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    SubjectRe: [PATCHv2 0/3] clocksource: add db8500 PRCMU timer
    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
    > clocksource.
    > 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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