linux-kernel - Re: [PATCH 23/23] clocksource tsc: add verify routine

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Date:	Wed, 31 Jan 2007 18:22:56 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	Ingo Molnar <mingo@...e.hu>, akpm@...l.org,
	linux-kernel@...r.kernel.org, johnstul@...ibm.com
Subject: Re: [PATCH 23/23] clocksource tsc: add verify routine

On Wed, 2007-01-31 at 09:02 -0800, Daniel Walker wrote:
> > well, this implementation is buggy in at least two ways:
> > 
> > firstly, it allows a circular verification dependency in highres+dyntick 
> > mode between the jiffies and tsc clocksources.
> 
> In the current implementation, it's only happens if the only clocks that
> exist are "tsc" and "jiffies" which makes verification impossible
> anyway .. It's unlikely, but it is possible , the fix is not that
> complex.

Wrong. You actually can verify against jiffies. Johns orginal code did
this and mine does too. Actually this _IS_ necessary otherwise you can
not detect that the BIOS changed the cpu freqeuncy behind the kernels
back. 

The circular problem is only relevant when you switch to highres/nohz.

	tglx


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