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[PATCH] time: fix time going backward w/ clock=pit

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Here is the PIT fix against the TOD patches that Tim pointed out.  Many
thanks to Tim for hunting this down.

Cc: Tim Mann <mann@vmware.com>
Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
john stultz 2006-06-26 00:25:16 -07:00 committed by Linus Torvalds
parent 7f9f303aa3
commit 6415ce9a92
1 changed files with 39 additions and 6 deletions
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45
arch/i386/kernel/i8253.c
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@ -41,9 +41,25 @@ static cycle_t pit_read(void)
{
unsigned long flags;
int count;
u64 jifs;
u32 jifs;
static int old_count;
static u32 old_jifs;
spin_lock_irqsave(&i8253_lock, flags);
/*
* Although our caller may have the read side of xtime_lock,
* this is now a seqlock, and we are cheating in this routine
* by having side effects on state that we cannot undo if
* there is a collision on the seqlock and our caller has to
* retry. (Namely, old_jifs and old_count.) So we must treat
* jiffies as volatile despite the lock. We read jiffies
* before latching the timer count to guarantee that although
* the jiffies value might be older than the count (that is,
* the counter may underflow between the last point where
* jiffies was incremented and the point where we latch the
* count), it cannot be newer.
*/
jifs = jiffies;
outb_p(0x00, PIT_MODE); /* latch the count ASAP */
count = inb_p(PIT_CH0); /* read the latched count */
count |= inb_p(PIT_CH0) << 8;
@ -55,12 +71,29 @@ static cycle_t pit_read(void)
outb(LATCH >> 8, PIT_CH0);
count = LATCH - 1;
}
/*
* It's possible for count to appear to go the wrong way for a
* couple of reasons:
*
* 1. The timer counter underflows, but we haven't handled the
* resulting interrupt and incremented jiffies yet.
* 2. Hardware problem with the timer, not giving us continuous time,
* the counter does small "jumps" upwards on some Pentium systems,
* (see c't 95/10 page 335 for Neptun bug.)
*
* Previous attempts to handle these cases intelligently were
* buggy, so we just do the simple thing now.
*/
if (count > old_count && jifs == old_jifs) {
count = old_count;
}
old_count = count;
old_jifs = jifs;
spin_unlock_irqrestore(&i8253_lock, flags);
jifs = jiffies_64;
jifs -= INITIAL_JIFFIES;
count = (LATCH-1) - count;
count = (LATCH - 1) - count;
return (cycle_t)(jifs * LATCH) + count;
}
@ -69,7 +102,7 @@ static struct clocksource clocksource_pit = {
.name = "pit",
.rating = 110,
.read = pit_read,
.mask = CLOCKSOURCE_MASK(64),
.mask = CLOCKSOURCE_MASK(32),
.mult = 0,
.shift = 20,
};