Just to update: I tried all the hints received here with no luck. Since others on this list managed to get uefifb working with the NVIDIA driver I believe the problem could be my mobo/UEFI FW/GPU combination. I found some rather old posts ([1], [2]) supporting this hypothesis. For the moment I switched to nouveau.

 

Thanks again to all,

 

raffaele

 

[1] https://forums.developer.nvidia.com/t/uefi-nvidia-vga-console-complaints/37690

[2] https://forums.developer.nvidia.com/t/nvidia-devs-any-eta-on-fbdev-console-mode-setting-implementation/47043