[PATCH 30/31] drm/i915: Make the stale cached active node available for any timeline

[Chris Wilson]

Rather than require the next timeline after idling to match the MRU
before idling, reset the index on the node and allow it to match the
first request. However, this requires cmpxchg(u64) and so is not trivial
on 32b, so for compatibility we just fallback to keeping the cached node
pointing to the MRU timeline.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_active.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index e1e7f1913c2a..b521494515d7 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -157,6 +157,10 @@ __active_retire(struct i915_active *ref)
 		rb_link_node(&ref->cache->node, NULL, &ref->tree.rb_node);
 		rb_insert_color(&ref->cache->node, &ref->tree);
 		GEM_BUG_ON(ref->tree.rb_node != &ref->cache->node);
+
+		/* Make the cached node available for reuse with any timeline */
+		if (IS_ENABLED(CONFIG_64BIT))
+			ref->cache->timeline = 0; /* needs cmpxchg(u64) */
 	}
 
 	spin_unlock_irqrestore(&ref->tree_lock, flags);
@@ -235,9 +239,22 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
 {
 	struct active_node *it;
 
+	GEM_BUG_ON(idx == 0); /* 0 is the unordered timeline, rsvd for cache */
+
 	it = READ_ONCE(ref->cache);
-	if (it && it->timeline == idx)
-		return it;
+	if (it) {
+		u64 cached = READ_ONCE(it->timeline);
+
+		if (cached == idx)
+			return it;
+
+#ifdef CONFIG_64BIT /* for cmpxchg(u64) */
+		if (!cached && !cmpxchg(&it->timeline, 0, idx)) {
+			GEM_BUG_ON(i915_active_fence_isset(&it->base));
+			return it;
+		}
+#endif
+	}
 
 	BUILD_BUG_ON(offsetof(typeof(*it), node));
 
-- 
2.20.1