[PATCH 1/2] dma-buf/dma-fence: Add dma_fence_init_noref()

[Rob Clark]

From: Rob Clark <robdclark at chromium.org>

Add a way to initialize a fence without touching the refcount.  This is
useful, for example, if the fence is embedded in a drm_sched_job.  In
this case the refcount will be initialized before the job is queued.
But the seqno of the hw_fence is not known until job_run().

Signed-off-by: Rob Clark <robdclark at chromium.org>
---
 drivers/dma-buf/dma-fence.c | 43 ++++++++++++++++++++++++++++---------
 include/linux/dma-fence.h   |  2 ++
 2 files changed, 35 insertions(+), 10 deletions(-)

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 74e36f6d05b0..97c05a465cb4 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -989,28 +989,27 @@ void dma_fence_describe(struct dma_fence *fence, struct seq_file *seq)
 EXPORT_SYMBOL(dma_fence_describe);
 
 /**
- * dma_fence_init - Initialize a custom fence.
+ * dma_fence_init_noref - Initialize a custom fence without initializing refcount.
  * @fence: the fence to initialize
  * @ops: the dma_fence_ops for operations on this fence
  * @lock: the irqsafe spinlock to use for locking this fence
  * @context: the execution context this fence is run on
  * @seqno: a linear increasing sequence number for this context
  *
- * Initializes an allocated fence, the caller doesn't have to keep its
- * refcount after committing with this fence, but it will need to hold a
- * refcount again if &dma_fence_ops.enable_signaling gets called.
- *
- * context and seqno are used for easy comparison between fences, allowing
- * to check which fence is later by simply using dma_fence_later().
+ * Like &dma_fence_init but does not initialize the refcount.  Suitable
+ * for cases where the fence is embedded in another struct which has it's
+ * refcount initialized before the fence is initialized.  Such as embedding
+ * in a &drm_sched_job, where the job is created before knowing the seqno
+ * of the hw_fence.
  */
 void
-dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
-	       spinlock_t *lock, u64 context, u64 seqno)
+dma_fence_init_noref(struct dma_fence *fence, const struct dma_fence_ops *ops,
+		     spinlock_t *lock, u64 context, u64 seqno)
 {
 	BUG_ON(!lock);
 	BUG_ON(!ops || !ops->get_driver_name || !ops->get_timeline_name);
+	BUG_ON(!kref_read(&fence->refcount));
 
-	kref_init(&fence->refcount);
 	fence->ops = ops;
 	INIT_LIST_HEAD(&fence->cb_list);
 	fence->lock = lock;
@@ -1021,4 +1020,28 @@ dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
 
 	trace_dma_fence_init(fence);
 }
+EXPORT_SYMBOL(dma_fence_init_noref);
+
+/**
+ * dma_fence_init - Initialize a custom fence.
+ * @fence: the fence to initialize
+ * @ops: the dma_fence_ops for operations on this fence
+ * @lock: the irqsafe spinlock to use for locking this fence
+ * @context: the execution context this fence is run on
+ * @seqno: a linear increasing sequence number for this context
+ *
+ * Initializes an allocated fence, the caller doesn't have to keep its
+ * refcount after committing with this fence, but it will need to hold a
+ * refcount again if &dma_fence_ops.enable_signaling gets called.
+ *
+ * context and seqno are used for easy comparison between fences, allowing
+ * to check which fence is later by simply using dma_fence_later().
+ */
+void
+dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
+	       spinlock_t *lock, u64 context, u64 seqno)
+{
+	kref_init(&fence->refcount);
+	dma_fence_init_noref(fence, ops, lock, context, seqno);
+}
 EXPORT_SYMBOL(dma_fence_init);
diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
index d54b595a0fe0..f617c78a2e0a 100644
--- a/include/linux/dma-fence.h
+++ b/include/linux/dma-fence.h
@@ -279,6 +279,8 @@ struct dma_fence_ops {
 	void (*set_deadline)(struct dma_fence *fence, ktime_t deadline);
 };
 
+void dma_fence_init_noref(struct dma_fence *fence, const struct dma_fence_ops *ops,
+			  spinlock_t *lock, u64 context, u64 seqno);
 void dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
 		    spinlock_t *lock, u64 context, u64 seqno);
 
-- 
2.39.2