eg:
1. mknod /dev/test b 9 512(any num more then 512) 
2. open /dev/test return ENXIO

what we expect:
1. mknod /dev/test b 9 512(or other numbers)
2.open /dev/test successfully

When I compared the source codes of kernel 4.9 and kernel 3.10, I found this :

static int __init md_init(void)
{
	...

	blk_register_region(MKDEV(MD_MAJOR, 0), 512, THIS_MODULE,
			    md_probe, NULL, NULL);// I don't know the reason of this change . 1UL<<MINORBITS, ===> 512
	blk_register_region(MKDEV(mdp_major, 0), 1UL<<MINORBITS, THIS_MODULE,
			    md_probe, NULL, NULL);

       ...

	md_geninit();
	return 0;

Why did we change the input parament to 512 ? any one can give me a favor?

Thanks and Regards