arch/powerpc/kernel/smp-tbsync.c

   1 /*
   2  * Smp timebase synchronization for ppc.
   3  *
   4  * Copyright (C) 2003 Samuel Rydh (samuel@ibrium.se)
   5  *
   6  */
   7
   8 #include <linux/kernel.h>
   9 #include <linux/sched.h>
  10 #include <linux/smp.h>
  11 #include <linux/unistd.h>
  12 #include <linux/slab.h>
  13 #include <linux/atomic.h>
  14 #include <asm/smp.h>
  15 #include <asm/time.h>
  16
  17 #define NUM_ITER                300
  18
  19 enum {
  20         kExit=0, kSetAndTest, kTest
  21 };
  22
  23 static struct {
  24         volatile u64            tb;
  25         volatile u64            mark;
  26         volatile int            cmd;
  27         volatile int            handshake;
  28         int                     filler[2];
  29
  30         volatile int            ack;
  31         int                     filler2[7];
  32
  33         volatile int            race_result;
  34 } *tbsync;
  35
  36 static volatile int             running;
  37
  38 static void enter_contest(u64 mark, long add)
  39 {
  40         while (get_tb() < mark)
  41                 tbsync->race_result = add;
  42 }
  43
  44 void smp_generic_take_timebase(void)
  45 {
  46         int cmd;
  47         u64 tb;
  48         unsigned long flags;
  49
  50         local_irq_save(flags);
  51         while (!running)
  52                 barrier();
  53         rmb();
  54
  55         for (;;) {
  56                 tbsync->ack = 1;
  57                 while (!tbsync->handshake)
  58                         barrier();
  59                 rmb();
  60
  61                 cmd = tbsync->cmd;
  62                 tb = tbsync->tb;
  63                 mb();
  64                 tbsync->ack = 0;
  65                 if (cmd == kExit)
  66                         break;
  67
  68                 while (tbsync->handshake)
  69                         barrier();
  70                 if (cmd == kSetAndTest)
  71                         set_tb(tb >> 32, tb & 0xfffffffful);
  72                 enter_contest(tbsync->mark, -1);
  73         }
  74         local_irq_restore(flags);
  75 }
  76
  77 static int start_contest(int cmd, long offset, int num)
  78 {
  79         int i, score=0;
  80         u64 tb;
  81         u64 mark;
  82
  83         tbsync->cmd = cmd;
  84
  85         local_irq_disable();
  86         for (i = -3; i < num; ) {
  87                 tb = get_tb() + 400;
  88                 tbsync->tb = tb + offset;
  89                 tbsync->mark = mark = tb + 400;
  90
  91                 wmb();
  92
  93                 tbsync->handshake = 1;
  94                 while (tbsync->ack)
  95                         barrier();
  96
  97                 while (get_tb() <= tb)
  98                         barrier();
  99                 tbsync->handshake = 0;
 100                 enter_contest(mark, 1);
 101
 102                 while (!tbsync->ack)
 103                         barrier();
 104
 105                 if (i++ > 0)
 106                         score += tbsync->race_result;
 107         }
 108         local_irq_enable();
 109         return score;
 110 }
 111
 112 void smp_generic_give_timebase(void)
 113 {
 114         int i, score, score2, old, min=0, max=5000, offset=1000;
 115
 116         pr_debug("Software timebase sync\n");
 117
 118         /* if this fails then this kernel won't work anyway... */
 119         tbsync = kzalloc( sizeof(*tbsync), GFP_KERNEL );
 120         mb();
 121         running = 1;
 122
 123         while (!tbsync->ack)
 124                 barrier();
 125
 126         pr_debug("Got ack\n");
 127
 128         /* binary search */
 129         for (old = -1; old != offset ; offset = (min+max) / 2) {
 130                 score = start_contest(kSetAndTest, offset, NUM_ITER);
 131
 132                 pr_debug("score %d, offset %d\n", score, offset );
 133
 134                 if( score > 0 )
 135                         max = offset;
 136                 else
 137                         min = offset;
 138                 old = offset;
 139         }
 140         score = start_contest(kSetAndTest, min, NUM_ITER);
 141         score2 = start_contest(kSetAndTest, max, NUM_ITER);
 142
 143         pr_debug("Min %d (score %d), Max %d (score %d)\n",
 144                  min, score, max, score2);
 145         score = abs(score);
 146         score2 = abs(score2);
 147         offset = (score < score2) ? min : max;
 148
 149         /* guard against inaccurate mttb */
 150         for (i = 0; i < 10; i++) {
 151                 start_contest(kSetAndTest, offset, NUM_ITER/10);
 152
 153                 if ((score2 = start_contest(kTest, offset, NUM_ITER)) < 0)
 154                         score2 = -score2;
 155                 if (score2 <= score || score2 < 20)
 156                         break;
 157         }
 158         pr_debug("Final offset: %d (%d/%d)\n", offset, score2, NUM_ITER );
 159
 160         /* exiting */
 161         tbsync->cmd = kExit;
 162         wmb();
 163         tbsync->handshake = 1;
 164         while (tbsync->ack)
 165                 barrier();
 166         tbsync->handshake = 0;
 167         kfree(tbsync);
 168         tbsync = NULL;
 169         running = 0;
 170 }