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smp-tbsync.c

/*
 * Smp timebase synchronization for ppc.
 *
 * Copyright (C) 2003 Samuel Rydh (samuel@ibrium.se)
 *
 */

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/unistd.h>
#include <linux/init.h>
#include <asm/atomic.h>
#include <asm/smp.h>
#include <asm/time.h>

#define NUM_ITER        300

enum {
      kExit=0, kSetAndTest, kTest
};

static struct {
      volatile u64            tb;
      volatile u64            mark;
      volatile int            cmd;
      volatile int            handshake;
      int               filler[2];

      volatile int            ack;
      int               filler2[7];

      volatile int            race_result;
} *tbsync;

static volatile int           running;

static void __devinit enter_contest(u64 mark, long add)
{
      while (get_tb() < mark)
            tbsync->race_result = add;
}

void __devinit smp_generic_take_timebase(void)
{
      int cmd;
      u64 tb;
      unsigned long flags;

      local_irq_save(flags);
      while (!running)
            barrier();
      rmb();

      for (;;) {
            tbsync->ack = 1;
            while (!tbsync->handshake)
                  barrier();
            rmb();

            cmd = tbsync->cmd;
            tb = tbsync->tb;
            mb();
            tbsync->ack = 0;
            if (cmd == kExit)
                  break;

            while (tbsync->handshake)
                  barrier();
            if (cmd == kSetAndTest)
                  set_tb(tb >> 32, tb & 0xfffffffful);
            enter_contest(tbsync->mark, -1);
      }
      local_irq_restore(flags);
}

static int __devinit start_contest(int cmd, long offset, int num)
{
      int i, score=0;
      u64 tb;
      u64 mark;

      tbsync->cmd = cmd;

      local_irq_disable();
      for (i = -3; i < num; ) {
            tb = get_tb() + 400;
            tbsync->tb = tb + offset;
            tbsync->mark = mark = tb + 400;

            wmb();

            tbsync->handshake = 1;
            while (tbsync->ack)
                  barrier();

            while (get_tb() <= tb)
                  barrier();
            tbsync->handshake = 0;
            enter_contest(mark, 1);

            while (!tbsync->ack)
                  barrier();

            if (i++ > 0)
                  score += tbsync->race_result;
      }
      local_irq_enable();
      return score;
}

void __devinit smp_generic_give_timebase(void)
{
      int i, score, score2, old, min=0, max=5000, offset=1000;

      printk("Synchronizing timebase\n");

      /* if this fails then this kernel won't work anyway... */
      tbsync = kzalloc( sizeof(*tbsync), GFP_KERNEL );
      mb();
      running = 1;

      while (!tbsync->ack)
            barrier();

      printk("Got ack\n");

      /* binary search */
      for (old = -1; old != offset ; offset = (min+max) / 2) {
            score = start_contest(kSetAndTest, offset, NUM_ITER);

            printk("score %d, offset %d\n", score, offset );

            if( score > 0 )
                  max = offset;
            else
                  min = offset;
            old = offset;
      }
      score = start_contest(kSetAndTest, min, NUM_ITER);
      score2 = start_contest(kSetAndTest, max, NUM_ITER);

      printk("Min %d (score %d), Max %d (score %d)\n",
             min, score, max, score2);
      score = abs(score);
      score2 = abs(score2);
      offset = (score < score2) ? min : max;

      /* guard against inaccurate mttb */
      for (i = 0; i < 10; i++) {
            start_contest(kSetAndTest, offset, NUM_ITER/10);

            if ((score2 = start_contest(kTest, offset, NUM_ITER)) < 0)
                  score2 = -score2;
            if (score2 <= score || score2 < 20)
                  break;
      }
      printk("Final offset: %d (%d/%d)\n", offset, score2, NUM_ITER );

      /* exiting */
      tbsync->cmd = kExit;
      wmb();
      tbsync->handshake = 1;
      while (tbsync->ack)
            barrier();
      tbsync->handshake = 0;
      kfree(tbsync);
      tbsync = NULL;
      running = 0;
}

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