Research RTC: which hardware RTC becomes rtc0 at startup?

[Marko Buršič]

When building a kernel the device driver configuration asks which rtc should a system use for NTP and startup synchronisation . By default this is rtc0.

 

The question is: In case there are more hardware RTCs in one embedded system ( in my case onboard RK808 and Dallas DS1307 over I2C), which one would be numbered  as rtc0? Threre are several RTC device drivers available and checked by default, some marked as M and other as '*' , supposing 'module' or 'driver' respectively.

Now if I have two hardware RTCs, I presume they should load as rtc0 and rtc1, but which part does numbering? I can't know this 'a priori' , I would like to use DS1307 for synchronisation.