TL;DR:

The DWMAC on Spacemit K3 is failled to register phy after the reload
the driver module (rmmod then insmod). Because the reset-gpio is
asserted while unloading the driver and is not desserted before reading
the c22 id, which leads to a fault.

Description

In a few days ago, E Shattow reports he has sufferred the a weird issue,
when unloading the spacemit-dwmac driver and then reloading it, he got
the following error:

[   60.713071] mdio_bus stmmac-0: MDIO device at address 1 is missing.

This only occurs when reloading the driver but the first initialization
successed. After some function tracking, I found it is failed at reading
c22 id. The call graph is as the follows:

stmmac_mdio_register
	mdiobus_alloc
	of_mdiobus_register
		__of_mdiobus_register
		__mdiobus_register
		__of_mdiobus_parse_phys
			of_mdiobus_child_is_phy
			of_mdiobus_register_phy
				fwnode_mdiobus_register_phy
					get_phy_device
						get_phy_c22_id

By checking the difference between the initialization process and the
reloading process, I found the reset gpio is asserted in the function
mdiobus_unregister(). And there is no any function desserted this reset
gpio in the loading stage. And in the initialization process, the reset
goio is deasserted.

This bug report is sent as I found it is hard to solve this problem
and ask for help to fix this issue as it is related to the framework
instead of a specific driver.

The possible workaround I found is as the following, just use the phy
id in the compatible string (Confirmed by dlan):
---

--- a/arch/riscv/boot/dts/spacemit/k3-pico-itx.dts
+++ b/arch/riscv/boot/dts/spacemit/k3-pico-itx.dts

@@ -196,7 +196,8 @@ &eth0 {

 	mdio {
 		phy0: phy@1 {
-			compatible = "ethernet-phy-ieee802.3-c22";
+			compatible = "ethernet-phy-id001c.c916",
+				     "ethernet-phy-ieee802.3-c22";
 			reg = <1>;
 			reset-gpios = <&gpio 0 15 GPIO_ACTIVE_LOW>;
 			reset-assert-us = <10000>;

---
An interesting thing is, moving the reset-gpio to the MDIO bus level
does not solve this problem. Only setting the right phy id can mitigate
the problem.

Regards,
Inochi