linux-kernel - Re: Not able to use HIGH_RES

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Date:	Thu, 22 Mar 2012 14:46:31 +0530
From:	Ajeet Yadav <ajeet.yadav.77@...il.com>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Not able to use HIGH_RES_TIMERS on ARM

Hi John,
On our target  ARCH_USES_GETTIMEOFFSET=y, can you please help me how
can I useIRQSOFF_TRACER, PREEMPT_TRACER ?


On Wed, Mar 21, 2012 at 3:46 PM, Ajeet Yadav <ajeet.yadav.77@...il.com> wrote:
> Hi John,
> Thank for clearing the confusion.
>
> On Wed, Mar 21, 2012 at 10:28 AM, John Stultz <john.stultz@...aro.org> 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.  That'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.  If you're using clockevents and clocksources, you should not
>>> select ARCH_USES_GETTIMEOFFSET.
>>>
>>
>> Hey Ajeet,
>>    As 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.  Some
>> 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.  If 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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