[PATCH] dma-buf/sync-file: Avoid enable fence signaling if poll(.timeout=0)

[Chris Wilson <chris@chris-wilson.co.uk>]

If we being polled with a timeout of zero, a nonblocking busy query,
we don't need to install any fence callbacks as we will not be waiting.
As we only install the callback once, the overhead comes from the atomic
bit test that also causes serialisation between threads.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: Gustavo Padovan <gustavo@padovan.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
---
 drivers/dma-buf/sync_file.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index 486d29c1a830..abb5fdab75fd 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -306,7 +306,8 @@ static unsigned int sync_file_poll(struct file *file, poll_table *wait)
 
 	poll_wait(file, &sync_file->wq, wait);
 
-	if (!test_and_set_bit(POLL_ENABLED, &sync_file->fence->flags)) {
+	if (!poll_does_not_wait(wait) &&
+	    !test_and_set_bit(POLL_ENABLED, &sync_file->fence->flags)) {
 		if (fence_add_callback(sync_file->fence, &sync_file->cb,
 				       fence_check_cb_func) < 0)
 			wake_up_all(&sync_file->wq);
-- 
2.9.3

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