[PATCH 2/3] dma-buf: sort fences in dma_fence_unwrap_merge

[Christian König]

The merge function initially handled only individual fences and
arrays which in turn were created by the merge function. This allowed
to create the new array by a simple merge sort based on the fence
context number.

The problem is now that since the addition of timeline sync objects
userspace can create chain containers in basically any fence context
order.

If those are merged together it can happen that we create really
large arrays since the merge sort algorithm doesn't work any more.

So put an insert sort behind the merge sort which kicks in when the
input fences are not in the expected order. This isn't as efficient
as a heap sort, but has better properties for the most common use
case.

Signed-off-by: Christian König <christian.koenig at amd.com>
---
 drivers/dma-buf/dma-fence-unwrap.c | 39 ++++++++++++++++++++++++++----
 1 file changed, 34 insertions(+), 5 deletions(-)

diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c
index 628af51c81af..d9aa280d9ff6 100644
--- a/drivers/dma-buf/dma-fence-unwrap.c
+++ b/drivers/dma-buf/dma-fence-unwrap.c
@@ -106,7 +106,7 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
 		fences[i] = dma_fence_unwrap_first(fences[i], &iter[i]);
 
 	count = 0;
-	do {
+	while (true) {
 		unsigned int sel;
 
 restart:
@@ -144,11 +144,40 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
 			}
 		}
 
-		if (tmp) {
-			array[count++] = dma_fence_get(tmp);
-			fences[sel] = dma_fence_unwrap_next(&iter[sel]);
+		if (!tmp)
+			break;
+
+		/*
+		 * We could use a binary search here, but since the assumption
+		 * is that the main input are already sorted dma_fence_arrays
+		 * just looking from end has a higher chance of finding the
+		 * right location on the first try
+		 */
+
+		for (i = count; i--;) {
+			if (likely(array[i]->context < tmp->context))
+				break;
+
+			if (array[i]->context == tmp->context) {
+				if (dma_fence_is_later(tmp, array[i])) {
+					dma_fence_put(array[i]);
+					array[i] = dma_fence_get(tmp);
+				}
+				fences[sel] = dma_fence_unwrap_next(&iter[sel]);
+				goto restart;
+			}
 		}
-	} while (tmp);
+
+		++i;
+		/*
+		 * Make room for the fence, this should be a nop most of the
+		 * time.
+		 */
+		memcpy(&array[i + 1], &array[i], (count - i) * sizeof(*array));
+		array[i] = dma_fence_get(tmp);
+		fences[sel] = dma_fence_unwrap_next(&iter[sel]);
+		count++;
+	};
 
 	if (count == 0) {
 		tmp = dma_fence_allocate_private_stub(ktime_get());
-- 
2.34.1