Specifying an iomem region on the kernel command line is done as follows:

iomem=name,file

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