[PATCH 08/26] dma-buf: add dma_resv_get_singleton

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

Add a function to simplify getting a single fence for all the fences in
the dma_resv object.

Signed-off-by: Christian König <christian.koenig@amd.com>
---
 drivers/dma-buf/dma-resv.c | 50 ++++++++++++++++++++++++++++++++++++++
 include/linux/dma-resv.h   |  2 ++
 2 files changed, 52 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 0a69f4b7e6b5..f91ca023b550 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -34,6 +34,7 @@
  */
 
 #include <linux/dma-resv.h>
+#include <linux/dma-fence-array.h>
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/sched/mm.h>
@@ -721,6 +722,55 @@ int dma_resv_get_fences(struct dma_resv *obj, bool write,
 }
 EXPORT_SYMBOL_GPL(dma_resv_get_fences);
 
+/**
+ * dma_resv_get_singleton - Get a single fence for all the fences
+ * @obj: the reservation object
+ * @write: true if we should return all fences
+ * @fence: the resulting fence
+ *
+ * Get a single fence representing all the fences inside the resv object.
+ * Returns either 0 for success or -ENOMEM.
+ *
+ * Warning: This can't be used like this when adding the fence back to the resv
+ * object since that can lead to stack corruption when finalizing the
+ * dma_fence_array.
+ */
+int dma_resv_get_singleton(struct dma_resv *obj, bool write,
+			   struct dma_fence **fence)
+{
+	struct dma_fence_array *array;
+	struct dma_fence **fences;
+	unsigned count;
+	int r;
+
+	r = dma_resv_get_fences(obj, write, &count, &fences);
+        if (r)
+		return r;
+
+	if (count == 0) {
+		*fence = NULL;
+		return 0;
+	}
+
+	if (count == 1) {
+		*fence = fences[0];
+		kfree(fences);
+		return 0;
+	}
+
+	array = dma_fence_array_create(count, fences,
+				       dma_fence_context_alloc(1),
+				       1, false);
+	if (!array) {
+		kfree(fences);
+		return -ENOMEM;
+	}
+
+	*fence = &array->base;
+	return 0;
+}
+EXPORT_SYMBOL_GPL(dma_resv_get_singleton);
+
 /**
  * dma_resv_wait_timeout - Wait on reservation's objects
  * shared and/or exclusive fences.
diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
index 09b676b87c35..082f77b7bc63 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -440,6 +440,8 @@ void dma_resv_prune(struct dma_resv *obj);
 void dma_resv_prune_unlocked(struct dma_resv *obj);
 int dma_resv_get_fences(struct dma_resv *obj, bool write,
 			unsigned int *num_fences, struct dma_fence ***fences);
+int dma_resv_get_singleton(struct dma_resv *obj, bool write,
+			   struct dma_fence **fence);
 int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src);
 long dma_resv_wait_timeout(struct dma_resv *obj, bool wait_all, bool intr,
 			   unsigned long timeout);
-- 
2.25.1