I was thinking of
   int bus_dmatag_subregion(bus_dma_tag_t dmat,
            bus_addr_t min_addr,
            bus_addr_t max_addr,
            bus_dma_tag_t *new_dmat);
   int bus_dmatag_destroy(bus_dma_tag_t);
   
   and then use the new dmatag with existing API.  Minimal change and no  
   massive changes to existing drivers. 


i like this idea.  then if the device has children, they also get
to inherit this limited tag...


.mrg.