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

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Message-ID: <4551348B.6070604@mvista.com>
Date:	Tue, 07 Nov 2006 17:36:11 -0800
From:	Kevin Hilman <khilman@...sta.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Subject: 2.6.18-rt7: rollover with 32-bit cycles_t

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?

Kevin

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