src/devices/timer.c

   1 #include "devices/timer.h"
   2 #include <debug.h>
   3 #include <inttypes.h>
   4 #include <round.h>
   5 #include <stdio.h>
   6 #include "threads/interrupt.h"
   7 #include "threads/io.h"
   8 #include "threads/synch.h"
   9 #include "threads/thread.h"
  10
  11 /* See [8254] for hardware details of the 8254 timer chip. */
  12
  13 #if TIMER_FREQ < 19
  14 #error 8254 timer requires TIMER_FREQ >= 19
  15 #endif
  16 #if TIMER_FREQ > 1000
  17 #error TIMER_FREQ <= 1000 recommended
  18 #endif
  19
  20 /* Number of timer ticks since OS booted. */
  21 static int64_t ticks;
  22
  23 /* Number of loops per timer tick.
  24    Initialized by timer_calibrate(). */
  25 static unsigned loops_per_tick;
  26
  27 static intr_handler_func timer_interrupt;
  28 static bool too_many_loops (unsigned loops);
  29 static void busy_wait (int64_t loops);
  30 static void real_time_sleep (int64_t num, int32_t denom);
  31
  32 /* Sets up the 8254 Programmable Interval Timer (PIT) to
  33    interrupt PIT_FREQ times per second, and registers the
  34    corresponding interrupt. */
  35 void
  36 timer_init (void)
  37 {
  38   /* 8254 input frequency divided by TIMER_FREQ, rounded to
  39      nearest. */
  40   uint16_t count = (1193180 + TIMER_FREQ / 2) / TIMER_FREQ;
  41
  42   outb (0x43, 0x34);    /* CW: counter 0, LSB then MSB, mode 2, binary. */
  43   outb (0x40, count & 0xff);
  44   outb (0x40, count >> 8);
  45
  46   intr_register_ext (0x20, timer_interrupt, "8254 Timer");
  47 }
  48
  49 /* Calibrates loops_per_tick, used to implement brief delays. */
  50 void
  51 timer_calibrate (void)
  52 {
  53   unsigned high_bit, test_bit;
  54
  55   ASSERT (intr_get_level () == INTR_ON);
  56   printf ("Calibrating timer...  ");
  57
  58   /* Approximate loops_per_tick as the largest power-of-two
  59      still less than one timer tick. */
  60   loops_per_tick = 1u << 10;
  61   while (!too_many_loops (loops_per_tick << 1))
  62     {
  63       loops_per_tick <<= 1;
  64       ASSERT (loops_per_tick != 0);
  65     }
  66
  67   /* Refine the next 8 bits of loops_per_tick. */
  68   high_bit = loops_per_tick;
  69   for (test_bit = high_bit >> 1; test_bit != high_bit >> 10; test_bit >>= 1)
  70     if (!too_many_loops (high_bit | test_bit))
  71       loops_per_tick |= test_bit;
  72
  73   printf ("%'"PRIu64" loops/s.\n", (uint64_t) loops_per_tick * TIMER_FREQ);
  74 }
  75
  76 /* Returns the number of timer ticks since the OS booted. */
  77 int64_t
  78 timer_ticks (void)
  79 {
  80   enum intr_level old_level = intr_disable ();
  81   int64_t t = ticks;
  82   intr_set_level (old_level);
  83   barrier ();
  84   return t;
  85 }
  86
  87 /* Returns the number of timer ticks elapsed since THEN, which
  88    should be a value once returned by timer_ticks(). */
  89 int64_t
  90 timer_elapsed (int64_t then)
  91 {
  92   return timer_ticks () - then;
  93 }
  94
  95 /* Suspends execution for approximately TICKS timer ticks. */
  96 void
  97 timer_sleep (int64_t ts)
  98 {
  99         int64_t deadline = ts + timer_ticks();
 100         thread_wait(thread_current(),deadline);
 101
 102         while(deadline > timer_ticks()){
 103                 thread_wait(thread_current(),deadline);
 104         }
 105 }
 106
 107
 108
 109 /* Suspends execution for approximately MS milliseconds. */
 110 void
 111 timer_msleep (int64_t ms)
 112 {
 113   real_time_sleep (ms, 1000);
 114 }
 115
 116 /* Suspends execution for approximately US microseconds. */
 117 void
 118 timer_usleep (int64_t us)
 119 {
 120   real_time_sleep (us, 1000 * 1000);
 121 }
 122
 123 /* Suspends execution for approximately NS nanoseconds. */
 124 void
 125 timer_nsleep (int64_t ns)
 126 {
 127   real_time_sleep (ns, 1000 * 1000 * 1000);
 128 }
 129
 130 /* Prints timer statistics. */
 131 void
 132 timer_print_stats (void)
 133 {
 134   printf ("Timer: %"PRId64" ticks\n", timer_ticks ());
 135 }
 136 \f
 137 /* Timer interrupt handler. */
 138 static void
 139 timer_interrupt (struct intr_frame *args UNUSED)
 140 {
 141   ticks++;
 142   thread_tick ();
 143 }
 144
 145 /* Returns true if LOOPS iterations waits for more than one timer
 146    tick, otherwise false. */
 147 static bool
 148 too_many_loops (unsigned loops)
 149 {
 150   /* Wait for a timer tick. */
 151   int64_t start = ticks;
 152   while (ticks == start)
 153     barrier ();
 154
 155   /* Run LOOPS loops. */
 156   start = ticks;
 157   busy_wait (loops);
 158
 159   /* If the tick count changed, we iterated too long. */
 160   barrier ();
 161   return start != ticks;
 162 }
 163
 164 /* Iterates through a simple loop LOOPS times, for implementing
 165    brief delays.
 166
 167    Marked NO_INLINE because code alignment can significantly
 168    affect timings, so that if this function was inlined
 169    differently in different places the results would be difficult
 170    to predict. */
 171 static void NO_INLINE
 172 busy_wait (int64_t loops)
 173 {
 174   while (loops-- > 0)
 175     barrier ();
 176 }
 177
 178 /* Sleep for approximately NUM/DENOM seconds. */
 179 static void
 180 real_time_sleep (int64_t num, int32_t denom)
 181 {
 182   /* Convert NUM/DENOM seconds into timer ticks, rounding down.
 183
 184         (NUM / DENOM) s
 185      ---------------------- = NUM * TIMER_FREQ / DENOM ticks.
 186      1 s / TIMER_FREQ ticks
 187   */
 188   int64_t ticks = num * TIMER_FREQ / denom;
 189
 190   ASSERT (intr_get_level () == INTR_ON);
 191   if (ticks > 0)
 192     {
 193       /* We're waiting for at least one full timer tick.  Use
 194          timer_sleep() because it will yield the CPU to other
 195          processes. */
 196       timer_sleep (ticks);
 197     }
 198   else
 199     {
 200       /* Otherwise, use a busy-wait loop for more accurate
 201          sub-tick timing.  We scale the numerator and denominator
 202          down by 1000 to avoid the possibility of overflow. */
 203       ASSERT (denom % 1000 == 0);
 204       busy_wait (loops_per_tick * num / 1000 * TIMER_FREQ / (denom / 1000));
 205     }
 206 }
 207