Override timer interrupt

[Paul Mackerras]

Rune Torgersen writes:

> The main couse is that our main bus frequency cannort be divided into
> 1kHz evently by the decrementer.
> Main bus freq = 99532800 Hz.
> Decrementer then becomes 24883, which gives us 999991.9624485600nsec per
> jiffy.
> That is not a number easilly converted into time without drift.

That shouldn't be a problem any more with the CONFIG_GENERIC_TIME
stuff.  What happens is that update_wall_time will accumulate
clock->xtime_interval to xtime for every clock->cycle_interval
timebase ticks.  The clock->xtime_interval is in units of 2^-22
nanoseconds, so it is very accurate.  It is computed from
clock->cycle_interval and clock->mult in
clocksource_calculate_interval() (in include/linux/clocksource.h).

Paul.