ADR-0005: Config slices by symbol ownership

Status: accepted Date: 2026-09-20

Context

The Buildroot .config is an input to every package. Declared whole, any change to it - one new package - reruns every action. Worse, each action ran Kconfig's syncconfig over the entire tree's Config.in files, which forced the whole tree into every action's inputs.

Decision

Each package receives a slice of the config: the lines whose symbol is global, owned by the package's directory or its closure's, mentioned in its own .mk, referenced by the infrastructure, or a PROVIDES/HAS option. Ownership comes from scanning every Config.in* (br2_owners). Only BR2_X=value lines are kept. The slice ships with its own auto.conf, so syncconfig never runs in a package action. The slice is a deterministic tar, so an unchanged slice has an unchanged digest and Buck2's early cutoff stops the invalidation there.

Consequences

• Easier: adding a package, or flipping one package's option, rebuilds that package and the rootfs (measured: 43 s and 81 s against a 5 min cold build).
• Easier: no Kconfig, no conf binary and no Config.in files in a package action.
• Harder: the ownership rules are static analysis of Kconfig and .mk text, and can be wrong: a symbol read indirectly (through a make variable built by concatenation) is not seen. check-narrow.py validates the slices against the current configuration; it must be rerun after adding packages.
• Deliberately conservative: all global symbols reach every package. A change to the architecture or optimization level rebuilds everything. That is right, and refining it (rootfs-only symbol groups) is a possible optimization, not a soundness fix.