we have run into a problem where some of our PCIe adapter models would not be detected by the system. It turned out that these are cards that use the +3.3V supply directly. Other cards, that derive it from the +12V supply work fine. When we measure the supply voltages on the slot we see +12V and +3.3Vaux but not +3.3V. Since Vaux is coming from the same power source we think the Load Switch U82 is not turned on.
For a working card the kernel reports “pciex8a yes” in /sys/kernel/debug/powergate.
For a non-working card it reports “pciex8a no”. But in neither case is +3.3V enabled.
Toggling the pciex8a power domain via /sys/kernel/debug/pg_domains/pciex8a/state has no effect either.