( ) enough time ,I would have gone into that subject more deeply.怎么填?If I had还是Had I had?

I would have gone into that subject more deeply.
过去时态的将来完成时.
那么前面分句的虚拟语气,自然就是过去完成时.
if I had had enough time,
也可以倒装,
Had I had enough time