2 * i8253.c 8253/PIT functions
5 #include <linux/clocksource.h>
6 #include <linux/spinlock.h>
7 #include <linux/jiffies.h>
8 #include <linux/sysdev.h>
9 #include <linux/module.h>
10 #include <linux/init.h>
13 #include <asm/delay.h>
14 #include <asm/i8253.h>
19 DEFINE_SPINLOCK(i8253_lock);
20 EXPORT_SYMBOL(i8253_lock);
22 void setup_pit_timer(void)
26 spin_lock_irqsave(&i8253_lock, flags);
27 outb_p(0x34,PIT_MODE); /* binary, mode 2, LSB/MSB, ch 0 */
29 outb_p(LATCH & 0xff , PIT_CH0); /* LSB */
31 outb(LATCH >> 8 , PIT_CH0); /* MSB */
32 spin_unlock_irqrestore(&i8253_lock, flags);
36 * Since the PIT overflows every tick, its not very useful
37 * to just read by itself. So use jiffies to emulate a free
40 static cycle_t pit_read(void)
46 spin_lock_irqsave(&i8253_lock, flags);
47 outb_p(0x00, PIT_MODE); /* latch the count ASAP */
48 count = inb_p(PIT_CH0); /* read the latched count */
49 count |= inb_p(PIT_CH0) << 8;
51 /* VIA686a test code... reset the latch if count > max + 1 */
53 outb_p(0x34, PIT_MODE);
54 outb_p(LATCH & 0xff, PIT_CH0);
55 outb(LATCH >> 8, PIT_CH0);
58 spin_unlock_irqrestore(&i8253_lock, flags);
62 jifs -= INITIAL_JIFFIES;
63 count = (LATCH-1) - count;
65 return (cycle_t)(jifs * LATCH) + count;
68 static struct clocksource clocksource_pit = {
77 static int __init init_pit_clocksource(void)
79 if (num_possible_cpus() > 4) /* PIT does not scale! */
82 clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);
83 return register_clocksource(&clocksource_pit);
85 module_init(init_pit_clocksource);