linux-kernel - Re: 2.6.18-rt7: rollover with 32-bit cycles

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Message-Id: <1162956221.20694.13.camel@dwalker1.mvista.com>
Date:	Tue, 07 Nov 2006 19:23:41 -0800
From:	Daniel Walker <dwalker@...sta.com>
To:	Kevin Hilman <khilman@...sta.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: 2.6.18-rt7: rollover with 32-bit cycles_t

On Tue, 2006-11-07 at 17:36 -0800, Kevin Hilman wrote:
> On ARM, I'm noticing the 'bug' message from check_critical_timing()
> where two calls to get_cycles() are compared and the 2nd is assumed to
> be >= the first.
> 
> This isn't properly handling the case of rollover which occurs
> relatively often with fast hardware clocks and 32-bit cycle counters.
> 
> Is this really a bug?  If the get_cycles() can be assumed to run between
> 0 and (cycles_t)~0, using the right unsigned math could get a proper
> delta even in the rollover case.  Is this a safe assumption?

Seems like the check should really be using something like time_before()
time_after() which takes the rollover into account .. What I don't
understand is why we don't see those on x86 ..

Daniel

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