Specifying an iomem region on the kernel command line is done as
The name is defined by the driver that will use the I/O area and the
file is the name of the host file that will be mapped in.
The driver finds its I/O area by calling find_iomem
unsigned long find_iomem(char *driver, unsigned long *len_out)
which returns the address of the beginning of the area and passes its
length out in len_out. The driver argument is the name of the I/O
area as defined on the command line.
Once the driver has located its I/O area, it can do whatever it wants
with it, including
providing a file interface to it
allowing processes to mmap it into their own address spaces
use it as a backing store for some other kind of interface, such as an
X server using it as a framebuffer and providing an X protocol
interface to it