drm/i915: Keep userpointer bindings if seqcount is unchanged, v2.

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@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-63-maarten.lankhorst@linux.intel.com

1 files 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 4afd08c1fe09..8ebdd252d6b8 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -281,12 +281,33 @@ 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);