Currently, to have full control of your AMD GPU while using the amdgpu driver, you need to append the boot parameter amdgpu.ppfeaturemask=0xffffffff to your bootloader configuration and reboot. NOTE: The following instructions are for guidance only. Check your distribution documentation on how to add boot parameters.
GRUB bootloader Edit the file /etc/default/grub as root and append amdgpu.ppfeaturemask=0xffffffff to GRUB_CMDLINE_LINUX_DEFAULT:
NOTE: In the above example, … represent other existing parameters. Do not add … to your GRUB_CMDLINE_LINUX_DEFAULT. You should only add amdgpu.ppfeaturemask=0xffffffff. Then regenerate the bootloader configuration file with the command:
sudo grub-mkconfig -o /boot/grub/grub.cfg
Reboot your system. You should have more controls when you select Advanced as Performance mode where you can tweak the mhz and voltage easily.