[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>

1 files changed, 39 insertions, 6 deletions

diff --git a/arch/i386/kernel/i8253.c b/arch/i386/kernel/i8253.c
index 38aa7f19f1d3..477b24daff53 100644
--- 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,
 };