[PATCH] sync_file: Return reasonable timestamp when merging signaled fences

[Michael Yang <michael.yang@imgtec.com>]

If all the sync points were signaled in both fences a and b,
there was only one sync point in merged fence which is a_fence[0].
The Fence structure in android framework might be confused about
timestamp if there were any sync points which were signaled after
a_fence[0]. It might be more reasonable to use timestamp of last signaled
sync point to represent the merged fence.
The issue can be found from EGL extension ANDROID_get_frame_timestamps.
Sometimes the return value of EGL_READS_DONE_TIME_ANDROID is head of
the return value of EGL_RENDERING_COMPLETE_TIME_ANDROID.
That means display/composition had been completed before rendering
was completed that is incorrect.

Some discussion can be found at:
https://android-review.googlesource.com/c/kernel/common/+/907009

Signed-off-by: Michael Yang <michael.yang@imgtec.com>
---
Hi,
I didn't get response since I previously sent this a month ago.
Could someone have a chance to look at it please?
Thanks.
 drivers/dma-buf/sync_file.c | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index 4f6305c..d46bfe1 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -274,8 +274,29 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 	for (; i_b < b_num_fences; i_b++)
 		add_fence(fences, &i, b_fences[i_b]);
 
-	if (i == 0)
-		fences[i++] = dma_fence_get(a_fences[0]);
+	/* If all the sync pts were signaled, then adding the sync_pt who
+	 * was the last signaled to the fence.
+	 */
+	if (i == 0) {
+		struct dma_fence *last_signaled_sync_pt = a_fences[0];
+		int iter;
+
+		for (iter = 1; iter < a_num_fences; iter++) {
+			if (ktime_compare(last_signaled_sync_pt->timestamp,
+				a_fences[iter]->timestamp) < 0) {
+				last_signaled_sync_pt = a_fences[iter];
+			}
+		}
+
+		for (iter = 0; iter < b_num_fences; iter++) {
+			if (ktime_compare(last_signaled_sync_pt->timestamp,
+				b_fences[iter]->timestamp) < 0) {
+				last_signaled_sync_pt = b_fences[iter];
+			}
+		}
+
+		fences[i++] = dma_fence_get(last_signaled_sync_pt);
+	}
 
 	if (num_fences > i) {
 		nfences = krealloc(fences, i * sizeof(*fences),
-- 
2.7.4