dma-buf: some dma_fence_chain improvements

The callback and the irq work are never used at the same
time. Putting them into an union saves us 24 bytes and
makes the structure only 120 bytes in size.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611120301.10595-2-christian.koenig@amd.com

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

@@ -137,6 +137,7 @@ static void dma_fence_chain_cb(struct dma_fence *f, struct dma_fence_cb *cb)
 	struct dma_fence_chain *chain;
 
 	chain = container_of(cb, typeof(*chain), cb);
+	init_irq_work(&chain->work, dma_fence_chain_irq_work);
 	irq_work_queue(&chain->work);
 	dma_fence_put(f);
 }
@@ -239,7 +240,6 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
 	rcu_assign_pointer(chain->prev, prev);
 	chain->fence = fence;
 	chain->prev_seqno = 0;
-	init_irq_work(&chain->work, dma_fence_chain_irq_work);
 
 	/* Try to reuse the context of the previous chain node. */
 	if (prev_chain && __dma_fence_is_later(seqno, prev->seqno, prev->ops)) {

include/linux/dma-fence-chain.h

@@ -16,21 +16,36 @@
 /**
  * struct dma_fence_chain - fence to represent an node of a fence chain
  * @base: fence base class
- * @lock: spinlock for fence handling
  * @prev: previous fence of the chain
  * @prev_seqno: original previous seqno before garbage collection
  * @fence: encapsulated fence
- * @cb: callback structure for signaling
- * @work: irq work item for signaling
+ * @lock: spinlock for fence handling
  */
 struct dma_fence_chain {
 	struct dma_fence base;
-	spinlock_t lock;
 	struct dma_fence __rcu *prev;
 	u64 prev_seqno;
 	struct dma_fence *fence;
-	struct dma_fence_cb cb;
-	struct irq_work work;
+	union {
+		/**
+		 * @cb: callback for signaling
+		 *
+		 * This is used to add the callback for signaling the
+		 * complection of the fence chain. Never used at the same time
+		 * as the irq work.
+		 */
+		struct dma_fence_cb cb;
+
+		/**
+		 * @work: irq work item for signaling
+		 *
+		 * Irq work structure to allow us to add the callback without
+		 * running into lock inversion. Never used at the same time as
+		 * the callback.
+		 */
+		struct irq_work work;
+	};
+	spinlock_t lock;
 };
 
 extern const struct dma_fence_ops dma_fence_chain_ops;