G14, unblocked by the frame measurement in efb7f991 — the plan deliberately left this
one alone until the up axis was known, on the grounds that a confidently wrong constant
would be worse than an obviously wrong zero. It is known now.
A virtual DualSense, DualShock 4 or Steam Deck that had received no motion reported
acceleration `[0, 0, 0]`. That is not "no data": zero proper acceleration means free
fall, which is a definite claim about the physical world and one that is never true of
a controller sitting on a desk or held in someone's hands — both read 1 g up. Anything
that interprets the accelerometer gets a confident wrong answer rather than a boring
right one.
It is worst exactly where it is least visible. A pad with no gyro at all — an X-Box
controller forwarded as a DualSense, which is what "Automatic" does for anything not
Sony or Valve — never sends motion, so it sits on that neutral for the entire session,
telling every game that reads it that the controller is falling. `switch_proto` has
always done this correctly on its own up axis, which is what made the gap visible in the
first place.
Which axis, and why it took a measurement. The wire is a unit passthrough into the
virtual pad's report, so the wire's up axis is the pad's own, and on 2026-08-07 a real
DualSense read over raw HID put `+0.997 g` on report axis 1 at rest, in a frame pinned
the same session as (Right, Up, Backward). So `MOTION_NEUTRAL_ACCEL` is `[0, 10000, 0]`
— NOT the z-up the notes had assumed from `switch_proto`'s documentation, which is why
guessing would have shipped a backend confidently disagreeing with the hardware.
The constant lives in punktfunk-core beside the units it is expressed in, and every
backend derives from it rather than restating it. The Deck's neutral in particular goes
through `steam_remap::motion_wire_to_deck`, the same rescale a real sample takes, so the
neutral and the live path can never end up with two opinions about what 1 g is — its
`hid-steam` resolution stays in exactly one place. The DS4 needs no separate change: it
reuses `DsState`.
`switch_proto` is deliberately NOT touched, and the test says so. It is a different
device on a different driver, its up axis is its own, and nobody has measured its frame
— aligning it to the DualSense for consistency would be the same unmeasured guess this
commit exists to avoid, just in the other direction.
Non-vacuity proven both ways rather than assumed. Moving the up axis to slot 2 (the old
z-up assumption) fails on the wire constant itself, which is what makes the measurement
load-bearing rather than decorative; reverting both neutrals to `[0, 0, 0]` fails on the
DualSense assertion with the message naming the defect. Each backend is checked in ITS
OWN units, because hard-coding "1 g" three times is how the halves of a unit contract
drift apart.
Gate (Linux CI image): fmt, build, `clippy --locked --all-targets -D warnings` across
punktfunk-core / pf-inject / pf-client-core, and both test suites — green, with
`Running tests/motion_contract.rs` and the new case's own `... ok` line observed in the
log rather than inferred from a green exit (`cargo test` stops after the first failing
binary, so a green-looking run can mean the contract test never executed at all).
enricobuehler
d996449a82