arch/i386/kernel/timers/timer_pit.c

   1 /*
   2  * This code largely moved from arch/i386/kernel/time.c.
   3  * See comments there for proper credits.
   4  */
   5
   6 #include <linux/spinlock.h>
   7 #include <linux/module.h>
   8 #include <linux/device.h>
   9 #include <linux/sysdev.h>
  10 #include <linux/timex.h>
  11 #include <asm/delay.h>
  12 #include <asm/mpspec.h>
  13 #include <asm/timer.h>
  14 #include <asm/smp.h>
  15 #include <asm/io.h>
  16 #include <asm/arch_hooks.h>
  17 #include <asm/i8253.h>
  18
  19 #include "do_timer.h"
  20 #include "io_ports.h"
  21
  22 static int count_p; /* counter in get_offset_pit() */
  23
  24 static int __init init_pit(char* override)
  25 {
  26         /* check clock override */
  27         if (override[0] && strncmp(override,"pit",3))
  28                 printk(KERN_ERR "Warning: clock= override failed. Defaulting to PIT\n");
  29
  30         count_p = LATCH;
  31         return 0;
  32 }
  33
  34 static void mark_offset_pit(void)
  35 {
  36         /* nothing needed */
  37 }
  38
  39 static unsigned long long monotonic_clock_pit(void)
  40 {
  41         return 0;
  42 }
  43
  44 static void delay_pit(unsigned long loops)
  45 {
  46         int d0;
  47         __asm__ __volatile__(
  48                 "\tjmp 1f\n"
  49                 ".align 16\n"
  50                 "1:\tjmp 2f\n"
  51                 ".align 16\n"
  52                 "2:\tdecl %0\n\tjns 2b"
  53                 :"=&a" (d0)
  54                 :"0" (loops));
  55 }
  56
  57
  58 /* This function must be called with xtime_lock held.
  59  * It was inspired by Steve McCanne's microtime-i386 for BSD.  -- jrs
  60  *
  61  * However, the pc-audio speaker driver changes the divisor so that
  62  * it gets interrupted rather more often - it loads 64 into the
  63  * counter rather than 11932! This has an adverse impact on
  64  * do_gettimeoffset() -- it stops working! What is also not
  65  * good is that the interval that our timer function gets called
  66  * is no longer 10.0002 ms, but 9.9767 ms. To get around this
  67  * would require using a different timing source. Maybe someone
  68  * could use the RTC - I know that this can interrupt at frequencies
  69  * ranging from 8192Hz to 2Hz. If I had the energy, I'd somehow fix
  70  * it so that at startup, the timer code in sched.c would select
  71  * using either the RTC or the 8253 timer. The decision would be
  72  * based on whether there was any other device around that needed
  73  * to trample on the 8253. I'd set up the RTC to interrupt at 1024 Hz,
  74  * and then do some jiggery to have a version of do_timer that
  75  * advanced the clock by 1/1024 s. Every time that reached over 1/100
  76  * of a second, then do all the old code. If the time was kept correct
  77  * then do_gettimeoffset could just return 0 - there is no low order
  78  * divider that can be accessed.
  79  *
  80  * Ideally, you would be able to use the RTC for the speaker driver,
  81  * but it appears that the speaker driver really needs interrupt more
  82  * often than every 120 us or so.
  83  *
  84  * Anyway, this needs more thought....          pjsg (1993-08-28)
  85  *
  86  * If you are really that interested, you should be reading
  87  * comp.protocols.time.ntp!
  88  */
  89
  90 static unsigned long get_offset_pit(void)
  91 {
  92         int count;
  93         unsigned long flags;
  94         static unsigned long jiffies_p = 0;
  95
  96         /*
  97          * cache volatile jiffies temporarily; we have xtime_lock.
  98          */
  99         unsigned long jiffies_t;
 100
 101         spin_lock_irqsave(&i8253_lock, flags);
 102         /* timer count may underflow right here */
 103         outb_p(0x00, PIT_MODE); /* latch the count ASAP */
 104
 105         count = inb_p(PIT_CH0); /* read the latched count */
 106
 107         /*
 108          * We do this guaranteed double memory access instead of a _p
 109          * postfix in the previous port access. Wheee, hackady hack
 110          */
 111         jiffies_t = jiffies;
 112
 113         count |= inb_p(PIT_CH0) << 8;
 114
 115         /* VIA686a test code... reset the latch if count > max + 1 */
 116         if (count > LATCH) {
 117                 outb_p(0x34, PIT_MODE);
 118                 outb_p(LATCH & 0xff, PIT_CH0);
 119                 outb(LATCH >> 8, PIT_CH0);
 120                 count = LATCH - 1;
 121         }
 122
 123         /*
 124          * avoiding timer inconsistencies (they are rare, but they happen)...
 125          * there are two kinds of problems that must be avoided here:
 126          *  1. the timer counter underflows
 127          *  2. hardware problem with the timer, not giving us continuous time,
 128          *     the counter does small "jumps" upwards on some Pentium systems,
 129          *     (see c't 95/10 page 335 for Neptun bug.)
 130          */
 131
 132         if( jiffies_t == jiffies_p ) {
 133                 if( count > count_p ) {
 134                         /* the nutcase */
 135                         count = do_timer_overflow(count);
 136                 }
 137         } else
 138                 jiffies_p = jiffies_t;
 139
 140         count_p = count;
 141
 142         spin_unlock_irqrestore(&i8253_lock, flags);
 143
 144         count = ((LATCH-1) - count) * TICK_SIZE;
 145         count = (count + LATCH/2) / LATCH;
 146
 147         return count;
 148 }
 149
 150
 151 /* tsc timer_opts struct */
 152 struct timer_opts timer_pit = {
 153         .name = "pit",
 154         .mark_offset = mark_offset_pit,
 155         .get_offset = get_offset_pit,
 156         .monotonic_clock = monotonic_clock_pit,
 157         .delay = delay_pit,
 158 };
 159
 160 struct init_timer_opts __initdata timer_pit_init = {
 161         .init = init_pit,
 162         .opts = &timer_pit,
 163 };
 164
 165 void setup_pit_timer(void)
 166 {
 167         unsigned long flags;
 168
 169         spin_lock_irqsave(&i8253_lock, flags);
 170         outb_p(0x34,PIT_MODE);          /* binary, mode 2, LSB/MSB, ch 0 */
 171         udelay(10);
 172         outb_p(LATCH & 0xff , PIT_CH0); /* LSB */
 173         udelay(10);
 174         outb(LATCH >> 8 , PIT_CH0);     /* MSB */
 175         spin_unlock_irqrestore(&i8253_lock, flags);
 176 }