diff options
| -rw-r--r-- | arch/i386/kernel/i8253.c | 45 |
1 files changed, 39 insertions, 6 deletions
diff --git a/arch/i386/kernel/i8253.c b/arch/i386/kernel/i8253.c index 38aa7f19f1d..477b24daff5 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, }; |