Every read came back doubled: seed REG_0C with 0x1248 and the firmware received
0x2490. The command byte's own trailing falling edge was being treated as a data
clock, so bit 15 was shifted away before the guest sampled it and the whole word
landed one place too high.
Each firmware bit is read/raise/lower (BK4819_ReadU16), which means the eighth
command bit is followed by a falling edge before the data phase begins. Skip that
one edge.
Why it went unnoticed: writes were always fine -- 52 registers held exactly what
the firmware wrote -- and the register the firmware polls hardest, REG_0C, was
legitimately 0 in this model. Reading zero and getting zero looks like success.
The skew only surfaced when something tried to report a value through it.
It also explains four failed attempts at the squelch interrupt. The model raised
REG_0C bit 0; the firmware received bit 1. So
while (BK4819_ReadRegister(BK4819_REG_0C) & 1u)
was never true, the acknowledging write inside it never ran, and 1719 polls saw a
flag the guest could not act on. Every one of those attempts was diagnosed as a
timing or gating problem and was not.
tools/test_bk4819_readback.sh locks it down. It seeds REG_0C -- read ~1700 times
per 30s, so a sample is guaranteed -- with a value carrying bits in both halves,
so a shift either way is unmistakable, and names the direction on failure. Bit 0
is left clear on purpose: with it set the firmware enters an acknowledge loop that
has no timeout, and this test is about alignment only.
Confirmed by A/B: committed code 0x2490, patched 0x1248.
