GPU hang on ThinkPad T420 with drm-kmod on 12.0-RELEASE

[Kevin Zheng]

Hi there,

I don't know if anyone else has been having similar issues.

On the ThinkPad T420 (i5-2540M, Sandybridge), the drm-kmod 
(drm-fbsd12.0-kmod-4.16.g20181215) hangs like this, infrequently, but 
often enough on GPU applications:

[drm] GPU HANG: ecode 6:0:0x61f7fffd, in chrome [100694], reason: Hang 
on rcs0, action: reset
drmn0: Resetting chip after gpu hang
last message repeated 1 times
drmn0: i915_reset_device timed out, cancelling all in-flight rendering.

It doesn't just hang with Chromium, it hangs on other apps as well.

Tricky thing is that things otherwise seem to work, and I haven't 
figured out how to reliably reproduce the hang.

Particularly, https://wiki.freebsd.org/Graphics/Intel-GPU-Matrix claims 
that other people have working Ivy Bridge with drm-kmod; maybe that has 
to be drm-kmod-legacy until I figure out what's going on.

Should I create a PR, or wait until I have more info?

-- 
Kevin Zheng
kevinz5000 at gmail.com | kevinz at berkeley.edu | PGP: 0xC22E1090