Comment 460 for bug 1690085

I have a Ryzen 1800X + Gigabyte GA-AX370-Gaming K7.

I was suffering from these hangs until I changed a few settings in my BIOS. The change that made the difference was the Power Supply Iddle Control that was added in the latest BIOS update. All other settings are left by default:
M.I.T->
  Advanced Frequency Settings ->
    Core Settings ->
      *Power Supply Iddle Control = Typical
      *SVM Mode = On
Peripherals ->
  *Above 4G Decoding = On
Chipset ->
  IOMMU = Auto (Disabled)

BIOS version: F23f AGESA 1.0.0.2a + SMU FW 43.18

I'm running Ubuntu x64 on stock kernel 4.15.0-23-generic with the following options
GRUB_CMDLINE_LINUX_DEFAULT="radeon.cik_support=1 amdgpu.cik_support=0 iommu=soft"

As an extra measure I completely blacklisted AMDGPU from my system because it was also a source of many of the kernel freezes (AMD 390x GPU), and I haven't seen a system freeze in almost two months. (they were _very_ frequent before, almost daily)

Good luck.