drm/i915: Before accessing an object via the cpu, flush GTT writes

If we want to read the pages directly via the CPU, we have to be sure
that we have to flush the writes via the GTT (as the CPU can not see
the address aliasing).

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-9-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -622,6 +622,8 @@ int i915_gem_obj_prepare_shmem_read(struct drm_i915_gem_object *obj,
 	if (ret)
 		return ret;
 
+	i915_gem_object_flush_gtt_write_domain(obj);
+
 	/* If we're not in the cpu read domain, set ourself into the gtt
 	 * read domain and manually flush cachelines (if required). This
 	 * optimizes for the case when the gpu will dirty the data
@@ -662,6 +664,8 @@ int i915_gem_obj_prepare_shmem_write(struct drm_i915_gem_object *obj,
 	if (ret)
 		return ret;
 
+	i915_gem_object_flush_gtt_write_domain(obj);
+
 	/* If we're not in the cpu write domain, set ourself into the
 	 * gtt write domain and manually flush cachelines (as required).
 	 * This optimizes for the case when the gpu will use the data