[PATCH] drm/syncobj: Deal with signalled fences in transfer.

[Bas Nieuwenhuizen]

See the comments in the code. Basically if the seqno is already
signalled then we get a NULL fence. If we then put the NULL fence
in a binary syncobj it counts as unsignalled, making that syncobj
pretty much useless for all expected uses.

Not 100% sure about the transfer to a timeline syncobj but I
believe it is needed there too, as AFAICT the add_point function
assumes the fence isn't NULL.

Fixes: ea569910cbab ("drm/syncobj: add transition iotcls between binary and timeline v2")
Cc: stable at vger.kernel.org
Signed-off-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
---
 drivers/gpu/drm/drm_syncobj.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index fdd2ec87cdd1..eb28a40400d2 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -861,6 +861,19 @@ static int drm_syncobj_transfer_to_timeline(struct drm_file *file_private,
 				     &fence);
 	if (ret)
 		goto err;
+
+	/* If the requested seqno is already signaled drm_syncobj_find_fence may
+	 * return a NULL fence. To make sure the recipient gets signalled, use
+	 * a new fence instead.
+	 */
+	if (!fence) {
+		fence = dma_fence_allocate_private_stub();
+		if (!fence) {
+			ret = -ENOMEM;
+			goto err;
+		}
+	}
+
 	chain = kzalloc(sizeof(struct dma_fence_chain), GFP_KERNEL);
 	if (!chain) {
 		ret = -ENOMEM;
@@ -890,6 +903,19 @@ drm_syncobj_transfer_to_binary(struct drm_file *file_private,
 				     args->src_point, args->flags, &fence);
 	if (ret)
 		goto err;
+
+	/* If the requested seqno is already signaled drm_syncobj_find_fence may
+	 * return a NULL fence. To make sure the recipient gets signalled, use
+	 * a new fence instead.
+	 */
+	if (!fence) {
+		fence = dma_fence_allocate_private_stub();
+		if (!fence) {
+			ret = -ENOMEM;
+			goto err;
+		}
+	}
+
 	drm_syncobj_replace_fence(binary_syncobj, fence);
 	dma_fence_put(fence);
 err:
-- 
2.34.1