drm/i915/execlists: Flush GTIIR on clearing CS interrupts during reset

Be paranoid and flush the GTIIR after clearing the CS interrupt to be
sure it has taken before we re-enable the interrupt handler. We still
see early interrupts following reset, the tasklet handling the mmio read
before it has been written by the CS. This hopefully reduces the
frequency to 0...

References: https://bugs.freedesktop.org/show_bug.cgi?id=104262
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Michel Thierry <michel.thierry@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180202145455.29876-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_lrc.c

@@ -1527,6 +1527,7 @@ static int gen9_init_render_ring(struct intel_engine_cs *engine)
 static void reset_irq(struct intel_engine_cs *engine)
 {
 	struct drm_i915_private *dev_priv = engine->i915;
+	int i;
 
 	/*
 	 * Clear any pending interrupt state.
@@ -1535,10 +1536,14 @@ static void reset_irq(struct intel_engine_cs *engine)
 	 * buffered, and if we only reset it once there may still be
 	 * an interrupt pending.
 	 */
-	I915_WRITE(GEN8_GT_IIR(gtiir[engine->id]),
-		   GT_CONTEXT_SWITCH_INTERRUPT << engine->irq_shift);
-	I915_WRITE(GEN8_GT_IIR(gtiir[engine->id]),
-		   GT_CONTEXT_SWITCH_INTERRUPT << engine->irq_shift);
+	for (i = 0; i < 2; i++) {
+		I915_WRITE(GEN8_GT_IIR(gtiir[engine->id]),
+			   GT_CONTEXT_SWITCH_INTERRUPT << engine->irq_shift);
+		POSTING_READ(GEN8_GT_IIR(gtiir[engine->id]));
+	}
+	GEM_BUG_ON(I915_READ(GEN8_GT_IIR(gtiir[engine->id])) &
+		   (GT_CONTEXT_SWITCH_INTERRUPT << engine->irq_shift));
+
 	clear_bit(ENGINE_IRQ_EXECLIST, &engine->irq_posted);
 }