pcie-link's retrain check compared post-retrain speed against a port's
max capability without checking whether anything was actually seated
downstream. A switch downstream port with no card plugged in retrains
to zero lanes but still reports a reset-state speed, so it was
misreported as "retrained to Gen1, capable of Gen5" — indistinguishable
from a real degraded link.
Now a port that was already empty before the retrain is skipped (same
treatment as a disabled device: legitimately unpopulated slots are
common and shouldn't fail the check), while a device that answered
before the retrain and disappeared right after it still fails, with an
honest "no device detected downstream" message instead of a fabricated
speed reading.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
