It’s your choice:
Anything before v4.5.x, you can use:
mmcroot=/dev/mmcblk0p1 ro
But on this platform after v4.5.3:
mmcroot=/dev/mmcblk2p1 ro
Here’s one of my previous write up’s on this issue: V4.5.x+ mmc re-numbering changes
By using mmcroot=PARTUUID=123456cc-01 ro (which u-boot detects the 123456cc-01 part) you don’t have to worry about random mmc node name changes…
Regards,