mxlen0 not showing up despite ConnectX apperaing in lspci -v

Was it working before for this particular NIC?  And the system?

I beleive the Driver Behavior changed from MLX from version 9.x to 10.x to 11.x now. 

Just another undocumented Vendor change throwing curveballs to those of use that use it. ;D 

Since you are the first from whom I see this tunable, but definitely not the first who I see using Mellanox NICs, I bet there must be some other method of switching ports role, such as OPROM, flash or something else.  But thanks for report any way.  It does not look like a problem then, otherwise Mellanox would set that tunable by default in their driver.

Actually the Firmware works prolem was articulated on this Forum here: 

https://forums.freebsd.org/threads/mellanox-mt26448.64350/

Fixed it with: 

 sysctl sys.device.mlx4_core0.mlx4_port1=eth

Regards,

Hazen