Re: Not able to use HIGH_RES_TIMERS on ARM

[Ajeet Yadav]

Hi John,
Thank for clearing the confusion.

On Wed, Mar 21, 2012 at 10:28 AM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> On 03/19/2012 06:26 AM, Russell King - ARM Linux wrote:
>>
>> On Mon, Mar 19, 2012 at 06:49:34PM +0530, Ajeet Yadav wrote:
>>>
>>> Therefore our target configuration with 2.6 kernel was
>>> GENERIC_CLOCKEVENTS=y, ARCH_USES_GETTIMEOFFSET=y,
>>
>> It's absolutely absurd to have a platform converted to use clockevents
>> and clocksources, and then select ARCH_USES_GETTIMEOFFSET. ÂThat's saying
>> "I provide the new infrastructure, but I want the dodgy old compatibility
>> which doesn't work properly with a set of other features as well".
>>
>>> I conclude that GENERIC_CLOCKEVENTS is supported, hence I must set
>>> ARCH_USES_GETTIMEOFFSET=n, in order to use NO_HZ, HIGH_RES_TIMERS,
>>> IRQSOFF_TRACER, PREEMPT_TRACER
>>
>> Correct. ÂIf you're using clockevents and clocksources, you should not
>> select ARCH_USES_GETTIMEOFFSET.
>>
>
> Hey Ajeet,
> Â ÂAs Russell pointed out, it looks like you're confused as to the use of
> ARCH_USES_GETTIMEOFFSET. That option is only for legacy systems that don't
> provide continuous clocksources that can be used for timekeeping.
>
> In the past, time was incremented by one tick every timer interrupt. ÂSome
> systems could use the timer hardware (usually PIT style decrementer) to
> calculate inter-tick times. Its only for this style of hardware, that either
> wraps or resets each tick, that GETTIMEOFFSET is needed. ÂIf you have a
> continuous counter that doesn't wrap for a reasonable number of ticks, you
> want to use the clocksource abstraction to represent that hardware. That has
> the benefits of allowing high res timers and nohz, since we don't need to
> keep a constant tick-beat to keep time (and also avoids lost-ticks and a
> host of problems that tick based timekeeping can run into).
>
> So I suspect you probably want to verify your hardware supports a
> clocksource and disable ARCH_USES_GETTIMEOFFSET.
>
> Sorry for any confusion!
>
> thanks
> -john
>
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