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         u32 jifs;
  45         static int old_count;
  46         static u32 old_jifs;
  47
  48         spin_lock_irqsave(&i8253_lock, flags);
  49         /*
  50          * Although our caller may have the read side of xtime_lock,
  51          * this is now a seqlock, and we are cheating in this routine
  52          * by having side effects on state that we cannot undo if
  53          * there is a collision on the seqlock and our caller has to
  54          * retry.  (Namely, old_jifs and old_count.)  So we must treat
  55          * jiffies as volatile despite the lock.  We read jiffies
  56          * before latching the timer count to guarantee that although
  57          * the jiffies value might be older than the count (that is,
  58          * the counter may underflow between the last point where
  59          * jiffies was incremented and the point where we latch the
  60          * count), it cannot be newer.
  61          */
  62         jifs = jiffies;
  63         outb_p(0x00, PIT_MODE); /* latch the count ASAP */
  64         count = inb_p(PIT_CH0); /* read the latched count */
  65         count |= inb_p(PIT_CH0) << 8;
  66
  67         /* VIA686a test code... reset the latch if count > max + 1 */
  68         if (count > LATCH) {
  69                 outb_p(0x34, PIT_MODE);
  70                 outb_p(LATCH & 0xff, PIT_CH0);
  71                 outb(LATCH >> 8, PIT_CH0);
  72                 count = LATCH - 1;
  73         }
  74
  75         /*
  76          * It's possible for count to appear to go the wrong way for a
  77          * couple of reasons:
  78          *
  79          *  1. The timer counter underflows, but we haven't handled the
  80          *     resulting interrupt and incremented jiffies yet.
  81          *  2. Hardware problem with the timer, not giving us continuous time,
  82          *     the counter does small "jumps" upwards on some Pentium systems,
  83          *     (see c't 95/10 page 335 for Neptun bug.)
  84          *
  85          * Previous attempts to handle these cases intelligently were
  86          * buggy, so we just do the simple thing now.
  87          */
  88         if (count > old_count && jifs == old_jifs) {
  89                 count = old_count;
  90         }
  91         old_count = count;
  92         old_jifs = jifs;
  93
  94         spin_unlock_irqrestore(&i8253_lock, flags);
  95
  96         count = (LATCH - 1) - count;
  97
  98         return (cycle_t)(jifs * LATCH) + count;
  99 }
 100
 101 static struct clocksource clocksource_pit = {
 102         .name   = "pit",
 103         .rating = 110,
 104         .read   = pit_read,
 105         .mask   = CLOCKSOURCE_MASK(32),
 106         .mult   = 0,
 107         .shift  = 20,
 108 };
 109
 110 static int __init init_pit_clocksource(void)
 111 {
 112         if (num_possible_cpus() > 4) /* PIT does not scale! */
 113                 return 0;
 114
 115         clocksource_pit.mult = clocksource_hz2mult(CLOCK_TICK_RATE, 20);
 116         return clocksource_register(&clocksource_pit);
 117 }
 118 module_init(init_pit_clocksource);