I was just trying to understand the call trace for mdio bus which got
me confused.

[0] https://lore.kernel.org/all/Z3fKkTSFFcU9gQLg@ryzen/ (local)

There is nothing particularly unusual in there. We see PCI bus
enumeration has found a device and bound a driver to it. The driver
has instantiated an MDIO bus, which has scanned the MDIO bus and found
a PHY. The phylib core then tried to load the kernel module needed to
drive the PHY.

Just because it is a PCI device does not mean firmware has to control
all the hardware. Linux has no problems controlling all this, and it
saves reinventing the wheel in firmware, avoids firmware bugs, and
allows new features to be added etc.

	Andrew