Resolved! Ethernet fails if cable plugged in during boot. If plugged in after boot, works.
This is an STM32MP157c based board design with Micrel KSZ9021GN PHY in 100 MII mode. Have you ever seen this situation before? Here are the relevant parts of the boot, the early kernel part looks normal and the LEDs are on.... [ 2.507568] stm32-dw...