ARM: fix delays

commit fb833b1fbb68461772dbf5e91bddea5e839187e9 upstream.

Commit 215e362dafed ("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: 215e362dafed ("ARM: 8306/1: loop_udelay: remove bogomips value limitation")
Reviewed-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/arm/include/asm/delay.h b/arch/arm/include/asm/delay.h
index b7a4281543556cfb82f716e0fc0337dae363187c..b1ce037e4380dcea92f6774c0f7dc1ad9c9569f3 100644 (file)
--- a/arch/arm/include/asm/delay.h
+++ b/arch/arm/include/asm/delay.h
@@ -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__