Subject: Re: 2.6.18-rt7: rollover with 32-bit cycles_t
From: Daniel Walker <dwalker@mvista.com>
Reply-To: dwalker@mvista.com
To: Kevin Hilman <khilman@mvista.com>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       linux-kernel@vger.kernel.org
In-Reply-To: <4551348B.6070604@mvista.com>
References: <4551348B.6070604@mvista.com>
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Date: Tue, 07 Nov 2006 19:23:41 -0800
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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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