+ * which turns out to be N = 51 - SHIFT_HZ.
+ * This gives the result as a 0.64 fixed-point fraction.
+ * That value is reduced by an offset amounting to 1 xsec per
+ * 2^31 timebase ticks to avoid problems with time going backwards
+ * by 1 xsec when we do timer_recalc_offset due to losing the
+ * fractional xsec. That offset is equal to ppc_tb_freq/2^51
+ * since there are 2^20 xsec in a second.