[PATCH] dma-buf/sync_file: only enable fence signalling during wait

[Gustavo Padovan <gustavo@padovan.org>]

From: Gustavo Padovan <gustavo.padovan@collabora.co.uk>

Signalling doesn't need to be enabled at sync_file creation, it is only
required if userspace waiting the fence to signal through poll().

Thus we delay fence_add_callback() until poll is called. It only adds the
callback the first time poll() is called. This avoid re-adding the same
callback multiple times.

v2: rebase and update to work with new fence support for sync_file

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
---
This patch applies on top of my latest sync_file changes to support
fence_array: https://lkml.org/lkml/2016/7/4/534

 drivers/dma-buf/sync_file.c | 23 ++++++++++++++---------
 include/linux/sync_file.h   |  2 ++
 2 files changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index 61a687c..1db4a64 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -86,8 +86,6 @@ struct sync_file *sync_file_create(struct fence *fence)
 		 fence->ops->get_timeline_name(fence), fence->context,
 		 fence->seqno);
 
-	fence_add_callback(fence, &sync_file->cb, fence_check_cb_func);
-
 	return sync_file;
 }
 EXPORT_SYMBOL(sync_file_create);
@@ -269,9 +267,6 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 		goto err;
 	}
 
-	fence_add_callback(sync_file->fence, &sync_file->cb,
-			   fence_check_cb_func);
-
 	strlcpy(sync_file->name, name, sizeof(sync_file->name));
 	return sync_file;
 
@@ -286,7 +281,6 @@ static void sync_file_free(struct kref *kref)
 	struct sync_file *sync_file = container_of(kref, struct sync_file,
 						     kref);
 
-	fence_remove_callback(sync_file->fence, &sync_file->cb);
 	fence_put(sync_file->fence);
 	kfree(sync_file);
 }
@@ -306,13 +300,24 @@ static unsigned int sync_file_poll(struct file *file, poll_table *wait)
 
 	poll_wait(file, &sync_file->wq, wait);
 
+	if (!sync_file->enabled) {
+		fence_add_callback(sync_file->fence, &sync_file->cb,
+				   fence_check_cb_func);
+		sync_file->enabled = true;
+	}
+
 	status = fence_is_signaled(sync_file->fence);
 
-	if (status)
-		return POLLIN;
+	if (!status)
+		return 0;
+
+	fence_remove_callback(sync_file->fence, &sync_file->cb);
+	sync_file->enabled = false;
+
 	if (status < 0)
 		return POLLERR;
-	return 0;
+
+	return POLLIN;
 }
 
 static long sync_file_ioctl_merge(struct sync_file *sync_file,
diff --git a/include/linux/sync_file.h b/include/linux/sync_file.h
index f7de5a0..16cbafd 100644
--- a/include/linux/sync_file.h
+++ b/include/linux/sync_file.h
@@ -28,6 +28,7 @@
  * @name:		name of sync_file.  Useful for debugging
  * @sync_file_list:	membership in global file list
  * @wq:			wait queue for fence signaling
+ * @enabled:		wether fence signal is enabled or not
  * @fence:		fence with the fences in the sync_file
  * @cb:			fence callback information
  */
@@ -40,6 +41,7 @@ struct sync_file {
 #endif
 
 	wait_queue_head_t	wq;
+	bool			enabled;
 
 	struct fence		*fence;
 	struct fence_cb cb;
-- 
2.5.5