drm/syncobj: use newly allocated stub fences

Allocate a new private stub fence in drm_syncobj_assign_null_handle,
instead of using a static stub fence.

When userspace creates a fence with DRM_SYNCOBJ_CREATE_SIGNALED or when
userspace signals a fence via DRM_IOCTL_SYNCOBJ_SIGNAL, the timestamp
obtained when the fence is exported and queried with SYNC_IOC_FILE_INFO
should match when the fence's status was changed from the perspective of
userspace, which is during the respective ioctl.

When a static stub fence started being used in by these ioctls, this
behavior changed. Instead, the timestamp returned by SYNC_IOC_FILE_INFO
became the first time anything used the static stub fence, which has no
meaning to userspace.

Signed-off-by: David Stevens <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]
Reviewed-by: Christian König <[e-mail]>
Signed-off-by: Christian König <[e-mail]>

drivers/dma-buf/dma-fence.c

···
 /**
  * dma_fence_get_stub - return a signaled fence
  *
- * Return a stub fence which is already signaled.
+ * Return a stub fence which is already signaled. The fence's
+ * timestamp corresponds to the first time after boot this
+ * function is called.
  */
 struct dma_fence *dma_fence_get_stub(void)
 {
···
 	return dma_fence_get(&dma_fence_stub);
 }
 EXPORT_SYMBOL(dma_fence_get_stub);
+
+/**
+ * dma_fence_allocate_private_stub - return a private, signaled fence
+ *
+ * Return a newly allocated and signaled stub fence.
+ */
+struct dma_fence *dma_fence_allocate_private_stub(void)
+{
+	struct dma_fence *fence;
+
+	fence = kzalloc(sizeof(*fence), GFP_KERNEL);
+	if (fence == NULL)
+		return ERR_PTR(-ENOMEM);
+
+	dma_fence_init(fence,
+		       &dma_fence_stub_ops,
+		       &dma_fence_stub_lock,
+		       0, 0);
+	dma_fence_signal(fence);
+
+	return fence;
+}
+EXPORT_SYMBOL(dma_fence_allocate_private_stub);
 
 /**
  * dma_fence_context_alloc - allocate an array of fence contexts

drivers/gpu/drm/drm_syncobj.c

···
  *
  * Assign a already signaled stub fence to the sync object.
  */
-static void drm_syncobj_assign_null_handle(struct drm_syncobj *syncobj)
+static int drm_syncobj_assign_null_handle(struct drm_syncobj *syncobj)
 {
-	struct dma_fence *fence = dma_fence_get_stub();
+	struct dma_fence *fence = dma_fence_allocate_private_stub();
+
+	if (IS_ERR(fence))
+		return PTR_ERR(fence);
 
 	drm_syncobj_replace_fence(syncobj, fence);
 	dma_fence_put(fence);
+	return 0;
 }
 
 /* 5s default for wait submission */
···
 int drm_syncobj_create(struct drm_syncobj **out_syncobj, uint32_t flags,
 		       struct dma_fence *fence)
 {
+	int ret;
 	struct drm_syncobj *syncobj;
 
 	syncobj = kzalloc(sizeof(struct drm_syncobj), GFP_KERNEL);
···
 	INIT_LIST_HEAD(&syncobj->cb_list);
 	spin_lock_init(&syncobj->lock);
 
-	if (flags & DRM_SYNCOBJ_CREATE_SIGNALED)
-		drm_syncobj_assign_null_handle(syncobj);
+	if (flags & DRM_SYNCOBJ_CREATE_SIGNALED) {
+		ret = drm_syncobj_assign_null_handle(syncobj);
+		if (ret < 0) {
+			drm_syncobj_put(syncobj);
+			return ret;
+		}
+	}
 
 	if (fence)
 		drm_syncobj_replace_fence(syncobj, fence);
···
 	if (ret < 0)
 		return ret;
 
-	for (i = 0; i < args->count_handles; i++)
-		drm_syncobj_assign_null_handle(syncobjs[i]);
+	for (i = 0; i < args->count_handles; i++) {
+		ret = drm_syncobj_assign_null_handle(syncobjs[i]);
+		if (ret < 0)
+			break;
+	}
 
 	drm_syncobj_array_free(syncobjs, args->count_handles);
 

include/linux/dma-fence.h

···
 }
 
 struct dma_fence *dma_fence_get_stub(void);
+struct dma_fence *dma_fence_allocate_private_stub(void);
 u64 dma_fence_context_alloc(unsigned num);
 
 #define DMA_FENCE_TRACE(f, fmt, args...) \