[PATCH v4 5/5] dma-buf/sync_file: only enable fence signalling on poll()

[Gustavo Padovan]

From: Gustavo Padovan <gustavo.pado...@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

v3: use atomic operation to set enabled and protect fence_add_callback()

Signed-off-by: Gustavo Padovan <gustavo.pado...@collabora.co.uk>
---
 drivers/dma-buf/sync_file.c | 22 +++++++++++++---------
 include/linux/sync_file.h   |  2 ++
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index 61a687c..240ef22 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 +281,8 @@ 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);
+       if (atomic_read(&sync_file->enabled))
+               fence_remove_callback(sync_file->fence, &sync_file->cb);
        fence_put(sync_file->fence);
        kfree(sync_file);
 }
@@ -306,13 +302,21 @@ static unsigned int sync_file_poll(struct file *file, 
poll_table *wait)
 
        poll_wait(file, &sync_file->wq, wait);
 
+       if (!atomic_xchg(&sync_file->enabled, 1)) {
+               if (fence_add_callback(sync_file->fence, &sync_file->cb,
+                                  fence_check_cb_func) < 0)
+                       wake_up_all(&sync_file->wq);
+       }
+
        status = fence_is_signaled(sync_file->fence);
 
-       if (status)
-               return POLLIN;
+       if (!status)
+               return 0;
+
        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..4ced13b 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;
+       atomic_t                enabled;
 
        struct fence            *fence;
        struct fence_cb cb;
-- 
2.5.5