dma-buf: Warn about dma_fence_chain container rules v2

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.

v2: fix comment style

Signed-off-by: Christian König <[e-mail]>
Reviewed-by: Daniel Vetter <[e-mail]>
Reviewed-by: Thomas Hellström <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/dma-buf/dma-fence-chain.c

···
 
 	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);