Comment # 26 on bug 1119621 from Tejas Guruswamy

(In reply to Takashi Iwai from comment #25)
> (In reply to Tejas Guruswamy from comment #24)
> > I finally had a chance to test again. Summary is unfortunately
> > kernel-default-4.20.5-1.1.ge544b4e.x86_64 from home:tiwai did not fix my
> > problem. i915.fastboot=1 still works though.
> 
> Thanks for testing.  That's what I was afraid of :-<
> So apparently we're looking at multiple issues.
> 
> > Note that
> > fastboot=Y appears to be the default in 4.20.6 and drm-tip.
> 
> Really?  I couldn't find anything that changed the default in the kernel
> source.

Sorry, this may have just been a side effect of running drm-tip.

> In theory we can flip fastboot=y as default.  It's been disabled due to the
> regression seen on a Chromebook with some funky firmware.
> But it's no right "fix", and the option can be given easily via boot option,
> let's try to track the right cause for now.

Progress at last! I found the same, or at least very similar, issue filed at
Arch (https://bugs.archlinux.org/task/60841) and it turns out
intel_idle.max_cstate=4 fixes my issue more reliably than i915.fastboot=1.

Not clear why cpu c-states are affecting the gpu but the root bug may in fact
be this one:
https://bugs.freedesktop.org/show_bug.cgi?id=103229

This issue has turned up on many Skylake systems with different possible causes
but the same symptoms (flickering/black screen) and workaround (reduce max
cstate from 9 to 4).
https://bugs.freedesktop.org/buglist.cgi?quicksearch=SKL%20flicker%20underrun&list_id=667366