I have a question about adding a device to ARM VM linux. When a device is added to the vmlinux, two functions, map_device () and map_ram () are invoked. I don't understand the purpose of map_ram(). 

In map_device(), the capacity to the device is used to set up the mapping between  a virtual address and the MMIO (of the device). Then,  in the map_ram (), a capacity to the virtual address is allocated. I am confused because the virtual address is already mapped to the mmio of the device. why do we need to allocate a capacity for the virtual address? Should the capacity be the same as the  one for the device?

Thanks

Peng