Quoting Mika Kuoppala (2017-08-08 14:36:39)
> Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> 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 <chris@xxxxxxxxxxxxxxxxxx>
> > Cc: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
>
> 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.
-Chris