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