[Intel-gfx] [PATCH 2/2] drm/i915: Postpone fake breadcrumb interrupt if a real interrupt is pending

[Chris Wilson]

On Thu, Feb 16, 2017 at 10:38:08AM +0000, Tvrtko Ursulin wrote:
> 
> On 16/02/2017 09:29, Chris Wilson wrote:
> >If the timer expires for enabling the fake interrupt, check to see
> >if there is a real interrupt queued before making the decision to start
> >polling. This helps in situations where we have a very slow irq-seqno
> >barrier that may accrue more breadcrumb interrupts before it is able to
> >catch up. It still leaves a hole for the timer to expire as we are
> >processing the last irq-seqno barrier, but it appears to help reduce the
> >frequency of "missed-interrupts" on Ironlake, at least.
> >
> >References: https://bugs.freedesktop.org/show_bug.cgi?id=99816
> >Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> >Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> >---
> > drivers/gpu/drm/i915/intel_breadcrumbs.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> >diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> >index ef3adfd37d7d..21269421bd2a 100644
> >--- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> >+++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> >@@ -41,6 +41,11 @@ static void intel_breadcrumbs_hangcheck(unsigned long data)
> > 		return;
> > 	}
> >
> >+	if (test_bit(ENGINE_IRQ_BREADCRUMB, &engine->irq_posted)) {
> >+		mod_timer(&b->hangcheck, jiffies + 1);
> >+		return;
> >+	}
> >+
> > 	DRM_DEBUG("Hangcheck timer elapsed... %s idle\n", engine->name);
> > 	set_bit(engine->id, &engine->i915->gpu_error.missed_irq_rings);
> > 	mod_timer(&engine->breadcrumbs.fake_irq, jiffies + 1);
> >
> 
> Another hmm :), barriers are so much shorter than the hangcheck
> interval (1500ms) so I don't quite understand what is the problem.

We may queue the wait many, many seconds before it is even being
processed. The point of this timer is to detect when we haven't seen an
interrupt for some time and that happens to be the waiter missed (backup
for seqno-barrier failing).
 
> Should we instead put a mod_timer when raising the user irq?

Frequency, mod_timer may require reprogramming of the apic and often
does when profiling ;)
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre