GENET and UNIMAC MDIO probe issue?

Hi Florian, Doug,

I'm currently trying to make the RaspberryPi CM4 IO board probe its
ethernet controller, and it looks like genet doesn't manage to find its
PHY and errors out with:

[    3.240435] libphy: Fixed MDIO Bus: probed
[    3.248849] bcmgenet fd580000.ethernet: GENET 5.0 EPHY: 0x0000
[    3.259118] libphy: bcmgenet MII bus: probed
[    3.278420] mdio_bus unimac-mdio--19: MDIO device at address 1 is missing.
[    3.285661] unimac-mdio unimac-mdio.-19: Broadcom UniMAC MDIO bus

....

[   13.082281] could not attach to PHY
[   13.089762] bcmgenet fd580000.ethernet eth0: failed to connect to PHY
[   74.739621] could not attach to PHY
[   74.746492] bcmgenet fd580000.ethernet eth0: failed to connect to PHY

Here's the full boot log:
https://pastebin.com/8RhuezSn

It looks like it's related to the following bugzilla entry:
https://bugzilla.kernel.org/show_bug.cgi?id=213485

However, that commit has been merged for a while apparently, and even
next-20211022 shows that behavior, both with the drivers built-in or as
modules. The other suggested fix to set probe_type to
PROBE_FORCE_SYNCHRONOUS doesn't seem to fix it either.

Maxime