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Subject[PATCH 1/3] mull'ify multiplication with HZ in __const_udelay() [Was: Re: Too much error in __const_udelay() ?]
John Stultz mentioned on lkml ( http://lkml.org/lkml/2004/6/5/15 ) that
calls to udelay() don't delay long enough, causing trouble e.g. in the USB
subsystem. The following patches address this issue.

Move the multiplication of (loops_per_jiffy * xloops) with HZ into
the "mull" asm operation. This increases the accuracy of the delay functions
largely:

n usec delay on a system with loops_per_jiffy = 1500000 :

n before after
1 1000 ticks 1499 ticks
10 14000 ticks 14999 ticks

n usec delay on a system with loops_per_jiffy = 100000 :

n before after
1 0 ticks 99 ticks
10 0 ticks 999 ticks
100 9000 ticks 9999 ticks

As noted by Kurt Garloff, it's necessary to adjust for large loops_per_jiffies,
as the multiplication of it with HZ fails for 4GHz or larger. So, John Stultz
suggested multiplying xloops with 4 first, and multiplying with (HZ/4).

Signed-off-by: Dominik Brodowski <linux@brodo.de>


diff -ruN linux-original/arch/i386/lib/delay.c linux/arch/i386/lib/delay.c
--- linux-original/arch/i386/lib/delay.c 2004-06-14 18:20:27.000000000 +0200
+++ linux/arch/i386/lib/delay.c 2004-06-15 07:48:57.302279400 +0200
@@ -31,10 +31,11 @@
inline void __const_udelay(unsigned long xloops)
{
int d0;
+ xloops *= 4;
__asm__("mull %0"
:"=d" (xloops), "=&a" (d0)
- :"1" (xloops),"0" (current_cpu_data.loops_per_jiffy));
- __delay(xloops * HZ);
+ :"1" (xloops),"0" (current_cpu_data.loops_per_jiffy * (HZ/4)));
+ __delay(xloops);
}

void __udelay(unsigned long usecs)
-
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