2 * i8253 PIT clocksource
4 #include <linux/clocksource.h>
5 #include <linux/init.h>
7 #include <linux/spinlock.h>
8 #include <linux/timex.h>
9 #include <linux/module.h>
10 #include <linux/i8253.h>
13 * Protects access to I/O ports
15 * 0040-0043 : timer0, i8253 / i8254
16 * 0061-0061 : NMI Control Register which contains two speaker control bits.
18 DEFINE_RAW_SPINLOCK(i8253_lock);
19 EXPORT_SYMBOL(i8253_lock);
21 #ifdef CONFIG_CLKSRC_I8253
23 * Since the PIT overflows every tick, its not very useful
24 * to just read by itself. So use jiffies to emulate a free
27 static cycle_t i8253_read(struct clocksource *cs)
35 raw_spin_lock_irqsave(&i8253_lock, flags);
37 * Although our caller may have the read side of xtime_lock,
38 * this is now a seqlock, and we are cheating in this routine
39 * by having side effects on state that we cannot undo if
40 * there is a collision on the seqlock and our caller has to
41 * retry. (Namely, old_jifs and old_count.) So we must treat
42 * jiffies as volatile despite the lock. We read jiffies
43 * before latching the timer count to guarantee that although
44 * the jiffies value might be older than the count (that is,
45 * the counter may underflow between the last point where
46 * jiffies was incremented and the point where we latch the
47 * count), it cannot be newer.
50 outb_pit(0x00, PIT_MODE); /* latch the count ASAP */
51 count = inb_pit(PIT_CH0); /* read the latched count */
52 count |= inb_pit(PIT_CH0) << 8;
54 /* VIA686a test code... reset the latch if count > max + 1 */
56 outb_pit(0x34, PIT_MODE);
57 outb_pit(PIT_LATCH & 0xff, PIT_CH0);
58 outb_pit(PIT_LATCH >> 8, PIT_CH0);
59 count = PIT_LATCH - 1;
63 * It's possible for count to appear to go the wrong way for a
66 * 1. The timer counter underflows, but we haven't handled the
67 * resulting interrupt and incremented jiffies yet.
68 * 2. Hardware problem with the timer, not giving us continuous time,
69 * the counter does small "jumps" upwards on some Pentium systems,
70 * (see c't 95/10 page 335 for Neptun bug.)
72 * Previous attempts to handle these cases intelligently were
73 * buggy, so we just do the simple thing now.
75 if (count > old_count && jifs == old_jifs)
81 raw_spin_unlock_irqrestore(&i8253_lock, flags);
83 count = (PIT_LATCH - 1) - count;
85 return (cycle_t)(jifs * PIT_LATCH) + count;
88 static struct clocksource i8253_cs = {
92 .mask = CLOCKSOURCE_MASK(32),
95 int __init clocksource_i8253_init(void)
97 return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE);