arch/mips/kernel/sync-r4k.c

   1 /*
   2  * Count register synchronisation.
   3  *
   4  * All CPUs will have their count registers synchronised to the CPU0 next time
   5  * value. This can cause a small timewarp for CPU0. All other CPU's should
   6  * not have done anything significant (but they may have had interrupts
   7  * enabled briefly - prom_smp_finish() should not be responsible for enabling
   8  * interrupts...)
   9  */
  10
  11 #include <linux/kernel.h>
  12 #include <linux/irqflags.h>
  13 #include <linux/cpumask.h>
  14
  15 #include <asm/r4k-timer.h>
  16 #include <linux/atomic.h>
  17 #include <asm/barrier.h>
  18 #include <asm/mipsregs.h>
  19
  20 static unsigned int initcount = 0;
  21 static atomic_t count_count_start = ATOMIC_INIT(0);
  22 static atomic_t count_count_stop = ATOMIC_INIT(0);
  23
  24 #define COUNTON 100
  25 #define NR_LOOPS 3
  26
  27 void synchronise_count_master(int cpu)
  28 {
  29         int i;
  30         unsigned long flags;
  31
  32         printk(KERN_INFO "Synchronize counters for CPU %u: ", cpu);
  33
  34         local_irq_save(flags);
  35
  36         /*
  37          * We loop a few times to get a primed instruction cache,
  38          * then the last pass is more or less synchronised and
  39          * the master and slaves each set their cycle counters to a known
  40          * value all at once. This reduces the chance of having random offsets
  41          * between the processors, and guarantees that the maximum
  42          * delay between the cycle counters is never bigger than
  43          * the latency of information-passing (cachelines) between
  44          * two CPUs.
  45          */
  46
  47         for (i = 0; i < NR_LOOPS; i++) {
  48                 /* slaves loop on '!= 2' */
  49                 while (atomic_read(&count_count_start) != 1)
  50                         mb();
  51                 atomic_set(&count_count_stop, 0);
  52                 smp_wmb();
  53
  54                 /* Let the slave writes its count register */
  55                 atomic_inc(&count_count_start);
  56
  57                 /* Count will be initialised to current timer */
  58                 if (i == 1)
  59                         initcount = read_c0_count();
  60
  61                 /*
  62                  * Everyone initialises count in the last loop:
  63                  */
  64                 if (i == NR_LOOPS-1)
  65                         write_c0_count(initcount);
  66
  67                 /*
  68                  * Wait for slave to leave the synchronization point:
  69                  */
  70                 while (atomic_read(&count_count_stop) != 1)
  71                         mb();
  72                 atomic_set(&count_count_start, 0);
  73                 smp_wmb();
  74                 atomic_inc(&count_count_stop);
  75         }
  76         /* Arrange for an interrupt in a short while */
  77         write_c0_compare(read_c0_count() + COUNTON);
  78
  79         local_irq_restore(flags);
  80
  81         /*
  82          * i386 code reported the skew here, but the
  83          * count registers were almost certainly out of sync
  84          * so no point in alarming people
  85          */
  86         printk("done.\n");
  87 }
  88
  89 void synchronise_count_slave(int cpu)
  90 {
  91         int i;
  92
  93         /*
  94          * Not every cpu is online at the time this gets called,
  95          * so we first wait for the master to say everyone is ready
  96          */
  97
  98         for (i = 0; i < NR_LOOPS; i++) {
  99                 atomic_inc(&count_count_start);
 100                 while (atomic_read(&count_count_start) != 2)
 101                         mb();
 102
 103                 /*
 104                  * Everyone initialises count in the last loop:
 105                  */
 106                 if (i == NR_LOOPS-1)
 107                         write_c0_count(initcount);
 108
 109                 atomic_inc(&count_count_stop);
 110                 while (atomic_read(&count_count_stop) != 2)
 111                         mb();
 112         }
 113         /* Arrange for an interrupt in a short while */
 114         write_c0_compare(read_c0_count() + COUNTON);
 115 }
 116 #undef NR_LOOPS