dma_map_page and dma_map_single

Hi ,

I was looking at tx_map ( igb-3.3.6  ) in igb driver. I could see that
dma_map_single is used for first buffer and dma_map_page is used for
the fragments.

What is the real difference  between these api ?  we can replace
dma_map_single with dma_map_page. then  why  we used dma_map_single
here ?.

Thanks,
Ratheesh