linux-kernel - RE: [PATCH] x86: Export tsc related information in sysfs

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Message-ID: <alpine.LFD.2.00.1005261631550.2995@localhost.localdomain>
Date:	Wed, 26 May 2010 16:41:06 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
cc:	Brian Bloniarz <bmb@...enacr.com>,
	john stultz <johnstul@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: RE: [PATCH] x86: Export tsc related information in sysfs

On Wed, 26 May 2010, Dan Magenheimer wrote:
> > From: Thomas Gleixner [mailto:tglx@...utronix.de]
> >
> > Arch independent solution is to provide information about the current
> > clock source in general. This is _NOT_ a TSC specific problem, you
> > have the same trouble with any other clocksource which gets calibrated
> > and does not take it's frequency as a constant value from boot loader,
> > configuration or some CPU/chipset register. The only missing piece is
> > a frequency member in struct clocksource which needs to be filled in
> > by the arch/machine specific code.
> 
> Actually there is already a frequency in struct clocksource except
> it's represented by the two components: mult and shift.  Maybe
> it would be best to expose these instead of khz (for all clocksources)
> so as to limit abuse by naive users.

You mean the TSC user space ones ?

> So, Thomas and John, if Brian's patch is modified to provide:
> 
> /sys/devices/system/clocksource/clocksource0/current_mult
> /sys/devices/system/clocksource/clocksource0/current_shift
> and/or
> /sys/devices/system/clocksource/clocksource0/current_khz
> 
> is that an acceptable arch-independent patch? (And which do
> you prefer?)

I'd rather prefer the frequency interface for a simple reason. It
allows to add a commandline option which provides the NTP folks with a
sensible solution to their calibration problem as I don't see that a
longer calibration time will reliably fix it.

So we'd get a "clocksource_freq=XXX" option which would be applied to
the clocksource which is selected on the command line with
"clocksource=NNN".

John ?

Thanks,

	tglx
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