You only need "do" support when there's inversion. So if the WH-word/phrase is the subject of the question (and so there's no inversion), then you don't need "do":
Which computer calculates faster?
On the other hand, if the QU phrase is the object (and there is inversion) then you need "do":
What did you drink with your meal?
Which computer did you buy?
Alternatively, if there's already an auxiliary verb (including a modal), you don't need add another one with inversion (and indeed, inversion or not, you can't usually use "do" alongside another modal/auxiliary):
Which computer can I buy?
When will you finish by?
An exception that can occur, in questions just like in ordinary statements, is that the auxiliary "do" can be used to mark a contrast. For example:
Speaker A: Mr Scarface didn't steal the car.
Speaker B: Well who did steal the car, then?
Another exception is so-called in situ questions, where you ask a question for confirmation/out of surprise, and where there's no inversion even though ordinarily there would have been. In this case, again because there's no inversion, you don't use do support:
You bought which computer? (implies: "I'm surprised you bought that computer!", or "please confirm again which computer you bought because I didn't hear you properly")