dma-buf: add dma_fence_get_stub

Extract of useful code from the timeline work. This provides a function
to return a stub or dummy fence which is always signaled.
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Link: https://patchwork.freedesktop.org/patch/265248/

drivers/dma-buf/dma-fence.c

@@ -30,13 +30,16 @@
 EXPORT_TRACEPOINT_SYMBOL(dma_fence_emit);
 EXPORT_TRACEPOINT_SYMBOL(dma_fence_enable_signal);
 
+static DEFINE_SPINLOCK(dma_fence_stub_lock);
+static struct dma_fence dma_fence_stub;
+
 /*
  * fence context counter: each execution context should have its own
  * fence context, this allows checking if fences belong to the same
  * context or not. One device can have multiple separate contexts,
  * and they're used if some engine can run independently of another.
  */
-static atomic64_t dma_fence_context_counter = ATOMIC64_INIT(0);
+static atomic64_t dma_fence_context_counter = ATOMIC64_INIT(1);
 
 /**
  * DOC: DMA fences overview
@@ -68,6 +71,37 @@ static atomic64_t dma_fence_context_counter = ATOMIC64_INIT(0);
  *   &dma_buf.resv pointer.
  */
 
+static const char *dma_fence_stub_get_name(struct dma_fence *fence)
+{
+        return "stub";
+}
+
+static const struct dma_fence_ops dma_fence_stub_ops = {
+	.get_driver_name = dma_fence_stub_get_name,
+	.get_timeline_name = dma_fence_stub_get_name,
+};
+
+/**
+ * dma_fence_get_stub - return a signaled fence
+ *
+ * Return a stub fence which is already signaled.
+ */
+struct dma_fence *dma_fence_get_stub(void)
+{
+	spin_lock(&dma_fence_stub_lock);
+	if (!dma_fence_stub.ops) {
+		dma_fence_init(&dma_fence_stub,
+			       &dma_fence_stub_ops,
+			       &dma_fence_stub_lock,
+			       0, 0);
+		dma_fence_signal_locked(&dma_fence_stub);
+	}
+	spin_unlock(&dma_fence_stub_lock);
+
+	return dma_fence_get(&dma_fence_stub);
+}
+EXPORT_SYMBOL(dma_fence_get_stub);
+
 /**
  * dma_fence_context_alloc - allocate an array of fence contexts
  * @num: amount of contexts to allocate

include/linux/dma-fence.h

@@ -541,6 +541,7 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr)
 	return ret < 0 ? ret : 0;
 }
 
+struct dma_fence *dma_fence_get_stub(void);
 u64 dma_fence_context_alloc(unsigned num);
 
 #define DMA_FENCE_TRACE(f, fmt, args...) \