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git.proxmox.com Git - mirror_ubuntu-bionic-kernel.git/blob - drivers/clocksource/i8253.c
2 * i8253 PIT clocksource
4 #include <linux/clocksource.h>
5 #include <linux/init.h>
7 #include <linux/spinlock.h>
8 #include <linux/timex.h>
10 #include <asm/i8253.h>
13 * Since the PIT overflows every tick, its not very useful
14 * to just read by itself. So use jiffies to emulate a free
17 static cycle_t
i8253_read(struct clocksource
*cs
)
25 raw_spin_lock_irqsave(&i8253_lock
, flags
);
27 * Although our caller may have the read side of xtime_lock,
28 * this is now a seqlock, and we are cheating in this routine
29 * by having side effects on state that we cannot undo if
30 * there is a collision on the seqlock and our caller has to
31 * retry. (Namely, old_jifs and old_count.) So we must treat
32 * jiffies as volatile despite the lock. We read jiffies
33 * before latching the timer count to guarantee that although
34 * the jiffies value might be older than the count (that is,
35 * the counter may underflow between the last point where
36 * jiffies was incremented and the point where we latch the
37 * count), it cannot be newer.
40 outb_pit(0x00, PIT_MODE
); /* latch the count ASAP */
41 count
= inb_pit(PIT_CH0
); /* read the latched count */
42 count
|= inb_pit(PIT_CH0
) << 8;
44 /* VIA686a test code... reset the latch if count > max + 1 */
46 outb_pit(0x34, PIT_MODE
);
47 outb_pit(PIT_LATCH
& 0xff, PIT_CH0
);
48 outb_pit(PIT_LATCH
>> 8, PIT_CH0
);
49 count
= PIT_LATCH
- 1;
53 * It's possible for count to appear to go the wrong way for a
56 * 1. The timer counter underflows, but we haven't handled the
57 * resulting interrupt and incremented jiffies yet.
58 * 2. Hardware problem with the timer, not giving us continuous time,
59 * the counter does small "jumps" upwards on some Pentium systems,
60 * (see c't 95/10 page 335 for Neptun bug.)
62 * Previous attempts to handle these cases intelligently were
63 * buggy, so we just do the simple thing now.
65 if (count
> old_count
&& jifs
== old_jifs
)
71 raw_spin_unlock_irqrestore(&i8253_lock
, flags
);
73 count
= (PIT_LATCH
- 1) - count
;
75 return (cycle_t
)(jifs
* PIT_LATCH
) + count
;
78 static struct clocksource i8253_cs
= {
82 .mask
= CLOCKSOURCE_MASK(32),
85 int __init
clocksource_i8253_init(void)
87 return clocksource_register_hz(&i8253_cs
, PIT_TICK_RATE
);