drm/i915: Unmask user interrupts writes into HWSP on snb/ivb/vlv/hsw

An oddity occurs on Sandybridge, Ivybridge and Haswell (and presumably
Valleyview) in that for the period following the GPU restart after a
reset, there are no GT interrupts received. From Ville's notes, bit 0 in
the HWSTAM corresponds to the render interrupt, and if we unmask it we
do see immediate resumption of GT interrupt delivery (via the master irq
handler) after the reset.

v2: Limit the w/a to the render interrupt from rcs

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107500
Fixes: c5498089 ("drm/i915: Mask everything in ring HWSTAM on gen6+ in ringbuffer mode")
References: d420a50c ("drm/i915: Clean up the HWSTAM mess")
Testcase: igt/gem_eio/reset-stress
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180808105101.913-2-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -387,8 +387,18 @@ static void intel_ring_setup_status_page(struct intel_engine_cs *engine)
 		mmio = RING_HWS_PGA(engine->mmio_base);
 	}
 
-	if (INTEL_GEN(dev_priv) >= 6)
-		I915_WRITE(RING_HWSTAM(engine->mmio_base), 0xffffffff);
+	if (INTEL_GEN(dev_priv) >= 6) {
+		u32 mask = ~0u;
+
+		/*
+		 * Keep the render interrupt unmasked as this papers over
+		 * lost interrupts following a reset.
+		 */
+		if (engine->id == RCS)
+			mask &= ~BIT(0);
+
+		I915_WRITE(RING_HWSTAM(engine->mmio_base), mask);
+	}
 
 	I915_WRITE(mmio, engine->status_page.ggtt_offset);
 	POSTING_READ(mmio);