bus_dma question

The bus_dma man page is good. but it there documentation of how to 
extract the physical page addresses from a map? In other words, if I 
defien a tag for a device and then create a map using bus_dmamap_create, 
and then load it using bus_dmamap_load, what is the recommended method 
of extracting the list of addresses to feed into a DMA scatter_gather 
list? Looking at various drivers I'm getting more and more confused.. 
There is  a lot of home-grown s/g list
generation code out there.. is there a "right" way to do this?