R9285, I notice that on both drm-next-4.15-dc and amd-staging-drm-next, when
using DC, vblank_mode=0 works windowed but not properly full screen.

Tried forcing TearFree from auto to off no difference.

There is no tearing where it would be expected - but it's not like it's just
being ignored, a bench mark test will cap fps at 2X refresh rate rather than
1X.

Maybe this is expected?

It doesn't happen with a months older DC kernel I have and looking at DC
benches on Phoronix I don't see any fps cap.