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Subject: Re: [PATCH]: fix 32bits integer overflow in loops_per_jiffy
	calculation
From: Yoann Vandoorselaere <yoann@prelude-ids.org>
To: Dominik Brodowski <devel@brodo.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Gabriel Paubert <paubert@iram.es>, cpufreq@lists.arm.linux.org.uk,
       cpufreq@www.linux.org.uk, linux-kernel@vger.kernel.org
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On Thu, 2002-08-22 at 19:46, Dominik Brodowski wrote:
> On Thu, Aug 22, 2002 at 09:35:16PM +0200, Benjamin Herrenschmidt wrote:
> > >IMHO per-arch functions are really not needed. The only architectures which
> > >have CPUFreq drivers by now are ARM and i386. This will change, hopefully;
> > >IMHO it should be enough to include some basic limit checking in 
> > >cpufreq_scale().
> > 
> > In this specific case, we were talking about PPC since the problem
> > occured when I implemented cpufreq support to switch the speed
> > of the latest powerbooks between 667 and 800Mhz
> 
> And the patch from Yoann solves this? 

Yep, the integer overflow resulted in an incorrectly computed
loops_per_jiffy :

Aug 21 19:50:41 titane kernel: adjust_jiffies: prechange cur=667000, new=800000
Aug 21 19:50:41 titane kernel: old loop_per_jiffy = 665.19 (cpufreq_ref_loops=3325952, cpufreq_ref_freq=667000).
Aug 21 19:50:41 titane kernel: new loop_per_jiffy = 669.02 (cpufreq_ref_loops=3325952, cpufreq_ref_freq=667000).

With the patch applied, it work fine :

Aug 22 11:33:40 titane kernel: adjust_jiffies: prechange cur=667000, new=800000
Aug 22 11:33:40 titane kernel: old loop_per_jiffy = 665.19 (cpufreq_ref_loops=3325952, cpufreq_ref_freq=667000).
Aug 22 11:33:40 titane kernel: new loop_per_jiffy = 797.82 (cpufreq_ref_loops=3325952, cpufreq_ref_freq=667000).

-- 
Yoann Vandoorselaere, http://www.prelude-ids.org

"Programming is a race between programmers, who try and make more and 
 more idiot-proof software, and universe, which produces more and more 
 remarkable idiots. Until now, universe leads the race"  -- R. Cook

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