linux-kernel - [PATCH 4.8 29/37] ARM: fix delays

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Date:   Fri, 14 Oct 2016 14:27:15 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Nicolas Pitre <nico@...aro.org>,
        Russell King <rmk+kernel@...linux.org.uk>
Subject: [PATCH 4.8 29/37] ARM: fix delays

4.8-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Russell King <rmk+kernel@...linux.org.uk>

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@...aro.org>
Signed-off-by: Russell King <rmk+kernel@...linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/arm/include/asm/delay.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- 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__