Patch "ARM: fix delays" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

ARM: fix delays

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm-fix-delays.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Russell King <[email protected]>
Date: Wed, 5 Oct 2016 23:40:43 +0100
Subject: ARM: fix delays

From: Russell King <[email protected]>

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.

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

@@ -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__

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/arm-fix-delays.patch