[Intel-gfx] [PATCH] drm/i915/gt: Try an extra flush on the Haswell blitter

[Chris Wilson]

Quoting Mika Kuoppala (2019-11-12 13:58:36)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > On gen7, including Haswell, the MI_FLUSH_DW command is not synchronous
> > with the command streamer nor is there an option to make it so. To hide
> > this we add a large delay on the CS so that the breadcrumb should always
> > be visible before the interrupt. However, that does not seem to be
> > enough to ensure the memory is actually coherent with the read of the
> > breadcrumb. The breadcrumb update is a post-sync op... Throw in a
> > preliminary MI_FLUSH_DW before the breadcrumb update in the hope that
> > helps.
> >
> > Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112147
> 
> If we are not sure, References

Either way, I'm closing the bug :)

> > Testcase: igt/i915_selftest/live_blt
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > ---
> >  drivers/gpu/drm/i915/gt/intel_ring_submission.c | 5 ++++-
> >  1 file changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/i915/gt/intel_ring_submission.c b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> > index a47d5a7c32c9..fc29df712810 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_ring_submission.c
> > @@ -454,6 +454,10 @@ static u32 *gen7_xcs_emit_breadcrumb(struct i915_request *rq, u32 *cs)
> >       GEM_BUG_ON(i915_request_active_timeline(rq)->hwsp_ggtt != rq->engine->status_page.vma);
> >       GEM_BUG_ON(offset_in_page(i915_request_active_timeline(rq)->hwsp_offset) != I915_GEM_HWS_SEQNO_ADDR);
> >  
> > +     *cs++ = MI_FLUSH_DW;
> > +     *cs++ = 0;
> > +     *cs++ = 0;
> 
> I see this as wmb wrt seqno write.
> 
> Massive hammer would be to include timestamp write in here with tlb
> flush?

I'm pretty sure it's meant to be a post-sync write op. However, the
evidence I think points towards this flush being ordered incorrect with
the breadcrumb write, so anything goes.
 
> But one can always hope.

Yeah, small changes until we can declare victory then go back to sleep.
-Chris