Quoting Mika Kuoppala (2017-08-08 14:36:39)
> Chris Wilson <ch...@chris-wilson.co.uk> writes:
> > As we may have just bound the renderstate into the GGTT for execution, we
> > need to ensure that the GTT TLB are also flushed.
> >
> > On snb-gt2, this would cause a random GPU hang at the start of a new
> > context (e.g. boot) and on snb-gt1, it was causing the renderstate batch
> > to take ~10s. It was the GPU hang that revealed the truth, as the CS
> > gleefully executed beyond the end of the golden renderstate batch, a good
> > indicator for a GTT TLB miss.
> >
> > Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> > Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
> > Cc: sta...@vger.kernel.org
> The flush has been there but got stomped by:
> Fixes: dc4be6071a24 ("drm/i915: Add explicit request management to 
> i915_gem_init_hw()")

Hmm, I think it is actually 20fe17aa52dc ("drm/i915: Remove
redundant TLB invalidate on switching contexts"). The importance is in
having the invalidate after the GTT bind and before the MI_BB_START.
Which just happens to be implied by the ordering of the context switch
emission, but given the indirect link, not guaranteed.

Commit dc4be6071a24 only modifies the sequence after the MI_BB_START, so
doesn't look like the culprit.
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