drm/i915: Introduce i915_gem_object_finish_gtt()

Like its siblings finish_gpu(), this function clears the object from the
GTT domain forcing it to be trigger a domain invalidation should we ever
need to use via the GTT again.

Note that the most important side-effect of finishing the GTT domain
(aside from clearing the tracking read/write domains) is that it imposes
an memory barrier so that all accesses are complete before it returns,
which is important if you intend to be modifying translation tables
shortly afterwards. The second most important side-effect is that it
tears down the GTT mappings forcing a page-fault and invalidation on
next user access to the object.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -2149,6 +2149,30 @@ i915_gem_object_wait_rendering(struct drm_i915_gem_object *obj)
 	return 0;
 }
 
+static void i915_gem_object_finish_gtt(struct drm_i915_gem_object *obj)
+{
+	u32 old_write_domain, old_read_domains;
+
+	if ((obj->base.read_domains & I915_GEM_DOMAIN_GTT) == 0)
+		return;
+
+	/* Act a barrier for all accesses through the GTT */
+	mb();
+
+	/* Force a pagefault for domain tracking on next user access */
+	i915_gem_release_mmap(obj);
+
+	old_read_domains = obj->base.read_domains;
+	old_write_domain = obj->base.write_domain;
+
+	obj->base.read_domains &= ~I915_GEM_DOMAIN_GTT;
+	obj->base.write_domain &= ~I915_GEM_DOMAIN_GTT;
+
+	trace_i915_gem_object_change_domain(obj,
+					    old_read_domains,
+					    old_write_domain);
+}
+
 /**
  * Unbinds an object from the GTT aperture.
  */
@@ -2173,8 +2197,7 @@ i915_gem_object_unbind(struct drm_i915_gem_object *obj)
 	 * cause memory corruption through use-after-free.
 	 */
 
-	/* blow away mappings if mapped through GTT */
-	i915_gem_release_mmap(obj);
+	i915_gem_object_finish_gtt(obj);
 
 	/* Move the object to the CPU domain to ensure that
 	 * any possible CPU writes while it's not in the GTT