aboutsummaryrefslogtreecommitdiffstats
path: root/arch/i386
diff options
context:
space:
mode:
authorjohn stultz <johnstul@us.ibm.com>2006-06-26 03:25:16 -0400
committerLinus Torvalds <torvalds@g5.osdl.org>2006-06-26 12:58:21 -0400
commit6415ce9a922a1446e7ee0ac9b016082232ebe373 (patch)
tree6f90fe4360ce23d54d23277b588730ccb5ba1d84 /arch/i386
parent7f9f303aa33c7acc7b4aa9ebea25cbd990bc707b (diff)
[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>
Diffstat (limited to 'arch/i386')
-rw-r--r--arch/i386/kernel/i8253.c45
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)
41{ 41{
42 unsigned long flags; 42 unsigned long flags;
43 int count; 43 int count;
44 u64 jifs; 44 u32 jifs;
45 static int old_count;
46 static u32 old_jifs;
45 47
46 spin_lock_irqsave(&i8253_lock, flags); 48 spin_lock_irqsave(&i8253_lock, flags);
49 /*
50 * Although our caller may have the read side of xtime_lock,
51 * this is now a seqlock, and we are cheating in this routine
52 * by having side effects on state that we cannot undo if
53 * there is a collision on the seqlock and our caller has to
54 * retry. (Namely, old_jifs and old_count.) So we must treat
55 * jiffies as volatile despite the lock. We read jiffies
56 * before latching the timer count to guarantee that although
57 * the jiffies value might be older than the count (that is,
58 * the counter may underflow between the last point where
59 * jiffies was incremented and the point where we latch the
60 * count), it cannot be newer.
61 */
62 jifs = jiffies;
47 outb_p(0x00, PIT_MODE); /* latch the count ASAP */ 63 outb_p(0x00, PIT_MODE); /* latch the count ASAP */
48 count = inb_p(PIT_CH0); /* read the latched count */ 64 count = inb_p(PIT_CH0); /* read the latched count */
49 count |= inb_p(PIT_CH0) << 8; 65 count |= inb_p(PIT_CH0) << 8;
@@ -55,12 +71,29 @@ static cycle_t pit_read(void)
55 outb(LATCH >> 8, PIT_CH0); 71 outb(LATCH >> 8, PIT_CH0);
56 count = LATCH - 1; 72 count = LATCH - 1;
57 } 73 }
58 spin_unlock_irqrestore(&i8253_lock, flags);
59 74
60 jifs = jiffies_64; 75 /*
76 * It's possible for count to appear to go the wrong way for a
77 * couple of reasons:
78 *
79 * 1. The timer counter underflows, but we haven't handled the
80 * resulting interrupt and incremented jiffies yet.
81 * 2. Hardware problem with the timer, not giving us continuous time,
82 * the counter does small "jumps" upwards on some Pentium systems,
83 * (see c't 95/10 page 335 for Neptun bug.)
84 *
85 * Previous attempts to handle these cases intelligently were
86 * buggy, so we just do the simple thing now.
87 */
88 if (count > old_count && jifs == old_jifs) {
89 count = old_count;
90 }
91 old_count = count;
92 old_jifs = jifs;
93
94 spin_unlock_irqrestore(&i8253_lock, flags);
61 95
62 jifs -= INITIAL_JIFFIES; 96 count = (LATCH - 1) - count;
63 count = (LATCH-1) - count;
64 97
65 return (cycle_t)(jifs * LATCH) + count; 98 return (cycle_t)(jifs * LATCH) + count;
66} 99}
@@ -69,7 +102,7 @@ static struct clocksource clocksource_pit = {
69 .name = "pit", 102 .name = "pit",
70 .rating = 110, 103 .rating = 110,
71 .read = pit_read, 104 .read = pit_read,
72 .mask = CLOCKSOURCE_MASK(64), 105 .mask = CLOCKSOURCE_MASK(32),
73 .mult = 0, 106 .mult = 0,
74 .shift = 20, 107 .shift = 20,
75}; 108};