Now this is just my own take of the film. I noticed that cobbs totem gets knocked off the sink just after he tries yusufs dream serum for the first time and from that moment on you never see it spin and STOP again in the whole film so I believe the rest of the film is cobbs dream in yusufs sleep chamber, from which he never also explains why cobs children are still as young as his memory of them at the end of the film.