61 lines
3.8 KiB
Markdown
61 lines
3.8 KiB
Markdown
# M4 boot integration development record
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Development reference and historical context. For current operating instructions, use the [user manual](../README.md). Acceptance applies only to the source and artifacts identified in each record.
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The user has authorized the remaining software through M12. This page records
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M4 development decisions. Software validation is complete; see [M4 evidence](m4-validation.md). Physical Pi validation remains deferred.
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The Pi kernel overlay starts from the exact source commit and archive checksum in
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Void's pinned `rpi5-kernel` recipe (6.12.87). It uses `bcm2712_defconfig` plus the
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small FDS delta, verifies Kconfig actually applied that delta, and exports the Pi 5
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Image, DTBs, overlays, and modules without modifying upstream Void files.
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The upstream Pi 5 16 KiB page configuration is retained.
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Critical storage, EROFS, keyboard, console and VC4 display support are built in.
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VC4 also requires the SND/SND_SOC core to be built in; optional sound devices can
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remain modules. CH341 SPI is disabled for the Dasung companion bridge. Generic
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ARM VM drivers are built into this same kernel so we can exercise software boot
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without claiming to emulate Pi firmware or RP1. Debug/BTF artifacts are omitted;
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module authentication is part of immutable image/release authentication rather
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than random build-generated module keys.
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`make kernel` uses the existing Void build container and cross compiler. Its
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additional host dependencies are Perl and Python for kernel generation, bc for
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build-time arithmetic, Bison/Flex for Kconfig, kmod for module indexes, OpenSSL
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headers for kernel build utilities, and archive tools. OpenSSL is a host build
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dependency, not a new Rust runtime dependency. The source and checksum are recorded
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in `packages/fds-kernel/template`.
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Image builders use a separate project-local prefix at `.host/image-tools` for
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signed Void x86_64 `erofs-utils`, `dosfstools` and `mtools`, plus their resolved
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libraries. They create filesystem images without privileged loop mounts and do
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not mutate the shared kernel build container. Versions and cached inputs are
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recorded under `out/manifests/image-tool-*`; full input locking is M12 work.
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Stage0 now has a Rust PID-1 implementation: direct filesystem
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syscalls, authenticated kernel uevents, explicit normal/recovery partition
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selection, early Dasung supervision, EROFS validation, mount transfer, and exec
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of native s6. Its normal path refuses to run outside root PID 1 on ramfs/tmpfs.
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The implementation reuses the locked `libc` crate for these Linux syscalls.
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The full initramfs matrix passed all four formats, missing-media USB insertion,
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ambiguous SYSTEM recovery, explicit recovery, invalid filesystems and foreign
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root rejection. A refreshed base package/rootfs build is being completed.
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QEMU uses its `max` CPU because the retained 16 KiB Pi kernel cannot execute
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on the old Cortex-A72 VM model.
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The FAT32 boot builder verifies every file after reading it back with mtools.
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Directory timestamps are normalized as well as file timestamps; two consecutive
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builds from the same inputs produced the same complete image SHA-256. This narrow
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result is not a claim of complete offline or release reproducibility.
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Two complete kernel builds also produced identical kernel Image, Pi 5 DTB and
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XBPS package digests. The helper now reuses a build after checking recorded
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source inputs, its package and all exported files. `--rebuild` bypasses reuse.
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The [boot guide](boot.md) documents usable build commands, image boundaries,
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waiting-console commands, display policy, and remaining physical checks.
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References: [Raspberry Pi cross-compilation](https://www.raspberrypi.com/documentation/computers/linux_kernel.html#cross-compile-the-kernel),
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[Linux initramfs](https://docs.kernel.org/filesystems/ramfs-rootfs-initramfs.html),
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and [root handoff semantics](https://man7.org/linux/man-pages/man8/switch_root.8.html).
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