linux/arch/i386/kernel/i8253.c

/*
 * i8253.c  8253/PIT functions
 *
 */
#include <linux/clocksource.h>
#include <linux/spinlock.h>
#include <linux/jiffies.h>
#include <linux/sysdev.h>
#include <linux/module.h>
#include <linux/init.h>

#include <asm/smp.h>
#include <asm/delay.h>
#include <asm/i8253.h>
#include <asm/io.h>

#include "io_ports.h"

DEFINE_SPINLOCK(i8253_lock);
EXPORT_SYMBOL(i8253_lock);

void setup_pit_timer(void)
{
	unsigned long flags;

	spin_lock_irqsave(&i8253_lock, flags);
	outb_p(0x34,PIT_MODE);		/* binary, mode 2, LSB/MSB, ch 0 */
	udelay(10);
	outb_p(LATCH & 0xff , PIT_CH0);	/* LSB */
	udelay(10);
	outb(LATCH >> 8 , PIT_CH0);	/* MSB */
	spin_unlock_irqrestore(&i8253_lock, flags);
}

/*
 * Since the PIT overflows every tick, its not very useful
 * to just read by itself. So use jiffies to emulate a free
 * running counter:
 */
static cycle_t pit_read(void)
{
	unsigned long flags;
	int count;
	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;

	/* VIA686a test code... reset the latch if count > max + 1 */
	if (count > LATCH) {
		outb_p(0x34, PIT_MODE);
		outb_p(LATCH & 0xff, PIT_CH0);
		outb(LATCH >> 8, PIT_CH0);
		count = LATCH - 1;
	}

	/*
	 * 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);

	count = (LATCH - 1) - count;

	return (cycle_t)(jifs * LATCH) + count;
}

static struct clocksource clocksource_pit = {
	.name	= "pit",
	.rating = 110,
	.read	= pit_read,
	.mask	= CLOCKSOURCE_MASK(32),
	.mult	= 0,
	.shift	= 20,
};

static int __init init_pit_clocksource(void)
{
	if (num_possible_cpus() > 4) /* PIT does not scale! */
		return 0;

	clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);
	return clocksource_register(&clocksource_pit);
}
module_init(init_pit_clocksource);