drm/i915: Reset semaphore page for gen8

An oversight is that when we wrap the seqno, we need to reset the hw
semaphore counters to 0. We did this for gen6 and gen7 and forgot to do
so for the new implementation required for gen8 (legacy).
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460010558-10705-6-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -2566,6 +2566,14 @@ void intel_ring_init_seqno(struct intel_engine_cs *engine, u32 seqno)
 		if (HAS_VEBOX(dev_priv))
 			I915_WRITE(RING_SYNC_2(engine->mmio_base), 0);
 	}
+	if (dev_priv->semaphore_obj) {
+		struct drm_i915_gem_object *obj = dev_priv->semaphore_obj;
+		struct page *page = i915_gem_object_get_dirty_page(obj, 0);
+		void *semaphores = kmap(page);
+		memset(semaphores + GEN8_SEMAPHORE_OFFSET(engine->id, 0),
+		       0, I915_NUM_ENGINES * gen8_semaphore_seqno_size);
+		kunmap(page);
+	}
 	memset(engine->semaphore.sync_seqno, 0,
 	       sizeof(engine->semaphore.sync_seqno));