dma-buf: add dma_fence_timestamp helper

When a fence signals there is a very small race window where the timestamp
isn't updated yet. sync_file solves this by busy waiting for the
timestamp to appear, but on other ocassions didn't handled this
correctly.

Provide a dma_fence_timestamp() helper function for this and use it in
all appropriate cases.

Another alternative would be to grab the spinlock when that happens.

v2 by teddy: add a wait parameter to wait for the timestamp to show up, in case
   the accurate timestamp is needed and/or the timestamp is not based on
   ktime (e.g. hw timestamp)
v3 chk: drop the parameter again for unified handling

Signed-off-by: Yunxiang Li <Yunxiang.Li@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 1774baa64f ("drm/scheduler: Change scheduled fence track v2")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
CC: stable@vger.kernel.org
Link: https://patchwork.freedesktop.org/patch/msgid/20230929104725.2358-1-christian.koenig@amd.com

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

@@ -76,16 +76,11 @@ struct dma_fence *__dma_fence_unwrap_merge(unsigned int num_fences,
 		dma_fence_unwrap_for_each(tmp, &iter[i], fences[i]) {
 			if (!dma_fence_is_signaled(tmp)) {
 				++count;
-			} else if (test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT,
-					    &tmp->flags)) {
-				if (ktime_after(tmp->timestamp, timestamp))
-					timestamp = tmp->timestamp;
 			} else {
-				/*
-				 * Use the current time if the fence is
-				 * currently signaling.
-				 */
-				timestamp = ktime_get();
+				ktime_t t = dma_fence_timestamp(tmp);
+
+				if (ktime_after(t, timestamp))
+					timestamp = t;
 			}
 		}
 	}

drivers/dma-buf/sync_file.c

@@ -268,13 +268,10 @@ static int sync_fill_fence_info(struct dma_fence *fence,
 		sizeof(info->driver_name));
 
 	info->status = dma_fence_get_status(fence);
-	while (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags) &&
-	       !test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags))
-		cpu_relax();
 	info->timestamp_ns =
-		test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags) ?
-		ktime_to_ns(fence->timestamp) :
-		ktime_set(0, 0);
+		dma_fence_is_signaled(fence) ?
+			ktime_to_ns(dma_fence_timestamp(fence)) :
+			ktime_set(0, 0);
 
 	return info->status;
 }

drivers/gpu/drm/scheduler/sched_main.c

@@ -929,7 +929,7 @@ drm_sched_get_cleanup_job(struct drm_gpu_scheduler *sched)
 
 		if (next) {
 			next->s_fence->scheduled.timestamp =
-				job->s_fence->finished.timestamp;
+				dma_fence_timestamp(&job->s_fence->finished);
 			/* start TO timer for next job */
 			drm_sched_start_timeout(sched);
 		}

include/linux/dma-fence.h

@@ -568,6 +568,25 @@ static inline void dma_fence_set_error(struct dma_fence *fence,
 	fence->error = error;
 }
 
+/**
+ * dma_fence_timestamp - helper to get the completion timestamp of a fence
+ * @fence: fence to get the timestamp from.
+ *
+ * After a fence is signaled the timestamp is updated with the signaling time,
+ * but setting the timestamp can race with tasks waiting for the signaling. This
+ * helper busy waits for the correct timestamp to appear.
+ */
+static inline ktime_t dma_fence_timestamp(struct dma_fence *fence)
+{
+	if (WARN_ON(!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)))
+		return ktime_get();
+
+	while (!test_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags))
+		cpu_relax();
+
+	return fence->timestamp;
+}
+
 signed long dma_fence_wait_timeout(struct dma_fence *,
 				   bool intr, signed long timeout);
 signed long dma_fence_wait_any_timeout(struct dma_fence **fences,