On 15/07/2020 12:50, Chris Wilson wrote:
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@xxxxxxxxxxxxxxxxxx>
---
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 0854b1552bc1..6737b5615c0c 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) */
Or when fence context wraps shock horror.
}
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;
cpmxchg suggests this needs to be atomic, however above the check for
equality comes from a separate read.
Since there is a lookup code path under the spinlock, perhaps the
unlocked lookup could just fail, and then locked lookup could re-assign
the timeline without the need for cmpxchg?
Regards,
Tvrtko
+ }
+#endif
+ }
BUILD_BUG_ON(offsetof(typeof(*it), node));
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