linux-kernel - Re: [RFC PATCH 17/30 v3] initialize the clock source to jiffies clock.

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Message-ID: <Pine.LNX.4.58.0801151621140.29090@gandalf.stny.rr.com>
Date:	Tue, 15 Jan 2008 16:27:08 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	John Stultz <johnstul@...ibm.com>
Subject: Re: [RFC PATCH 17/30 v3] initialize the clock source to jiffies
 clock.


On Tue, 15 Jan 2008, Mathieu Desnoyers wrote:
> >
> > This patch simply initializes the clock to use clocksource_jiffies, so
> > that any early user of clocksource_read will not crash.
> >
>
> Hrm, is it sane at all to use the jiffies as a clocksource at early
> boot? I thought it was updated by the timer interrupt, which is only
> activated late in the boot process.

It gives us a bogus value, but we know it's always there. This was
discovered in the -rt patch where we had a hard hang at early bootup.
Seems that the tracer was calling for a clock source before it was
initialized, and it called a null pointer.

The original fix was to just make a dummy timer source that returned zero.
But using jiffies seemed a better solution.

-- Steve
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