[PATCH 1/3] ARM: OMAP2+: 32k-counter: Use hwmod lookup to check presence of 32k timer

[Russell King - ARM Linux]

On Thu, Apr 05, 2012 at 09:36:00AM +0000, Hiremath, Vaibhav wrote:
> There seems to be limitation for ARM architecture, it is restricted by
> sched_clock implementation present in "arch/arm/kernel/sched_clock.c".
> Natively, clocksource framework does support change in rate/frequency for
> registered timer, using,

Any kind of switching of timing sources introduces loss of time issues
by the mere fact that you can't instantly know the counter values of
both timing sources at precisely the same instant - because CPUs can
only do one thing at a time.  So any kind of repeated dynamic switching
_will_ result in a gradual loss of time - which will be indeterminant
as it depends on the frequency of the switching and the relative delta
between the two.

To put it another way - it is not possible to maintain high precision
and accuracy while dynamically switching your timing sources.

I'm not about to lift the restriction that there's only one source for
sched_clock() just for OMAP.  That'd require a lot of additional code -
it's not just about adjusting the multiplier and shift, you also have to
correct the returned ns value as well, which means synchronizing against
two counters.  (And as I've pointed out above, that's impossible to do
accurately.)