[PATCH 2/3] dma-buf: add dma_resv_get_singleton_rcu (v3)

[Jason Ekstrand]

Add a helper function to get a single fence representing
all fences in a dma_resv object.

This fence is either the only one in the object or all not
signaled fences of the object in a flatted out dma_fence_array.

v2 (Jason Ekstrand):
 - Take reference of fences both for creating the dma_fence_array and in
   the case where we return one fence.
 - Handle the case where dma_resv_get_list() returns NULL

v3 (Jason Ekstrand):
 - Add an _rcu suffix because it is read-only
 - Rewrite to use dma_resv_get_fences_rcu so it's RCU-safe
 - Add an EXPORT_SYMBOL_GPL declaration
 - Re-author the patch to Jason since very little is left of Christian
   König's original patch

Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
---
 drivers/dma-buf/dma-resv.c | 104 +++++++++++++++++++++++++++++++++++++
 include/linux/dma-resv.h   |   2 +
 2 files changed, 106 insertions(+)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
index 6ddbeb5dfbf65..5dd4c38bd9cb4 100644
--- a/drivers/dma-buf/dma-resv.c
+++ b/drivers/dma-buf/dma-resv.c
@@ -33,6 +33,8 @@
  */
 
 #include <linux/dma-resv.h>
+#include <linux/dma-fence-chain.h>
+#include <linux/dma-fence-array.h>
 #include <linux/export.h>
 #include <linux/mm.h>
 #include <linux/sched/mm.h>
@@ -49,6 +51,19 @@
  * write-side updates.
  */
 
+/**
+ * dma_fence_deep_dive_for_each - deep dive into the fence containers
+ * @fence: resulting fence
+ * @chain: variable for a dma_fence_chain
+ * @index: index into a dma_fence_array
+ * @head: starting point
+ *
+ * Helper to deep dive into the fence containers for flattening them.
+ */
+#define dma_fence_deep_dive_for_each(fence, chain, index, head)	\
+	dma_fence_chain_for_each(chain, head)			\
+		dma_fence_array_for_each(fence, index, chain)
+
 DEFINE_WD_CLASS(reservation_ww_class);
 EXPORT_SYMBOL(reservation_ww_class);
 
@@ -517,6 +532,95 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 }
 EXPORT_SYMBOL_GPL(dma_resv_get_fences_rcu);
 
+/**
+ * dma_resv_get_singleton - get a single fence for the dma_resv object
+ * @obj: the reservation object
+ * @extra: extra fence to add to the resulting array
+ * @result: resulting dma_fence
+ *
+ * Get a single fence representing all unsignaled fences in the dma_resv object
+ * plus the given extra fence. If we got only one fence return a new
+ * reference to that, otherwise return a dma_fence_array object.
+ *
+ * RETURNS
+ * Returns -NOMEM if allocations fail, zero otherwise.
+ */
+int dma_resv_get_singleton_rcu(struct dma_resv *obj, struct dma_fence **result)
+{
+	struct dma_fence **resv_fences, *fence, *chain, **fences;
+	struct dma_fence_array *array;
+	unsigned int num_resv_fences, num_fences;
+	unsigned int ret, i, j;
+
+	ret = dma_resv_get_fences_rcu(obj, NULL, &num_resv_fences, &resv_fences);
+	if (ret)
+		return ret;
+
+	num_fences = 0;
+	*result = NULL;
+
+	if (num_resv_fences == 0)
+		return 0;
+
+	for (i = 0; i < num_resv_fences; ++i) {
+		dma_fence_deep_dive_for_each(fence, chain, j, resv_fences[i]) {
+			if (dma_fence_is_signaled(fence))
+				continue;
+
+			*result = fence;
+			++num_fences;
+		}
+	}
+
+	if (num_fences <= 1) {
+		*result = dma_fence_get(*result);
+		goto put_resv_fences;
+	}
+
+	fences = kmalloc_array(num_fences, sizeof(struct dma_fence*),
+			       GFP_KERNEL);
+	if (!fences) {
+		*result = NULL;
+		ret = -ENOMEM;
+		goto put_resv_fences;
+	}
+
+	num_fences = 0;
+	for (i = 0; i < num_resv_fences; ++i) {
+		dma_fence_deep_dive_for_each(fence, chain, j, resv_fences[i]) {
+			if (!dma_fence_is_signaled(fence))
+				fences[num_fences++] = dma_fence_get(fence);
+		}
+	}
+
+	if (num_fences <= 1) {
+		*result = num_fences ? fences[0] : NULL;
+		kfree(fences);
+		goto put_resv_fences;
+	}
+
+	array = dma_fence_array_create(num_fences, fences,
+				       dma_fence_context_alloc(1),
+				       1, false);
+	if (array) {
+		*result = &array->base;
+	} else {
+		*result = NULL;
+		while (num_fences--)
+			dma_fence_put(fences[num_fences]);
+		kfree(fences);
+		ret = -ENOMEM;
+	}
+
+put_resv_fences:
+	while (num_resv_fences--)
+		dma_fence_put(resv_fences[num_resv_fences]);
+	kfree(resv_fences);
+
+	return ret;
+}
+EXPORT_SYMBOL_GPL(dma_resv_get_singleton_rcu);
+
 /**
  * dma_resv_wait_timeout_rcu - 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 d44a77e8a7e34..5f82894fed0b9 100644
--- a/include/linux/dma-resv.h
+++ b/include/linux/dma-resv.h
@@ -285,6 +285,8 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
 
 int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src);
 
+int dma_resv_get_singleton_rcu(struct dma_resv *obj, struct dma_fence **result);
+
 long dma_resv_wait_timeout_rcu(struct dma_resv *obj, bool wait_all, bool intr,
 			       unsigned long timeout);
 
-- 
2.29.2