jcp-energy: the Neural Engine is reported only once its rail is proved live #158
2026-10-05
jcp-energy: the Neural Engine is reported only once its rail is proved live
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FabricMeter listed the Neural Engine as readable whenever IOReport opened, and reported its `ANE0` delta over every span. A rail that did not advance was read as zero, "on an idle fabric that is the truth". On macOS 27.0.1 an unprivileged reader's ANE0 is frozen. It did not advance in any of 25 sample intervals over 3 s of Core ML inference on the ANE, while root powermetrics read 0.47 J of ANE energy over the same span. So real Neural Engine work was reported as 0 J, which is what the ANE0 fix set out to stop. - `npu_rail_liveness(load)`, cached once per process. It runs the shared liveness rule over the ANE rails the counter sums, the `ANE Energy` aggregate or else every `ANE<n>` rail, with the caller's Neural-Engine load between samples. A sample with no ANE rail is no sample, so a part without one is Unavailable, never a vacuous Live. - FabricMeter lists and reports the Neural Engine only once that proof says Live. - The survey now uses the shared rule instead of a single advance over 60 ms. Its CPU status comes from `ioreport_status()`, and its Neural Engine status from the proof. A rail republished in lumps passes a one-advance test. Tests: - the ANE rails the probe watches match the counter's sum; - the scripted probe gives Frozen for a holding ANE0, Live for an advancing one, Frozen for a lump and for one engine of two, and Unavailable with no ANE rail; - on this Mac the meter lists only the CPU and the GPU, and a load that never touches the ANE leaves its rail Frozen. Mutations caught: emitting the Neural Engine figure unproved, and dropping the empty-rail filter. Co-Authored-By: Claude Opus 5.5 <[email protected]>