DMA buffer sharing in 3.3

One question that is not very clear to me is what is considered DMA buffer. Could some one clarify which scheme is operational upon using DMA-BUF Api's.

Scheme 1:
device-mem -> Camera -> buffer-one-sysmem
buffer-one-sysmem -> CPU -> buffer-two-sysmem
buffer-two-sysmem -> Render -> device-mem

Scheme 2:
device-mem -> Camera -> buffer-common-sysmem
buffer-common-sysmem -> Render -> device-mem

Scheme 3:
device-mem -> Camera -> Render -> device-mem

I suspect scheme 2. Can someone comment.