Problem solved :).

As stated in https://wiki.archlinux.org/index.php/AMDGPU#Selecting_the_right_driver

The parameters must be added to the kernel but the parameters depend on what cart you have installed in the case the card is Southern Islands **or** Sea Islands.

For Southern Islands parameters are: radeon.si_support=0 amdgpu.si_support=1
For Sea Islands parameters are: radeon.cik_support=0 amdgpu.cik_support=1

but not both, or you will end with a frozen display during the boot as in my case.

I hope this help others.

On Sat, Feb 17, 2018 at 12:56 AM, Pietro Pesci Feltri <ppesci@gmail.com> wrote:

I add the parameters manually pressing "e" in the grub menu 2 days ago. One hour later the initialization process was not ended.
I was forced to reboot the machine.

Anyway thanks.