It's expensive; it spins for a millisecond to obtain one bit.

Suppose this is running on a quiescent system with no interrupts or other tasks running. I can see it degenerating into deterministic behavior. Suppose the RTC counter and the CPU clock are from the same master clock, and the code is executing cleanly to the clock from on-chip caches.

Ultimately, the question is: is there really one bit of entropy from each call to get_bit(), and under what conditions?