[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 38aa7f1..477b24d 100644
--- a/arch/i386/kernel/i8253.c
+++ b/arch/i386/kernel/i8253.c
@@ -41,9 +41,25 @@
{
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 @@
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 @@
.name = "pit",
.rating = 110,
.read = pit_read,
- .mask = CLOCKSOURCE_MASK(64),
+ .mask = CLOCKSOURCE_MASK(32),
.mult = 0,
.shift = 20,
};