From: David Mosberger <davidm@napali.hpl.hp.com>

Basically, what the patch does is provide two hooks such that platforms
(and subplatforms) can provide time-interpolation in a way that guarantees
that two causally related gettimeofday() calls will never see time going
backwards (unless there is a settimeofday() call, of course).

There is some evidence that the current scheme does work: we use it on ia64
both for cycle-counter-based interpolation and the SGI folks use it with a
chipset-based high-performance counter.


It seems like enough platforms do this sort of thing to provide _some_
support in the core, especially because it's rather tricky to guarantee
that time never goes backwards (short of a settimeofday, of course).

This patch is based on something Jes Sorensen wrote for the SGI Itanium 2
platform (which has a chipset-internal high-res clock).  I adapted it so it
can be used for cycle-counter interpolation also.  The net effect is that
"last_time_offset" can be removed completely from the kernel.

The basic idea behind the patch is simply: every time you advance xtime by
N nanoseconds, you call update_wall_time_hook(NSEC).  Every time the time
gets set (i.e., discontinuity is OK), reset_wall_time_hook() is called.