linux-kernel - Re: [PATCH 1/3] clocksource: clean up clocksource

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Date:	Thu, 4 Jul 2013 12:27:28 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alex Shi <alex.shi@...el.com>
cc:	hpa@...ux.intel.com, tim.c.chen@...ux.intel.com,
	linux-kernel@...r.kernel.org, andi.kleen@...el.com,
	a.p.zijlstra@...llo.nl, mingo@...e.hu
Subject: Re: [PATCH 1/3] clocksource: clean up clocksource_select

On Thu, 4 Jul 2013, Alex Shi wrote:
> > We need this check and it is completely unrelated to the problem
> > you're trying to solve.
> > 
> > Assume the following:
> > 
> >        System boots with clocksource A and switches into highres mode.
> >        Now clocksource B gets registered and B is not highres capable.
> > 
> >        clocksource_find_best() selects again A, but we have
> >        clocksource=B on the kernel command line to override the kernel
> >        decision.
> > 
> > By removing the check, you install he non highres capable clocksource
> > B and kill the machine.
> > 
> 
> You'r right. my bad, Sorry!
> 
> BTW, why we allow user override a second best clocksource? I mean user
> can override the tsc with hpet. because undetected unstable tsc?

The user can decide to override with clocksource=jiffies if he wants
for testing purposes. That wont switch into highres ever if done from
the kernel command line. Now we have a sysfs interface, so we need to
sanity check the user override against highres.

The override is quite useful to test hpet, pmtimer on a machine which
would always select TSC.

Thanks,

	tglx
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