kernel/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 atomic_t count_start_flag = ATOMIC_INIT(0);
  21 static atomic_t count_count_start = ATOMIC_INIT(0);
  22 static atomic_t count_count_stop = ATOMIC_INIT(0);
  23 static atomic_t count_reference = ATOMIC_INIT(0);
  24
  25 #define COUNTON 100
  26 #define NR_LOOPS 5
  27
  28 void synchronise_count_master(int cpu)
  29 {
  30         int i;
  31         unsigned long flags;
  32         unsigned int initcount;
  33
  34         printk(KERN_INFO "Synchronize counters for CPU %u: ", cpu);
  35
  36         local_irq_save(flags);
  37
  38         /*
  39          * Notify the slaves that it's time to start
  40          */
  41         atomic_set(&count_reference, read_c0_count());
  42         atomic_set(&count_start_flag, cpu);
  43         smp_wmb();
  44
  45         /* Count will be initialised to current timer for all CPU's */
  46         initcount = read_c0_count();
  47
  48         /*
  49          * We loop a few times to get a primed instruction cache,
  50          * then the last pass is more or less synchronised and
  51          * the master and slaves each set their cycle counters to a known
  52          * value all at once. This reduces the chance of having random offsets
  53          * between the processors, and guarantees that the maximum
  54          * delay between the cycle counters is never bigger than
  55          * the latency of information-passing (cachelines) between
  56          * two CPUs.
  57          */
  58
  59         for (i = 0; i < NR_LOOPS; i++) {
  60                 /* slaves loop on '!= 2' */
  61                 while (atomic_read(&count_count_start) != 1)
  62                         mb();
  63                 atomic_set(&count_count_stop, 0);
  64                 smp_wmb();
  65
  66                 /* this lets the slaves write their count register */
  67                 atomic_inc(&count_count_start);
  68
  69                 /*
  70                  * Everyone initialises count in the last loop:
  71                  */
  72                 if (i == NR_LOOPS-1)
  73                         write_c0_count(initcount);
  74
  75                 /*
  76                  * Wait for all slaves to leave the synchronization point:
  77                  */
  78                 while (atomic_read(&count_count_stop) != 1)
  79                         mb();
  80                 atomic_set(&count_count_start, 0);
  81                 smp_wmb();
  82                 atomic_inc(&count_count_stop);
  83         }
  84         /* Arrange for an interrupt in a short while */
  85         write_c0_compare(read_c0_count() + COUNTON);
  86         atomic_set(&count_start_flag, 0);
  87
  88         local_irq_restore(flags);
  89
  90         /*
  91          * i386 code reported the skew here, but the
  92          * count registers were almost certainly out of sync
  93          * so no point in alarming people
  94          */
  95         printk("done.\n");
  96 }
  97
  98 void synchronise_count_slave(int cpu)
  99 {
 100         int i;
 101         unsigned int initcount;
 102
 103         /*
 104          * Not every cpu is online at the time this gets called,
 105          * so we first wait for the master to say everyone is ready
 106          */
 107
 108         while (atomic_read(&count_start_flag) != cpu)
 109                 mb();
 110
 111         /* Count will be initialised to next expire for all CPU's */
 112         initcount = atomic_read(&count_reference);
 113
 114         for (i = 0; i < NR_LOOPS; i++) {
 115                 atomic_inc(&count_count_start);
 116                 while (atomic_read(&count_count_start) != 2)
 117                         mb();
 118
 119                 /*
 120                  * Everyone initialises count in the last loop:
 121                  */
 122                 if (i == NR_LOOPS-1)
 123                         write_c0_count(initcount);
 124
 125                 atomic_inc(&count_count_stop);
 126                 while (atomic_read(&count_count_stop) != 2)
 127                         mb();
 128         }
 129         /* Arrange for an interrupt in a short while */
 130         write_c0_compare(read_c0_count() + COUNTON);
 131 }
 132 #undef NR_LOOPS