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[ARM] 3980/1: extend the ARM Versatile sched_clock implementation from 32 to 63 bit

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period

This provides a 63 bit clock counter guaranteed to be monotonic over a
period of 35583 days instead of a clock wrap every 179 seconds, as long
as sched_clock() is called at least once every 89 seconds.  This should
not be a problem in practice, although a kernel timer could be scheduled
every 80 seconds for example simply to call sched_clock() making sure
top bits are always synchronized if need be.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
This commit is contained in:
Nicolas Pitre 2006-12-04 20:29:21 +01:00 committed by Russell King
parent 2f1675c11a
commit 752bee178e
1 changed files with 10 additions and 4 deletions
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14
arch/arm/mach-versatile/core.c
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@ -27,6 +27,7 @@
#include <linux/amba/bus.h>
#include <linux/amba/clcd.h>
#include <asm/cnt32_to_63.h>
#include <asm/system.h>
#include <asm/hardware.h>
#include <asm/io.h>
@ -228,14 +229,19 @@ void __init versatile_map_io(void)
/*
* This is the Versatile sched_clock implementation. This has
* a resolution of 41.7ns, and a maximum value of about 179s.
* a resolution of 41.7ns, and a maximum value of about 35583 days.
*
* The return value is guaranteed to be monotonic in that range as
* long as there is always less than 89 seconds between successive
* calls to this function.
*/
unsigned long long sched_clock(void)
{
unsigned long long v;
unsigned long long v = cnt32_to_63(readl(VERSATILE_REFCOUNTER));
v = (unsigned long long)readl(VERSATILE_REFCOUNTER) * 125;
do_div(v, 3);
/* the <<1 gets rid of the cnt_32_to_63 top bit saving on a bic insn */
v *= 125<<1;
do_div(v, 3<<1);
return v;
}