On pointer type and the return type of ioremap

Hi all, I'm writing my little module now a days and want to ioremap some space. I learnt from LDD3 that "Therefore, generic memory addresses in the kernel are usually unsigned long"(ch11). So I declared a pointer like "usigned long p2dev;" But when I feed it with ioremap, it yields a warning: warning: assignment makes integer from pointer without a cast [enabled by default] Do I really have to add a cast? What is the reason that while "generic pointers are unsigned long" but the return type of ioremap is void *? Thanks in advance. -- Cheers, Grissiom