Created attachment 732108 [details]
HWInfo output

Motherboard is a ASUS CROSSHAIR V FORMULA-Z, BIOS 2201 03/23/2015 (latest 
available)
The CPU is an AMD FX(tm)-8350 Eight-Core Processor

With 4.4.72 everything was working (since years the hardware was used before 
with 13.1 and 42.1), even with the nvidia blob ;-)

Since kernel 4.4.74 with iommu enabled in bios, I got freeze, like one of the 
trace I finally found in journal. (Attached)

Today I retry with 4.4.75 and get immediately after the system is online a
AMD-Vi buffer timeout error.

I since then disabled iommu in the bios and the computer doesn't show bugs.
(But as the computer should run VM iommu is quite mandatory)

I know there was a kernel-firmware and ucode-amd update between the 4.4.72 and 
now.