Looking for pointers on diagnosing ring test failure in amdgpu

[Matthew Macy]

I'm trying to bring up amdgpu an Carrizo A10 (Thinkpad e565 in case it matters) on FreeBSD. The driver is essentially unmodified from what is found in Linux 4.6 - relying on an extended version of FreeBSD's linuxkpi shims. The shims work well enough that i915/drm from 4.6 works extremely well on most hardware (I have yet to diagnose / fix the severe artifacts on Cherry Trail and Atom).

On my A10 ring 11 test is failing:
  https://gist.github.com/mattmacy/8e4a85072648eceb2445ad227dcc447c

On my friend's A12 based EliteBook ring initialization succeeds:
https://gist.github.com/mattmacy/d1fac64ab5190bb2568d6480dfbd7ee6

With minor timing perturbations ring tests  will fail as early as ring 0.

I'm hoping that one of the amdgpu developers might give me pointers on how to diagnose further and or what bugs in the linuxkpi might be causing this. I know that I can selectively disable the rings, but that doesn't help fix the underlying problem.

Thanks in advance.

-M