tracked down another issue with BPI/RCM4. i was wondering why the ethernet was working only at 100mbit/s and not reliably. turns out it had to do with the connection/polarity of the LEDs. the bpi-cm4 expects LED1 to be active high, but on the Reform motherboard, it's wired as active low. and: the LED pins also double as config straps for the RTL8211F PHY on the bpi-cm4. so one or two config bits must have been unexpected. cutting the traces on RCM4 made 1gpbs work for the first time.