arch/i386/kernel/i8253.c

   1 /*
   2  * i8253.c  8253/PIT functions
   3  *
   4  */
   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>
  11
  12 #include <asm/smp.h>
  13 #include <asm/delay.h>
  14 #include <asm/i8253.h>
  15 #include <asm/io.h>
  16
  17 #include "io_ports.h"
  18
  19 DEFINE_SPINLOCK(i8253_lock);
  20 EXPORT_SYMBOL(i8253_lock);
  21
  22 void setup_pit_timer(void)
  23 {
  24         unsigned long flags;
  25
  26         spin_lock_irqsave(&i8253_lock, flags);
  27         outb_p(0x34,PIT_MODE);          /* binary, mode 2, LSB/MSB, ch 0 */
  28         udelay(10);
  29         outb_p(LATCH & 0xff , PIT_CH0); /* LSB */
  30         udelay(10);
  31         outb(LATCH >> 8 , PIT_CH0);     /* MSB */
  32         spin_unlock_irqrestore(&i8253_lock, flags);
  33 }
  34
  35 /*
  36  * Since the PIT overflows every tick, its not very useful
  37  * to just read by itself. So use jiffies to emulate a free
  38  * running counter:
  39  */
  40 static cycle_t pit_read(void)
  41 {
  42         unsigned long flags;
  43         int count;
  44         u64 jifs;
  45
  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;
  50
  51         /* VIA686a test code... reset the latch if count > max + 1 */
  52         if (count > LATCH) {
  53                 outb_p(0x34, PIT_MODE);
  54                 outb_p(LATCH & 0xff, PIT_CH0);
  55                 outb(LATCH >> 8, PIT_CH0);
  56                 count = LATCH - 1;
  57         }
  58         spin_unlock_irqrestore(&i8253_lock, flags);
  59
  60         jifs = jiffies_64;
  61
  62         jifs -= INITIAL_JIFFIES;
  63         count = (LATCH-1) - count;
  64
  65         return (cycle_t)(jifs * LATCH) + count;
  66 }
  67
  68 static struct clocksource clocksource_pit = {
  69         .name   = "pit",
  70         .rating = 110,
  71         .read   = pit_read,
  72         .mask   = CLOCKSOURCE_MASK(64),
  73         .mult   = 0,
  74         .shift  = 20,
  75 };
  76
  77 static int __init init_pit_clocksource(void)
  78 {
  79         if (num_possible_cpus() > 4) /* PIT does not scale! */
  80                 return 0;
  81
  82         clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);
  83         return clocksource_register(&clocksource_pit);
  84 }
  85 module_init(init_pit_clocksource);