[Intel-gfx] [PATCH 2/3] drm/i915/execlists: Delay updating ring register state after resume

[Chris Wilson]

Quoting Chris Wilson (2018-03-27 22:01:56)
> Now that we reload both RING_HEAD and RING_TAIL when rebinding the
> context, we do not need to scrub those registers immediately on resume.

So CI reports that contrary to my belief, we didn't do a
i915_gem_contexts_lost() across suspend. Hmm, nope that's definitely
there in i915_gem_suspend, so all contexts should have been unpinned.
Oh, silly me, that doesn't account for the extra perma-pin we keep on
the kernel contexts to avoid them being evicted.

Ok, that's annoying and has some ramifications for the first patch if we
can contrive a wedging to be injected into the kernel context. Hmm, an
outside possibility to be sure as it would need a full device reset at
precisely the same time as a i915_gem_switch_to_kernel_context,
exceedingly rare. Not so rare as having the kernel context be an
innocent victim (the last context on an idle engine) - that's not
impacted by this, as there we know the breadcrumb has already been
emitted so RING_HEAD will not roll back and reemit on recovery.
-Chris