Daniel Walker writes:

If you switch to the rtc do the shift and mult need to change?

You can't switch; any given CPU chip will have either the RTC or the
timebase but not both.

quoted

+       /* XXX this assumes clock->shift == 22 */
+       /* 4611686018 ~= 2^(20+64-22) / 1e9 */
+       t2x = (u64) clock->mult * 4611686018ULL;

It might make the code more readable if you put that constant into a
macro that gives it a name.

The comment already gives more information than some arbitrary name
would.

Paul.