[PATCH]: fix 32bits integer overflow in loops_per_jiffy calculation

From: Yoann Vandoorselaere <yoann@prelude-ids.org>
To: cpufreq@lists.arm.linux.org
Cc: cpufreq@www.linux.org.uk, linux-kernel@vger.kernel.org,
	"benh@kernel.crashing.org" <benh@kernel.crashing.org>
Subject: [PATCH]: fix 32bits integer overflow in loops_per_jiffy calculation
Date: 22 Aug 2002 11:50:40 +0200	[thread overview]
Message-ID: <1030009840.15429.109.camel@alph> (raw)

[-- Attachment #1: Type: text/plain, Size: 513 bytes --]

Hi,

The "low_part * mult" multiplication of the old function may overflow a
32bits integer...

This patch both fix the overflow issue (tested with frequencies up to
20Ghz), and make the result of the function lose less precision.

Please apply, 

-- 
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

[-- Attachment #2: cpufreq-overflow.diff --]
[-- Type: text/plain, Size: 747 bytes --]

--- linux-benh/kernel/cpufreq.c	2002-08-21 17:27:52.000000000 +0200
+++ linux-yoann/kernel/cpufreq.c	2002-08-22 11:27:09.000000000 +0200
@@ -78,14 +78,16 @@ static unsigned int             cpufreq_
  */
 static unsigned long scale(unsigned long old, u_int div, u_int mult)
 {
-	unsigned long low_part, high_part;
-
-	high_part  = old / div;
-	low_part   = (old % div) / 100;
-	high_part *= mult;
-	low_part   = low_part * mult / div;
-
-	return high_part + low_part * 100;
+        unsigned long val, carry = 0;
+        
+        mult /= 100;
+        div  /= 100;
+        val = old / div * mult;
+
+        carry = old % div;
+        carry = carry * mult / div;
+                
+        return val + carry;
 }
 
 
