[RFC PATCH 071/162] drm/i915: Keep userpointer bindings if seqcount is unchanged, v2.

[Matthew Auld]

From: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>

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.

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>
Cc: Thomas Hellström <thomas.hellstrom 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 fb4bc30fbd9a..d1ecc31b5e90 100644
--- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
+++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
@@ -275,12 +275,33 @@ 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);
-- 
2.26.2