drivers/clocksource/i8253.c

   1 /*
   2  * i8253 PIT clocksource
   3  */
   4 #include <linux/clocksource.h>
   5 #include <linux/init.h>
   6 #include <linux/io.h>
   7 #include <linux/spinlock.h>
   8 #include <linux/timex.h>
   9
  10 #include <asm/i8253.h>
  11
  12 /*
  13  * Since the PIT overflows every tick, its not very useful
  14  * to just read by itself. So use jiffies to emulate a free
  15  * running counter:
  16  */
  17 static cycle_t i8253_read(struct clocksource *cs)
  18 {
  19         static int old_count;
  20         static u32 old_jifs;
  21         unsigned long flags;
  22         int count;
  23         u32 jifs;
  24
  25         raw_spin_lock_irqsave(&i8253_lock, flags);
  26         /*
  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.
  38          */
  39         jifs = jiffies;
  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;
  43
  44         /* VIA686a test code... reset the latch if count > max + 1 */
  45         if (count > LATCH) {
  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;
  50         }
  51
  52         /*
  53          * It's possible for count to appear to go the wrong way for a
  54          * couple of reasons:
  55          *
  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.)
  61          *
  62          * Previous attempts to handle these cases intelligently were
  63          * buggy, so we just do the simple thing now.
  64          */
  65         if (count > old_count && jifs == old_jifs)
  66                 count = old_count;
  67
  68         old_count = count;
  69         old_jifs = jifs;
  70
  71         raw_spin_unlock_irqrestore(&i8253_lock, flags);
  72
  73         count = (PIT_LATCH - 1) - count;
  74
  75         return (cycle_t)(jifs * PIT_LATCH) + count;
  76 }
  77
  78 static struct clocksource i8253_cs = {
  79         .name           = "pit",
  80         .rating         = 110,
  81         .read           = i8253_read,
  82         .mask           = CLOCKSOURCE_MASK(32),
  83 };
  84
  85 int __init clocksource_i8253_init(void)
  86 {
  87         return clocksource_register_hz(&i8253_cs, PIT_TICK_RATE);
  88 }