linux-kernel - Re: [PATCH] lapic-timer: increase the max

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Date:	Mon, 10 Jan 2011 09:36:31 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Pierre Tardy <pierre.tardy@...el.com>
cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Arjan van de Ven <arjan@...radead.org>,
	Adrian Bunk <bunk@...sta.de>, Andrew Morton <akpm@...l.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>, john stultz <johnstul@...ibm.com>,
	Roman Zippel <zippel@...ux-m68k.org>, Andi Kleen <ak@...e.de>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCH] lapic-timer: increase the max_delta to 31 bits

On Thu, 6 Jan 2011, Pierre Tardy wrote:

> Latest atom socs(penwell) does not have hpet timer.
> 
> As their local APIC timer is clocked at 400KHZ, and the current code
> limit their Initial Counter register to 23 bits, they cannot sleep
> more than 1.34 seconds which leads to ~2 spurious wakeup per second (1 per thread)
> 
> These SOCs support 32bit timer so we change the max_delta to at least 31bits.
> So we can at least sleep for 300 seconds.
> 
> We could not find any previous chip errata where lapic would only have 23 bit precision
> As powertop is suggesting to activate HPET to "sleep longer", this could mean this problem is
> already known.

I have no idea why I put this limit there back then. If there is no
erratum then there is no reason to keep it.

Acked-by: Thomas Gleixner <tglx@...utronix.de>

 
> Problem is here since very first implementation of lapic timer as a clock event
> e9e2cdb [PATCH] clockevents: i386 drivers
> cc contributors of the first patch and x86 maintainers
> diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
> index 7821813..c031f1d 100644
> --- a/arch/x86/kernel/apic/apic.c
> +++ b/arch/x86/kernel/apic/apic.c
> @@ -684,7 +684,7 @@ static int __init calibrate_APIC_clock(void)
>  	lapic_clockevent.mult = div_sc(delta, TICK_NSEC * LAPIC_CAL_LOOPS,
>  				       lapic_clockevent.shift);
>  	lapic_clockevent.max_delta_ns =
> -		clockevent_delta2ns(0x7FFFFF, &lapic_clockevent);
> +		clockevent_delta2ns(0x7FFFFFFF, &lapic_clockevent);
>  	lapic_clockevent.min_delta_ns =
>  		clockevent_delta2ns(0xF, &lapic_clockevent);
>  
> -- 
> 1.7.0.4
> 
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