drm/i915: Don't clflush before release phys object

When we teardown the backing storage for the phys object, we copy from
the coherent contiguous block back to the shmemfs object, clflushing as
we go. Trying to clflush the invalid sg beforehand just oops and would
be redundant (due to it already being coherent, and clflushed
afterwards).
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161223145804.6605-3-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -246,14 +246,16 @@ i915_gem_object_get_pages_phys(struct drm_i915_gem_object *obj)
 
 static void
 __i915_gem_object_release_shmem(struct drm_i915_gem_object *obj,
-				struct sg_table *pages)
+				struct sg_table *pages,
+				bool needs_clflush)
 {
 	GEM_BUG_ON(obj->mm.madv == __I915_MADV_PURGED);
 
 	if (obj->mm.madv == I915_MADV_DONTNEED)
 		obj->mm.dirty = false;
 
-	if ((obj->base.read_domains & I915_GEM_DOMAIN_CPU) == 0 &&
+	if (needs_clflush &&
+	    (obj->base.read_domains & I915_GEM_DOMAIN_CPU) == 0 &&
 	    !cpu_cache_is_coherent(obj->base.dev, obj->cache_level))
 		drm_clflush_sg(pages);
 
@@ -265,7 +267,7 @@ static void
 i915_gem_object_put_pages_phys(struct drm_i915_gem_object *obj,
 			       struct sg_table *pages)
 {
-	__i915_gem_object_release_shmem(obj, pages);
+	__i915_gem_object_release_shmem(obj, pages, false);
 
 	if (obj->mm.dirty) {
 		struct address_space *mapping = obj->base.filp->f_mapping;
@@ -2233,7 +2235,7 @@ i915_gem_object_put_pages_gtt(struct drm_i915_gem_object *obj,
 	struct sgt_iter sgt_iter;
 	struct page *page;
 
-	__i915_gem_object_release_shmem(obj, pages);
+	__i915_gem_object_release_shmem(obj, pages, true);
 
 	i915_gem_gtt_finish_pages(obj, pages);