Commit 5b13d863 authored by Ingo Molnar's avatar Ingo Molnar
Browse files

revert "x86: tsc prevent time going backwards" 



revert:

| commit 47001d60
| Author: Thomas Gleixner <tglx@linutronix.de>
| Date:   Tue Apr 1 19:45:18 2008 +0200
|
|     x86: tsc prevent time going backwards

it has been identified to cause suspend regression - and the
commit fixes a longstanding bug that existed before 2.6.25 was
opened - so it can wait some more until the effects are better
understood.

Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
parent 4cac04dd

arch/x86/kernel/tsc_32.c

+1 −14
Original line number Diff line number Diff line
@@ -287,27 +287,14 @@ core_initcall(cpufreq_tsc); 
/* clock source code */



static unsigned long current_tsc_khz = 0;

static struct clocksource clocksource_tsc;



/*

 * We compare the TSC to the cycle_last value in the clocksource

 * structure to avoid a nasty time-warp issue. This can be observed in

 * a very small window right after one CPU updated cycle_last under

 * xtime lock and the other CPU reads a TSC value which is smaller

 * than the cycle_last reference value due to a TSC which is slighty

 * behind. This delta is nowhere else observable, but in that case it

 * results in a forward time jump in the range of hours due to the

 * unsigned delta calculation of the time keeping core code, which is

 * necessary to support wrapping clocksources like pm timer.

 */

static cycle_t read_tsc(void)

{

	cycle_t ret;



	rdtscll(ret);



	return ret >= clocksource_tsc.cycle_last ?

		ret : clocksource_tsc.cycle_last;

	return ret;

}



static struct clocksource clocksource_tsc = {

arch/x86/kernel/tsc_64.c

+3 −20
Original line number Diff line number Diff line
@@ -11,7 +11,6 @@ 
#include <asm/hpet.h>

#include <asm/timex.h>

#include <asm/timer.h>

#include <asm/vgtod.h>



static int notsc __initdata = 0;
@@ -291,34 +290,18 @@ int __init notsc_setup(char *s) 


__setup("notsc", notsc_setup);



static struct clocksource clocksource_tsc;



/*

 * We compare the TSC to the cycle_last value in the clocksource

 * structure to avoid a nasty time-warp. This can be observed in a

 * very small window right after one CPU updated cycle_last under

 * xtime/vsyscall_gtod lock and the other CPU reads a TSC value which

 * is smaller than the cycle_last reference value due to a TSC which

 * is slighty behind. This delta is nowhere else observable, but in

 * that case it results in a forward time jump in the range of hours

 * due to the unsigned delta calculation of the time keeping core

 * code, which is necessary to support wrapping clocksources like pm

 * timer.

 */

/* clock source code: */

static cycle_t read_tsc(void)

{

	cycle_t ret = (cycle_t)get_cycles();



	return ret >= clocksource_tsc.cycle_last ?

		ret : clocksource_tsc.cycle_last;

	return ret;

}



static cycle_t __vsyscall_fn vread_tsc(void)

{

	cycle_t ret = (cycle_t)vget_cycles();



	return ret >= __vsyscall_gtod_data.clock.cycle_last ?

		ret : __vsyscall_gtod_data.clock.cycle_last;

	return ret;

}



static struct clocksource clocksource_tsc = {

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