drm/i915: Protect active fences on i915

The i915 also uses the fence registers for GPU access to tiled buffers so
we cannot reallocate one whilst it is on the active list. By performing a
LRU scan of the fenced buffers we also avoid waiting the possibility of
waiting on a pinned, or otherwise unusable, buffer.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_gem.c

@@ -1595,18 +1595,32 @@ i915_gem_object_get_fence_reg(struct drm_gem_object *obj, bool write)
 
 	/* None available, try to steal one or wait for a user to finish */
 	if (i == dev_priv->num_fence_regs) {
+		uint32_t seqno = dev_priv->mm.next_gem_seqno;
 		loff_t offset;
 
 		if (avail == 0)
 			return -ENOMEM;
 
-		/* Could try to use LRU here instead... */
 		for (i = dev_priv->fence_reg_start;
 		     i < dev_priv->num_fence_regs; i++) {
+			uint32_t this_seqno;
+
 			reg = &dev_priv->fence_regs[i];
 			old_obj_priv = reg->obj->driver_private;
-			if (!old_obj_priv->pin_count)
+
+			if (old_obj_priv->pin_count)
+				continue;
+
+			/* i915 uses fences for GPU access to tiled buffers */
+			if (IS_I965G(dev) || !old_obj_priv->active)
 				break;
+
+			/* find the seqno of the first available fence */
+			this_seqno = old_obj_priv->last_rendering_seqno;
+			if (this_seqno != 0 &&
+			    reg->obj->write_domain == 0 &&
+			    i915_seqno_passed(seqno, this_seqno))
+				seqno = this_seqno;
 		}
 
 		/*
@@ -1614,15 +1628,25 @@ i915_gem_object_get_fence_reg(struct drm_gem_object *obj, bool write)
 		 * objects to finish before trying again.
 		 */
 		if (i == dev_priv->num_fence_regs) {
-			ret = i915_gem_object_set_to_gtt_domain(reg->obj, 0);
-			if (ret) {
-				WARN(ret != -ERESTARTSYS,
-				     "switch to GTT domain failed: %d\n", ret);
-				return ret;
+			if (seqno == dev_priv->mm.next_gem_seqno) {
+				i915_gem_flush(dev,
+					       I915_GEM_GPU_DOMAINS,
+					       I915_GEM_GPU_DOMAINS);
+				seqno = i915_add_request(dev,
+							 I915_GEM_GPU_DOMAINS);
+				if (seqno == 0)
+					return -ENOMEM;
 			}
+
+			ret = i915_wait_request(dev, seqno);
+			if (ret)
+				return ret;
 			goto try_again;
 		}
 
+		BUG_ON(old_obj_priv->active ||
+		       (reg->obj->write_domain & I915_GEM_GPU_DOMAINS));
+
 		/*
 		 * Zap this virtual mapping so we can set up a fence again
 		 * for this object next time we need it.