[Intel-gfx] [PATCH 5/7] 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>
Reviewed-by: Thomas Hellström <thomas.hellstrom at intel.com>
---
 drivers/gpu/drm/i915/i915_active.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index b9bd5578ff54..03b246cb8466 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);
@@ -235,6 +239,8 @@ 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 */
+
 	/*
 	 * We track the most recently used timeline to skip a rbtree search
 	 * for the common case, under typical loads we never need the rbtree
@@ -243,8 +249,17 @@ static struct active_node *__active_lookup(struct i915_active *ref, u64 idx)
 	 * current timeline.
 	 */
 	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))
+			return it;
+#endif
+	}
 
 	BUILD_BUG_ON(offsetof(typeof(*it), node));
 
-- 
2.20.1