[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: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/i386/kernel/i8253.c b/arch/i386/kernel/i8253.c
index 38aa7f19f1d30ef006377a0b80cc3cd6f15209a0..477b24daff539bf3d5a1ab6be6e10f6f10dedc22 100644 (file)
--- a/arch/i386/kernel/i8253.c
+++ b/arch/i386/kernel/i8253.c
@@ -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;
        }
-       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;
+       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,
 };