Why need seprate IOREMAP and KMAP, when they both map cpu physical address space to kernel virtual address

In case of memory mapped IO : Both Memory mapped IO and RAM are in same CPU physical address space. When we map any higmem memory we use kmap and when we need to map memory mapped IO we use ioremap So my question is why we need to separate functions to get kernel virtual address, since both (memory mapped IO and RAM) are in same physical address space . Why cant we have single function, as both are creating virtual address for some CPU physical address