docs: walk-free STM32 plan (read-side freshness decision + migration batches)#514
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Design doc for making the per-cycle peripheral walk structurally unnecessary so derive_walk_deletable() flips on user-compiled firmware, not just curated demos with a hand walk_deleted flag. Part 1 recommends batch-boundary read freshness (< one-interval staleness) implemented via interior-mutability sync-on-read against a bus-published Arc<AtomicU64> clock; rejects the &mut read choke point on blast radius. Includes a SPIKE integration test proving the mechanism compiles against the real Peripheral trait and is byte-exact at batch boundaries, bounded by < interval mid-batch. Part 2 inventories the 49 invaders-bus walkers (28 inert Class-A + 21 real-tick Class-B across 8 model types) with per-class exemplars, migration order, and PR batches B0-B8 each with a walk-on@1 vs scheduler@64 differential gate. Part 3 gives the walk_deleted deprecation path.
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Design doc for the walk-free campaign: make the per-cycle peripheral walk structurally unnecessary so ANY firmware gets interval-64 batching, not just curated demos with a hand walk_deleted flag.
Decision: batch-boundary read freshness via interior mutability (Arc published cycle clock; ≤ one-interval staleness on raw counter reads, event-observables byte-exact via absolute deadlines + batch clamps). The &mut-read alternative is rejected on measured blast radius (134 Peripheral impls, ~1800 read call sites) and, fatally, the fetch_slice/observer aliasing model. Includes a committed spike test proving the pattern compiles and holds its bounds against the real trait.
Inventory: 49 walkers = 28 inert Class-A instances (needs_legacy_walk=false assertions only) + 21 real-work instances across 8 model types (systick, timers, dma, i2c, adc, exti, scb, bxcan), each mapped to an existing migration exemplar. Batches B0-B8 with per-batch differential gates (walk-on@interval-1 vs scheduler@64, armed-peripheral firmware).
Endgame: derive_walk_deletable flips true per-board as batches land; walk_deleted becomes a deprecated escape hatch.