If there are 2 GPUs in a PC, one processing only graphics and the other dedicated to PhysX, what is the flow of the physics-related data?
Let's say we're simulating an explosion and the PhysX calculates exploded elements (position, size, number, whatnot) while graphics GPU renders them. Does CPU send input to PhysX GPU, gets the results and sends (some of) them to graphics GPU? Or is something like this possible: CPU sends input data to PhysX GPU and, after performing calculations, data from PhysX GPU goes straight to graphics GPU (omitting CPU altogether) since they're just used to render stuff anyway and CPU doesn't need to do anything with them?
Edit: Actually, as a bonus question, I'd like to know the same in case of just one GPU (used for both PhysX and graphics). Does the data flow to GPU, gets calculated, returns to CPU and then goes back to GPU for rendering or can it "stay" on GPU?