drm/i915/selftests: Provide full mb() around clflush

clflush is an unserialised instruction and the IA manual strongly advises
you to serialise it with a mb. To be cautious, apply one before and one
after, so that it is serialised with both writes and reads without
worrying too much about the required direction.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706174926.4712-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_coherency.c

@@ -42,11 +42,21 @@ static int cpu_set(struct drm_i915_gem_object *obj,
 
 	page = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
 	map = kmap_atomic(page);
-	if (needs_clflush & CLFLUSH_BEFORE)
+
+	if (needs_clflush & CLFLUSH_BEFORE) {
+		mb();
 		clflush(map+offset_in_page(offset) / sizeof(*map));
+		mb();
+	}
+
 	map[offset_in_page(offset) / sizeof(*map)] = v;
-	if (needs_clflush & CLFLUSH_AFTER)
+
+	if (needs_clflush & CLFLUSH_AFTER) {
+		mb();
 		clflush(map+offset_in_page(offset) / sizeof(*map));
+		mb();
+	}
+
 	kunmap_atomic(map);
 
 	i915_gem_obj_finish_shmem_access(obj);
@@ -68,8 +78,13 @@ static int cpu_get(struct drm_i915_gem_object *obj,
 
 	page = i915_gem_object_get_page(obj, offset >> PAGE_SHIFT);
 	map = kmap_atomic(page);
-	if (needs_clflush & CLFLUSH_BEFORE)
+
+	if (needs_clflush & CLFLUSH_BEFORE) {
+		mb();
 		clflush(map+offset_in_page(offset) / sizeof(*map));
+		mb();
+	}
+
 	*v = map[offset_in_page(offset) / sizeof(*map)];
 	kunmap_atomic(map);