89 lines
2.5 KiB
C
89 lines
2.5 KiB
C
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/*
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* i8253 PIT clocksource
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*/
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#include <linux/clocksource.h>
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#include <linux/init.h>
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#include <linux/io.h>
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#include <linux/spinlock.h>
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#include <linux/timex.h>
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#include <asm/i8253.h>
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/*
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* Since the PIT overflows every tick, its not very useful
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* to just read by itself. So use jiffies to emulate a free
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* running counter:
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*/
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static cycle_t i8253_read(struct clocksource *cs)
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{
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static int old_count;
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static u32 old_jifs;
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unsigned long flags;
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int count;
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u32 jifs;
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raw_spin_lock_irqsave(&i8253_lock, flags);
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/*
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* Although our caller may have the read side of xtime_lock,
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* this is now a seqlock, and we are cheating in this routine
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* by having side effects on state that we cannot undo if
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* there is a collision on the seqlock and our caller has to
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* retry. (Namely, old_jifs and old_count.) So we must treat
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* jiffies as volatile despite the lock. We read jiffies
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* before latching the timer count to guarantee that although
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* the jiffies value might be older than the count (that is,
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* the counter may underflow between the last point where
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* jiffies was incremented and the point where we latch the
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* count), it cannot be newer.
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*/
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jifs = jiffies;
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outb_pit(0x00, PIT_MODE); /* latch the count ASAP */
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count = inb_pit(PIT_CH0); /* read the latched count */
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count |= inb_pit(PIT_CH0) << 8;
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/* VIA686a test code... reset the latch if count > max + 1 */
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if (count > LATCH) {
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outb_pit(0x34, PIT_MODE);
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outb_pit(PIT_LATCH & 0xff, PIT_CH0);
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outb_pit(PIT_LATCH >> 8, PIT_CH0);
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count = PIT_LATCH - 1;
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}
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/*
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* It's possible for count to appear to go the wrong way for a
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* couple of reasons:
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*
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* 1. The timer counter underflows, but we haven't handled the
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* resulting interrupt and incremented jiffies yet.
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* 2. Hardware problem with the timer, not giving us continuous time,
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* the counter does small "jumps" upwards on some Pentium systems,
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* (see c't 95/10 page 335 for Neptun bug.)
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*
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* Previous attempts to handle these cases intelligently were
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* buggy, so we just do the simple thing now.
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*/
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if (count > old_count && jifs == old_jifs)
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count = old_count;
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old_count = count;
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old_jifs = jifs;
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raw_spin_unlock_irqrestore(&i8253_lock, flags);
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count = (PIT_LATCH - 1) - count;
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return (cycle_t)(jifs * PIT_LATCH) + count;
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}
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static struct clocksource i8253_cs = {
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.name = "pit",
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.rating = 110,
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.read = i8253_read,
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.mask = CLOCKSOURCE_MASK(32),
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};
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int __init clocksource_i8253_init(void)
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{
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return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE);
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}
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