[PATCH 3/4] dma-buf: Warn about dma_fence_chain container rules

["Christian König" <ckoenig.leichtzumerken@gmail.com>]

Chaining of dma_fence_chain objects is only allowed through the prev
fence and not through the contained fence.

Warn about that when we create a dma_fence_chain.

Signed-off-by: Christian König <christian.koenig@amd.com>
---
 drivers/dma-buf/dma-fence-chain.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
index 1b4cb3e5cec9..fa33f6b7f77b 100644
--- a/drivers/dma-buf/dma-fence-chain.c
+++ b/drivers/dma-buf/dma-fence-chain.c
@@ -254,5 +254,13 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
 
 	dma_fence_init(&chain->base, &dma_fence_chain_ops,
 		       &chain->lock, context, seqno);
+
+	/* Chaining dma_fence_chain container together is only allowed through
+	 * the prev fence and not through the contained fence.
+	 *
+	 * The correct way of handling this is to flatten out the fence
+	 * structure into a dma_fence_array by the caller instead.
+	 */
+	WARN_ON(dma_fence_is_chain(fence));
 }
 EXPORT_SYMBOL(dma_fence_chain_init);
-- 
2.25.1