drm/i915: Only insert the mb() before updating the fence parameter

With a fence, we only need to insert a memory barrier around the actual
fence alteration for CPU accesses through the GTT. Performing the
barrier in flush-fence was inserting unnecessary and expensive barriers
for never fenced objects.

Note removing the barriers from flush-fence, which was effectively a
barrier before every direct access through the GTT, revealed that we
where missing a barrier before the first access through the GTT. Lack of
that barrier was sufficient to cause GPU hangs.

v2: Add a couple more comments to explain the new barriers

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

drivers/gpu/drm/i915/i915_gem.c

@@ -2611,9 +2611,22 @@ static void i830_write_fence_reg(struct drm_device *dev, int reg,
 	POSTING_READ(FENCE_REG_830_0 + reg * 4);
 }
 
+inline static bool i915_gem_object_needs_mb(struct drm_i915_gem_object *obj)
+{
+	return obj && obj->base.read_domains & I915_GEM_DOMAIN_GTT;
+}
+
 static void i915_gem_write_fence(struct drm_device *dev, int reg,
 				 struct drm_i915_gem_object *obj)
 {
+	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	/* Ensure that all CPU reads are completed before installing a fence
+	 * and all writes before removing the fence.
+	 */
+	if (i915_gem_object_needs_mb(dev_priv->fence_regs[reg].obj))
+		mb();
+
 	switch (INTEL_INFO(dev)->gen) {
 	case 7:
 	case 6:
@@ -2623,6 +2636,12 @@ static void i915_gem_write_fence(struct drm_device *dev, int reg,
 	case 2: i830_write_fence_reg(dev, reg, obj); break;
 	default: BUG();
 	}
+
+	/* And similarly be paranoid that no direct access to this region
+	 * is reordered to before the fence is installed.
+	 */
+	if (i915_gem_object_needs_mb(obj))
+		mb();
 }
 
 static inline int fence_number(struct drm_i915_private *dev_priv,
@@ -2652,7 +2671,7 @@ static void i915_gem_object_update_fence(struct drm_i915_gem_object *obj,
 }
 
 static int
-i915_gem_object_flush_fence(struct drm_i915_gem_object *obj)
+i915_gem_object_wait_fence(struct drm_i915_gem_object *obj)
 {
 	if (obj->last_fenced_seqno) {
 		int ret = i915_wait_seqno(obj->ring, obj->last_fenced_seqno);
@@ -2662,12 +2681,6 @@ i915_gem_object_flush_fence(struct drm_i915_gem_object *obj)
 		obj->last_fenced_seqno = 0;
 	}
 
-	/* Ensure that all CPU reads are completed before installing a fence
-	 * and all writes before removing the fence.
-	 */
-	if (obj->base.read_domains & I915_GEM_DOMAIN_GTT)
-		mb();
-
 	obj->fenced_gpu_access = false;
 	return 0;
 }
@@ -2678,7 +2691,7 @@ i915_gem_object_put_fence(struct drm_i915_gem_object *obj)
 	struct drm_i915_private *dev_priv = obj->base.dev->dev_private;
 	int ret;
 
-	ret = i915_gem_object_flush_fence(obj);
+	ret = i915_gem_object_wait_fence(obj);
 	if (ret)
 		return ret;
 
@@ -2752,7 +2765,7 @@ i915_gem_object_get_fence(struct drm_i915_gem_object *obj)
 	 * will need to serialise the write to the associated fence register?
 	 */
 	if (obj->fence_dirty) {
-		ret = i915_gem_object_flush_fence(obj);
+		ret = i915_gem_object_wait_fence(obj);
 		if (ret)
 			return ret;
 	}
@@ -2773,7 +2786,7 @@ i915_gem_object_get_fence(struct drm_i915_gem_object *obj)
 		if (reg->obj) {
 			struct drm_i915_gem_object *old = reg->obj;
 
-			ret = i915_gem_object_flush_fence(old);
+			ret = i915_gem_object_wait_fence(old);
 			if (ret)
 				return ret;
 
@@ -3068,6 +3081,13 @@ i915_gem_object_set_to_gtt_domain(struct drm_i915_gem_object *obj, bool write)
 
 	i915_gem_object_flush_cpu_write_domain(obj);
 
+	/* Serialise direct access to this object with the barriers for
+	 * coherent writes from the GPU, by effectively invalidating the
+	 * GTT domain upon first access.
+	 */
+	if ((obj->base.read_domains & I915_GEM_DOMAIN_GTT) == 0)
+		mb();
+
 	old_write_domain = obj->base.write_domain;
 	old_read_domains = obj->base.read_domains;