linux-kernel - Re: PM-Timer clock source is slow. Try something else: How slow? What other source(s)?

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Date:	Wed, 29 Nov 2006 17:32:09 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Linda Walsh <lkml@...nx.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: PM-Timer clock source is slow. Try something else: How slow?
	What other source(s)?

On Wed, 2006-11-29 at 16:56 -0800, Linda Walsh wrote:
> I recently noticed this message in my bootup that I don't remember
> from before:
> 
> PCI: Probing PCI hardware (bus 00)
> * Found PM-Timer Bug on the chipset. Due to workarounds for a bug,
> * this clock source is slow. Consider trying other clock sources

This basically means that your chipset has a bug which requires the ACPI
PM timer to be read three times in order to get a valid reading.

This will cause gettimeofday/clock_gettime to take longer to execute,
which is what is meant by "slow" (rather then the counter's frequency
being incorrect).

>     How would this affect my clock?  It says to try another
> clock source, what type of clock source would it be suggesting I
> use? Another chip already in the computer? It is an Intel 440BX
> chipset; on an Dell motherboard. Would that be likely to have
> another chip source that is compensating?
> 
> I don't notice a significant clock slowdown, but I'm running NTP,
> so that could be masking the problem.

Unless you're running performance critical programs that utilize
gettimeofday/clock_gettime, you probably won't notice anything. Time
should still function properly.  If you are having performance issues,
you can try using a different clocksource (the TSC is probably safe, but
not necessarily).

thanks
-john

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