[PATCH] dma-buf: avoid scheduling on fence status query v2

[Andres Rodriguez <andresx7@gmail.com>]

When a timeout of zero is specified, the caller is only interested in
the fence status.

In the current implementation, dma_fence_default_wait will always call
schedule_timeout() at least once for an unsignaled fence. This adds a
significant overhead to a fence status query.

Avoid this overhead by returning early if a zero timeout is specified.

v2: move early return after enable_signaling

Signed-off-by: Andres Rodriguez <andresx7@gmail.com>
---

 If I'm understanding correctly, I don't think we need to register the
 default wait callback. But if that isn't the case please let me know.

 This patch has the same perf improvements as v1.

 drivers/dma-buf/dma-fence.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 0918d3f..57da14c 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -402,6 +402,11 @@ dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)
 		}
 	}
 
+	if (!timeout) {
+		ret = 0;
+		goto out;
+	}
+
 	cb.base.func = dma_fence_default_wait_cb;
 	cb.task = current;
 	list_add(&cb.base.node, &fence->cb_list);
-- 
2.9.3