Commit 79c62cf1 authored Jan 11, 2006 by Ross Biro Committed by Linus Torvalds Jan 11, 2006

[PATCH] x86_64: Make udelay more accurate



The attempt to avoid overflow in __delay caused varying precision
on different CPUs depending on differences in the CPU speed.

We should be able to do this multiplication with out overflowing
provided the
cpu is running at less than about 128 GHz.  xloops < 20000 * 0x10c6.
loops_per_jiffy * HZ <= cpu_clock_speed.  So if the cpu clock speed
< 2^64/(20000 * 0x10c6) = 2^64/ 51E6CC0 < 2^64/2^27 = 2^37 = 128G we
will not overflow the calculation.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent e4e94072

arch/x86_64/lib/delay.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -39,7 +39,7 @@ void __delay(unsigned long loops)

		inline void __const_udelay(unsigned long xloops)
		{
		__delay(((xloops * cpu_data[raw_smp_processor_id()].loops_per_jiffy) >> 32) * HZ);
		__delay((xloops * HZ * cpu_data[raw_smp_processor_id()].loops_per_jiffy) >> 32);
		}

		void __udelay(unsigned long usecs)

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