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 unsigned long flags; 94 95 local_irq_save(flags); 96 97 /* 98 * Not every cpu is online at the time this gets called, 99 * so we first wait for the master to say everyone is ready 100 */ 101 102 for (i = 0; i < NR_LOOPS; i++) { 103 atomic_inc(&count_count_start); 104 while (atomic_read(&count_count_start) != 2) 105 mb(); 106 107 /* 108 * Everyone initialises count in the last loop: 109 */ 110 if (i == NR_LOOPS-1) 111 write_c0_count(initcount); 112 113 atomic_inc(&count_count_stop); 114 while (atomic_read(&count_count_stop) != 2) 115 mb(); 116 } 117 /* Arrange for an interrupt in a short while */ 118 write_c0_compare(read_c0_count() + COUNTON); 119 120 local_irq_restore(flags); 121 } 122 #undef NR_LOOPS 123