Re: [PATCHv2 0/3] clocksource: add db8500 PRCMU timer

From: Santosh Shilimkar
Date: Thu Jun 02 2011 - 09:16:44 EST

On 6/2/2011 6:34 PM, Russell King - ARM Linux wrote:
On Thu, Jun 02, 2011 at 06:27:22PM +0530, Santosh Shilimkar wrote:
Earlier this year at ELC SFO, I had a discussion with
John and Thomas on how to have a high-resolution clock-source
and a low-resolution clock-source working together to cover
the low power scenario and still manage to get the highest
timer resolution.
The idea was to do dynamic switching of clock-source
which initially looked simple. Here the idea was to
have this working for suspend and as well as cupidle.

I don't think you can do this, because you'll lose precision whenever you
switch from the high resolution clocksource to the low resolution

While you can quickly update the current time of day from the highres one,
the lowres may or may not be a long way from its next tick rollover. So
you lose the precision whenever you switch.

However, over a suspend/resume cycle, the precision loss is normally very
small compared to the time which you have been suspended, so the %age
error also becomes very small.

With cpuidle, it becomes a completely different matter. Here, the %age
error is much larger because of the smaller sleep periods, and chances
are we can't wait for the low-res timer to change.

So if you're using cpuidle, you really need a clocksource which runs
continuously, even in whatever states cpuidle drops you into.

According to John, that's what the meta-clock source will for.
It will be continuous and will make use of underneath high res,
low res clock-sources based on the availability.

I let John comment on this on details but I think any other
method would have shortcoming.


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