkan for the queryable QPU substrate; the no-QPU
constructor still works at runtime but the loader needs
libvulkan.so.1 present to dlopen libavcodec.so.62).
- arch/PKGBUILD depends gains vulkan-icd-loader, makedepends gains
cmake / ninja / vulkan-headers.
## Why
The recipe layer picks the substrate; for cycle 6 (H.264 IDCT 4×4)
the recipe is CPU NEON, so this is effectively a NEON-to-NEON
substitution with one extra dispatch call and recipe-table lookup.
The point of this first cycle isn't perf wins — it's plumbing. Once
the path is wired and stable, follow-up patches batch through the
bulk paths (idct_add16 / idct_add16intra / idct_add8) and stack
cycles 7/8/9 (IDCT 8×8, luma-v deblock, qpel mc20).
Bit-exact against ff_h264_idct_add_neon (daedalus-fourier cycle 6
green; FFmpeg's 4×4 block storage matches daedalus's column-major
convention).
## Scope NOT covered
- Bulk paths (idct_add16 / idct_add16intra / idct_add8) — most IDCT
4×4 calls in real H.264 streams go through these, not the per-
block c->idct_add path; intra-4×4-only macroblocks are a minority.
Batched substitution lands in a follow-up.
- High-bit-depth (10-bit) path — not touched; 8-bit only.
- Cycles 7/8/9 — separate PRs.
## SONAME
Unchanged. libavcodec.so.62 / libavformat.so.62 / libavutil.so.60.
No daedalus-v4l2-dkms or daedalus-v4l2 bump required.
## Refs
- reauktion/daedalus-v4l2 issue #11 (substitution arc): reauktion/daedalus-v4l2#11
- marfrit/daedalus-fourier cycle 6 close (H.264 IDCT 4×4 NEON green)
