arch/mips/kernel/sync-r4k.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 /*
   3  * Count register synchronisation.
   4  *
   5  * All CPUs will have their count registers synchronised to the CPU0 next time
   6  * value. This can cause a small timewarp for CPU0. All other CPU's should
   7  * not have done anything significant (but they may have had interrupts
   8  * enabled briefly - prom_smp_finish() should not be responsible for enabling
   9  * interrupts...)
  10  */
  11
  12 #include <linux/kernel.h>
  13 #include <linux/irqflags.h>
  14 #include <linux/cpumask.h>
  15
  16 #include <asm/r4k-timer.h>
  17 #include <linux/atomic.h>
  18 #include <asm/barrier.h>
  19 #include <asm/mipsregs.h>
  20
  21 static unsigned int initcount = 0;
  22 static atomic_t count_count_start = ATOMIC_INIT(0);
  23 static atomic_t count_count_stop = ATOMIC_INIT(0);
  24
  25 #define COUNTON 100
  26 #define NR_LOOPS 3
  27
  28 void synchronise_count_master(int cpu)
  29 {
  30         int i;
  31         unsigned long flags;
  32
  33         pr_info("Synchronize counters for CPU %u: ", cpu);
  34
  35         local_irq_save(flags);
  36
  37         /*
  38          * We loop a few times to get a primed instruction cache,
  39          * then the last pass is more or less synchronised and
  40          * the master and slaves each set their cycle counters to a known
  41          * value all at once. This reduces the chance of having random offsets
  42          * between the processors, and guarantees that the maximum
  43          * delay between the cycle counters is never bigger than
  44          * the latency of information-passing (cachelines) between
  45          * two CPUs.
  46          */
  47
  48         for (i = 0; i < NR_LOOPS; i++) {
  49                 /* slaves loop on '!= 2' */
  50                 while (atomic_read(&count_count_start) != 1)
  51                         mb();
  52                 atomic_set(&count_count_stop, 0);
  53                 smp_wmb();
  54
  55                 /* Let the slave writes its count register */
  56                 atomic_inc(&count_count_start);
  57
  58                 /* Count will be initialised to current timer */
  59                 if (i == 1)
  60                         initcount = read_c0_count();
  61
  62                 /*
  63                  * Everyone initialises count in the last loop:
  64                  */
  65                 if (i == NR_LOOPS-1)
  66                         write_c0_count(initcount);
  67
  68                 /*
  69                  * Wait for slave to leave the synchronization point:
  70                  */
  71                 while (atomic_read(&count_count_stop) != 1)
  72                         mb();
  73                 atomic_set(&count_count_start, 0);
  74                 smp_wmb();
  75                 atomic_inc(&count_count_stop);
  76         }
  77         /* Arrange for an interrupt in a short while */
  78         write_c0_compare(read_c0_count() + COUNTON);
  79
  80         local_irq_restore(flags);
  81
  82         /*
  83          * i386 code reported the skew here, but the
  84          * count registers were almost certainly out of sync
  85          * so no point in alarming people
  86          */
  87         pr_cont("done.\n");
  88 }
  89
  90 void synchronise_count_slave(int cpu)
  91 {
  92         int i;
  93
  94         /*
  95          * Not every cpu is online at the time this gets called,
  96          * so we first wait for the master to say everyone is ready
  97          */
  98
  99         for (i = 0; i < NR_LOOPS; i++) {
 100                 atomic_inc(&count_count_start);
 101                 while (atomic_read(&count_count_start) != 2)
 102                         mb();
 103
 104                 /*
 105                  * Everyone initialises count in the last loop:
 106                  */
 107                 if (i == NR_LOOPS-1)
 108                         write_c0_count(initcount);
 109
 110                 atomic_inc(&count_count_stop);
 111                 while (atomic_read(&count_count_stop) != 2)
 112                         mb();
 113         }
 114         /* Arrange for an interrupt in a short while */
 115         write_c0_compare(read_c0_count() + COUNTON);
 116 }
 117 #undef NR_LOOPS