[PATCH 62/64] drm/i915: Keep userpointer bindings if seqcount is unchanged, v2.

[Maarten Lankhorst]

Instead of force unbinding and rebinding every time, we try to check
if our notifier seqcount is still correct when pages are bound. This
way we only rebind userptr when we need to, and prevent stalls.

Changes since v1:
- Missing mutex_unlock, reported by kbuild.

Reported-by: kernel test robot <lkp at intel.com>
Reported-by: Dan Carpenter <dan.carpenter at oracle.com>

Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
---
 drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 27 ++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
index 84c3e8dc1491..44fc0df89784 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -274,12 +274,33 @@ int i915_gem_object_userptr_submit_init(struct drm_i915_gem_object *obj)
 	if (ret)
 		return ret;
 
-	/* Make sure userptr is unbound for next attempt, so we don't use stale pages. */
-	ret = i915_gem_object_userptr_unbind(obj, false);
+	/* optimistically try to preserve current pages while unlocked */
+	if (i915_gem_object_has_pages(obj) &&
+	    !mmu_interval_check_retry(&obj->userptr.notifier,
+				      obj->userptr.notifier_seq)) {
+		spin_lock(&i915->mm.notifier_lock);
+		if (obj->userptr.pvec &&
+		    !mmu_interval_read_retry(&obj->userptr.notifier,
+					     obj->userptr.notifier_seq)) {
+			obj->userptr.page_ref++;
+
+			/* We can keep using the current binding, this is the fastpath */
+			ret = 1;
+		}
+		spin_unlock(&i915->mm.notifier_lock);
+	}
+
+	if (!ret) {
+		/* Make sure userptr is unbound for next attempt, so we don't use stale pages. */
+		ret = i915_gem_object_userptr_unbind(obj, false);
+	}
 	i915_gem_object_unlock(obj);
-	if (ret)
+	if (ret < 0)
 		return ret;
 
+	if (ret > 0)
+		return 0;
+
 	notifier_seq = mmu_interval_read_begin(&obj->userptr.notifier);
 
 	pvec = kvmalloc_array(num_pages, sizeof(struct page *), GFP_KERNEL);
-- 
2.30.0.rc1