drm/amdgpu: no updates shouldn't cause vm flush v2

v2 (chk): split fix from original patch
Signed-off-by: monk.liu <monk.liu@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c

@@ -200,13 +200,15 @@ void amdgpu_vm_flush(struct amdgpu_ring *ring,
 {
 	uint64_t pd_addr = amdgpu_bo_gpu_offset(vm->page_directory);
 	struct amdgpu_vm_id *vm_id = &vm->ids[ring->idx];
+	struct amdgpu_fence *flushed_updates = vm_id->flushed_updates;
 
-	if (pd_addr != vm_id->pd_gpu_addr || !vm_id->flushed_updates ||
-	    amdgpu_fence_is_earlier(vm_id->flushed_updates, updates)) {
+	if (pd_addr != vm_id->pd_gpu_addr || !flushed_updates ||
+	    (updates && amdgpu_fence_is_earlier(flushed_updates, updates))) {
 
 		trace_amdgpu_vm_flush(pd_addr, ring->idx, vm_id->id);
-		amdgpu_fence_unref(&vm_id->flushed_updates);
-		vm_id->flushed_updates = amdgpu_fence_ref(updates);
+		vm_id->flushed_updates = amdgpu_fence_ref(
+			amdgpu_fence_later(flushed_updates, updates));
+		amdgpu_fence_unref(&flushed_updates);
 		vm_id->pd_gpu_addr = pd_addr;
 		amdgpu_ring_emit_vm_flush(ring, vm_id->id, vm_id->pd_gpu_addr);
 	}