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Date: Mon, 19 Mar 2001 12:39:42 +0100
From: Jamie Lokier <lk@tantalophile.demon.co.uk>
To: Russell King <rmk@arm.linux.org.uk>
Cc: linux-kernel@vger.kernel.org
Subject: Re: gettimeofday question
Message-ID: <20010319123941.A18699@pcep-jamie.cern.ch>
In-Reply-To: <200103031249.f23Cn4R01208@flint.arm.linux.org.uk> <20010319073356.A16622@flint.arm.linux.org.uk>
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In-Reply-To: <20010319073356.A16622@flint.arm.linux.org.uk>; from rmk@arm.linux.org.uk on Mon, Mar 19, 2001 at 07:33:56AM +0000
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Russell King wrote:
> > I've noticed that one of my machines here suffers from the "time going
> > backwards problem" and so started thinking about the x86 solution.
> > 
> > I've come to the conclusion that it has a hole which could cause it
> > to return the wrong time in one specific case:
> > 
> > - in do_gettimeofday(), we disable irqs (read_lock_irqsave)
> > - the ISA timer wraps, but we've got interrupts disabled, so no update
> >   of xtime or jiffies occurs
> > - in do_slow_gettimeoffset(), we read the timer, which has wrapped
> > - since jiffies_p != jiffies, we do not apply any correction
> > - our idea of time is now one jiffy slow.
> 
> I never heard any response to this.  Could some knowledgeable person please
> take a look at it?

do_slow_gettimeoffset() is supposed to correct for wrapping.
But where it says:

	#define BUGGY_NEPTUN_TIMER

		if( jiffies_t == jiffies_p ) {
		    if( count > count_p ) {
			      /* the nutcase */

Shouldn't that say "count < count_p"?

-- Jamie
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