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/irq.h>
  10 #include <linux/sysdev.h>
  11 #include <linux/timex.h>
  12 #include <asm/delay.h>
  13 #include <asm/mpspec.h>
  14 #include <asm/timer.h>
  15 #include <asm/smp.h>
  16 #include <asm/io.h>
  17 #include <asm/arch_hooks.h>
  18 #include <asm/i8253.h>
  19
  20 #include "do_timer.h"
  21 #include "io_ports.h"
  22
  23 static int count_p; /* counter in get_offset_pit() */
  24
  25 static int __init init_pit(char* override)
  26 {
  27         /* check clock override */
  28         if (override[0] && strncmp(override,"pit",3))
  29                 printk(KERN_ERR "Warning: clock= override failed. Defaulting to PIT\n");
  30
  31         count_p = LATCH;
  32         return 0;
  33 }
  34
  35 static void mark_offset_pit(void)
  36 {
  37         /* nothing needed */
  38 }
  39
  40 static unsigned long long monotonic_clock_pit(void)
  41 {
  42         return 0;
  43 }
  44
  45 static void delay_pit(unsigned long loops)
  46 {
  47         int d0;
  48         __asm__ __volatile__(
  49                 "\tjmp 1f\n"
  50                 ".align 16\n"
  51                 "1:\tjmp 2f\n"
  52                 ".align 16\n"
  53                 "2:\tdecl %0\n\tjns 2b"
  54                 :"=&a" (d0)
  55                 :"0" (loops));
  56 }
  57
  58
  59 /* This function must be called with xtime_lock held.
  60  * It was inspired by Steve McCanne's microtime-i386 for BSD.  -- jrs
  61  *
  62  * However, the pc-audio speaker driver changes the divisor so that
  63  * it gets interrupted rather more often - it loads 64 into the
  64  * counter rather than 11932! This has an adverse impact on
  65  * do_gettimeoffset() -- it stops working! What is also not
  66  * good is that the interval that our timer function gets called
  67  * is no longer 10.0002 ms, but 9.9767 ms. To get around this
  68  * would require using a different timing source. Maybe someone
  69  * could use the RTC - I know that this can interrupt at frequencies
  70  * ranging from 8192Hz to 2Hz. If I had the energy, I'd somehow fix
  71  * it so that at startup, the timer code in sched.c would select
  72  * using either the RTC or the 8253 timer. The decision would be
  73  * based on whether there was any other device around that needed
  74  * to trample on the 8253. I'd set up the RTC to interrupt at 1024 Hz,
  75  * and then do some jiggery to have a version of do_timer that
  76  * advanced the clock by 1/1024 s. Every time that reached over 1/100
  77  * of a second, then do all the old code. If the time was kept correct
  78  * then do_gettimeoffset could just return 0 - there is no low order
  79  * divider that can be accessed.
  80  *
  81  * Ideally, you would be able to use the RTC for the speaker driver,
  82  * but it appears that the speaker driver really needs interrupt more
  83  * often than every 120 us or so.
  84  *
  85  * Anyway, this needs more thought....          pjsg (1993-08-28)
  86  *
  87  * If you are really that interested, you should be reading
  88  * comp.protocols.time.ntp!
  89  */
  90
  91 static unsigned long get_offset_pit(void)
  92 {
  93         int count;
  94         unsigned long flags;
  95         static unsigned long jiffies_p = 0;
  96
  97         /*
  98          * cache volatile jiffies temporarily; we have xtime_lock.
  99          */
 100         unsigned long jiffies_t;
 101
 102         spin_lock_irqsave(&i8253_lock, flags);
 103         /* timer count may underflow right here */
 104         outb_p(0x00, PIT_MODE); /* latch the count ASAP */
 105
 106         count = inb_p(PIT_CH0); /* read the latched count */
 107
 108         /*
 109          * We do this guaranteed double memory access instead of a _p
 110          * postfix in the previous port access. Wheee, hackady hack
 111          */
 112         jiffies_t = jiffies;
 113
 114         count |= inb_p(PIT_CH0) << 8;
 115
 116         /* VIA686a test code... reset the latch if count > max + 1 */
 117         if (count > LATCH) {
 118                 outb_p(0x34, PIT_MODE);
 119                 outb_p(LATCH & 0xff, PIT_CH0);
 120                 outb(LATCH >> 8, PIT_CH0);
 121                 count = LATCH - 1;
 122         }
 123
 124         /*
 125          * avoiding timer inconsistencies (they are rare, but they happen)...
 126          * there are two kinds of problems that must be avoided here:
 127          *  1. the timer counter underflows
 128          *  2. hardware problem with the timer, not giving us continuous time,
 129          *     the counter does small "jumps" upwards on some Pentium systems,
 130          *     (see c't 95/10 page 335 for Neptun bug.)
 131          */
 132
 133         if( jiffies_t == jiffies_p ) {
 134                 if( count > count_p ) {
 135                         /* the nutcase */
 136                         count = do_timer_overflow(count);
 137                 }
 138         } else
 139                 jiffies_p = jiffies_t;
 140
 141         count_p = count;
 142
 143         spin_unlock_irqrestore(&i8253_lock, flags);
 144
 145         count = ((LATCH-1) - count) * TICK_SIZE;
 146         count = (count + LATCH/2) / LATCH;
 147
 148         return count;
 149 }
 150
 151
 152 /* tsc timer_opts struct */
 153 struct timer_opts timer_pit = {
 154         .name = "pit",
 155         .mark_offset = mark_offset_pit,
 156         .get_offset = get_offset_pit,
 157         .monotonic_clock = monotonic_clock_pit,
 158         .delay = delay_pit,
 159 };
 160
 161 struct init_timer_opts __initdata timer_pit_init = {
 162         .init = init_pit,
 163         .opts = &timer_pit,
 164 };
 165
 166 void setup_pit_timer(void)
 167 {
 168         unsigned long flags;
 169
 170         spin_lock_irqsave(&i8253_lock, flags);
 171         outb_p(0x34,PIT_MODE);          /* binary, mode 2, LSB/MSB, ch 0 */
 172         udelay(10);
 173         outb_p(LATCH & 0xff , PIT_CH0); /* LSB */
 174         udelay(10);
 175         outb(LATCH >> 8 , PIT_CH0);     /* MSB */
 176         spin_unlock_irqrestore(&i8253_lock, flags);
 177 }