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ARM: fix delays 

Commit 215e362daf ("ARM: 8306/1: loop_udelay: remove bogomips value
limitation") tried to increase the bogomips limitation, but in doing
so messed up udelay such that it always gives about a 5% error in the
delay, even if we use a timer.

The calculation is:

	loops = UDELAY_MULT * us_delay * ticks_per_jiffy >> UDELAY_SHIFT

Originally, UDELAY_MULT was ((UL(2199023) * HZ) >> 11) and UDELAY_SHIFT
30.  Assuming HZ=100, us_delay of 1000 and ticks_per_jiffy of 1660000
(eg, 166MHz timer, 1ms delay) this would calculate:

	((UL(2199023) * HZ) >> 11) * 1000 * 1660000 >> 30
		=> 165999

With the new values of 2047 * HZ + 483648 * HZ / 1000000 and 31, we get:

	(2047 * HZ + 483648 * HZ / 1000000) * 1000 * 1660000 >> 31
		=> 158269

which is incorrect.  This is due to a typo - correcting it gives:

	(2147 * HZ + 483648 * HZ / 1000000) * 1000 * 1660000 >> 31
		=> 165999

i.o.w, the original value.

Fixes: 215e362daf ("ARM: 8306/1: loop_udelay: remove bogomips value limitation")
Cc: <stable@vger.kernel.org>
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
This commit is contained in:
Russell King 2016-10-05 23:40:43 +01:00
parent 117e5e9c4c
commit fb833b1fbb
1 changed files with 1 additions and 1 deletions
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2
arch/arm/include/asm/delay.h
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@ -10,7 +10,7 @@
#include <asm/param.h> /* HZ */
#define MAX_UDELAY_MS 2
#define UDELAY_MULT UL(2047 * HZ + 483648 * HZ / 1000000)
#define UDELAY_MULT UL(2147 * HZ + 483648 * HZ / 1000000)
#define UDELAY_SHIFT 31
#ifndef __ASSEMBLY__