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

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Date:	Wed, 26 May 2010 14:35:23 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Brian Bloniarz <bmb@...enacr.com>
cc:	john stultz <johnstul@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dan Magenheimer <dan.magenheimer@...cle.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 Tue, 25 May 2010, Brian Bloniarz wrote:

> john stultz wrote:
> > On Tue, 2010-05-25 at 20:16 -0400, Brian Bloniarz wrote:
> >> On 05/24/2010 09:33 PM, Brian Bloniarz wrote:
> >>> So what's wrong with just adding a
> >>> /sys/devices/system/clocksource/clocksource0/tsc_khz?
> >> As an RFC:
> >>
> >> Add clocksource.sys_register & sys_unregister so the
> >> current clocksource can add supplemental information to
> >> /sys/devices/system/clocksource/clocksource0/
> >>
> >> Export tsc_khz when current_clocksource==tsc so that
> >> daemons like NTP can account for the variability of
> >> calibration results.
> > 
> > I think this is a bad idea, as it creates an ABI that is arch AND
> > machine specific, which will cause portability problems in applications
> > that expect the interface to be there.
> 
> It's an arch-independent ABI that returns ENOENT on
> unsupported platforms ;)
> 
> Could you please explain what you envision as an
> arch-independent solution to this problem?
> I guess the tsc_long_calibration=1 alternative is
> one.

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.

Thanks,

	tglx
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