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Commit 6415ce9a authored by John Stultz's avatar John Stultz Committed by Linus Torvalds
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[PATCH] time: fix time going backward w/ clock=pit 



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: default avatarJohn Stultz <johnstul@us.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 7f9f303a

arch/i386/kernel/i8253.c

+39 −6
Original line number Diff line number Diff line
@@ -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;
@@ -55,11 +71,28 @@ static cycle_t pit_read(void) 
		outb(LATCH >> 8, PIT_CH0);

		count = LATCH - 1;

	}

	spin_unlock_irqrestore(&i8253_lock, flags);



	jifs = jiffies_64;

	/*

	 * 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 -= INITIAL_JIFFIES;

	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,

};