Compute Module • Ethernet problem (Porting from CM3B+ to CM4s)

Hello Everyone,

I am currently working on porting a custom board that was originally designed for Raspberry Pi 3B+ CM to the Raspberry Pi 4S CM. I successfully completed the port, but I am encountering a strange problem with the Ethernet interface.

The Ethernet works only when I power on the image for the first time. However, if I perform a reboot or a shutdown and then power it on immediately, the Ethernet does not blink at all. To make it work again, I need to shut down, wait about 10 seconds, and then power it on.

This behavior did not occur on the 3B+ CM.

When it works, I see the following in the logs:

[ 8.678677] smsc95xx v2.0.0
[ 8.804629] smsc95xx ... Failed to read reg index 0x00000114: -71
[ 8.823344] smsc95xx ... smsc95xx_mdio_read: MII is busy
[ 8.823362] SMSC LAN8700 usb-001:003:01: phy_poll_reset failed: -71
[ 8.840107] smsc95xx ... can't attach PHY to usb-001:003
[ 8.855935] smsc95xx: probe of 1-1.1:1.0 failed with error -71


When it doesn’t work, the Ethernet is not detected at all.

Can anyone help me understand the cause of this issue? Why does it work on the first boot but fail on subsequent reboots with the CM4S?

Thank you in advance for your help.

Statistics: Posted by Alie-Eldeen.Omar — Thu Aug 21, 2025 3:34 pm

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