dma-buf: Add dma_fence_array_for_each (v2)

Add a helper to iterate over all fences in a dma_fence_array object.

v2 (Jason Ekstrand)
- Return NULL from dma_fence_array_first if head == NULL. This matches
the iterator behavior of dma_fence_chain_for_each in that it iterates
zero times if head == NULL.
- Return NULL from dma_fence_array_next if index > array->num_fences.

Signed-off-by: Jason Ekstrand <[e-mail]>
Reviewed-by: Jason Ekstrand <[e-mail]>
Reviewed-by: Christian König <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Maarten Lankhorst <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Signed-off-by: Christian König <[e-mail]>

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

···
 	return true;
 }
 EXPORT_SYMBOL(dma_fence_match_context);
+
+struct dma_fence *dma_fence_array_first(struct dma_fence *head)
+{
+	struct dma_fence_array *array;
+
+	if (!head)
+		return NULL;
+
+	array = to_dma_fence_array(head);
+	if (!array)
+		return head;
+
+	return array->fences[0];
+}
+EXPORT_SYMBOL(dma_fence_array_first);
+
+struct dma_fence *dma_fence_array_next(struct dma_fence *head,
+				       unsigned int index)
+{
+	struct dma_fence_array *array = to_dma_fence_array(head);
+
+	if (!array || index >= array->num_fences)
+		return NULL;
+
+	return array->fences[index];
+}
+EXPORT_SYMBOL(dma_fence_array_next);

include/linux/dma-fence-array.h

···
 	return container_of(fence, struct dma_fence_array, base);
 }
 
+/**
+ * dma_fence_array_for_each - iterate over all fences in array
+ * @fence: current fence
+ * @index: index into the array
+ * @head: potential dma_fence_array object
+ *
+ * Test if @array is a dma_fence_array object and if yes iterate over all fences
+ * in the array. If not just iterate over the fence in @array itself.
+ */
+#define dma_fence_array_for_each(fence, index, head)			\
+	for (index = 0, fence = dma_fence_array_first(head); fence;	\
+	     ++(index), fence = dma_fence_array_next(head, index))
+
 struct dma_fence_array *dma_fence_array_create(int num_fences,
 					       struct dma_fence **fences,
 					       u64 context, unsigned seqno,
 					       bool signal_on_any);
 
 bool dma_fence_match_context(struct dma_fence *fence, u64 context);
+
+struct dma_fence *dma_fence_array_first(struct dma_fence *head);
+struct dma_fence *dma_fence_array_next(struct dma_fence *head,
+				       unsigned int index);
 
 #endif /* __LINUX_DMA_FENCE_ARRAY_H */