mmap failed if address is not the start of GPIO group

Hi,

I amusing mmap to map the GPIO register so that I can monitor their setting like the input/output direction:

mmap_addr = mmap(NULL, mem_size, PROT_READ|PROT_WRITE, MAP_SHARED, gpio_fd,mem_start);
    if (mmap_addr == MAP_FAILED) {
            printf("gpio mmap() failed!\n");
            exit(1);
    }

If I set mem_start to the start of any of the GPIO groups, it works fine. Otherwise mmap() fails. For example

mem_start = 0x49052000 (GPIO group 3) mmap() works fine

mem_start = 0x49052034 (Group 3 CTRL register) mmap() fails.

Does anyone have any idea why it does that? I am very confused.

Platform: Omap3530, Linux

Thanks,

Max