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1// SPDX-License-Identifier: GPL-2.0
2/*
3 * Count register synchronisation.
4 *
5 * Derived from arch/x86/kernel/tsc_sync.c
6 * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
7 */
8
9#include <linux/kernel.h>
10#include <linux/irqflags.h>
11#include <linux/cpumask.h>
12#include <linux/atomic.h>
13#include <linux/nmi.h>
14#include <linux/smp.h>
15#include <linux/spinlock.h>
16
17#include <asm/r4k-timer.h>
18#include <asm/mipsregs.h>
19#include <asm/time.h>
20
21#define COUNTON 100
22#define NR_LOOPS 3
23#define LOOP_TIMEOUT 20
24
25/*
26 * Entry/exit counters that make sure that both CPUs
27 * run the measurement code at once:
28 */
29static atomic_t start_count;
30static atomic_t stop_count;
31static atomic_t test_runs;
32
33/*
34 * We use a raw spinlock in this exceptional case, because
35 * we want to have the fastest, inlined, non-debug version
36 * of a critical section, to be able to prove counter time-warps:
37 */
38static arch_spinlock_t sync_lock = __ARCH_SPIN_LOCK_UNLOCKED;
39
40static uint32_t last_counter;
41static uint32_t max_warp;
42static int nr_warps;
43static int random_warps;
44
45/*
46 * Counter warp measurement loop running on both CPUs.
47 */
48static uint32_t check_counter_warp(void)
49{
50 uint32_t start, now, prev, end, cur_max_warp = 0;
51 int i, cur_warps = 0;
52
53 start = read_c0_count();
54 end = start + (uint32_t) mips_hpt_frequency / 1000 * LOOP_TIMEOUT;
55
56 for (i = 0; ; i++) {
57 /*
58 * We take the global lock, measure counter, save the
59 * previous counter that was measured (possibly on
60 * another CPU) and update the previous counter timestamp.
61 */
62 arch_spin_lock(&sync_lock);
63 prev = last_counter;
64 now = read_c0_count();
65 last_counter = now;
66 arch_spin_unlock(&sync_lock);
67
68 /*
69 * Be nice every now and then (and also check whether
70 * measurement is done [we also insert a 10 million
71 * loops safety exit, so we dont lock up in case the
72 * counter is totally broken]):
73 */
74 if (unlikely(!(i & 7))) {
75 if (now > end || i > 10000000)
76 break;
77 cpu_relax();
78 touch_nmi_watchdog();
79 }
80 /*
81 * Outside the critical section we can now see whether
82 * we saw a time-warp of the counter going backwards:
83 */
84 if (unlikely(prev > now)) {
85 arch_spin_lock(&sync_lock);
86 max_warp = max(max_warp, prev - now);
87 cur_max_warp = max_warp;
88 /*
89 * Check whether this bounces back and forth. Only
90 * one CPU should observe time going backwards.
91 */
92 if (cur_warps != nr_warps)
93 random_warps++;
94 nr_warps++;
95 cur_warps = nr_warps;
96 arch_spin_unlock(&sync_lock);
97 }
98 }
99 WARN(!(now-start),
100 "Warning: zero counter calibration delta: %d [max: %d]\n",
101 now-start, end-start);
102 return cur_max_warp;
103}
104
105/*
106 * The freshly booted CPU initiates this via an async SMP function call.
107 */
108static void check_counter_sync_source(void *__cpu)
109{
110 unsigned int cpu = (unsigned long)__cpu;
111 int cpus = 2;
112
113 atomic_set(&test_runs, NR_LOOPS);
114retry:
115 /* Wait for the target to start. */
116 while (atomic_read(&start_count) != cpus - 1)
117 cpu_relax();
118
119 /*
120 * Trigger the target to continue into the measurement too:
121 */
122 atomic_inc(&start_count);
123
124 check_counter_warp();
125
126 while (atomic_read(&stop_count) != cpus-1)
127 cpu_relax();
128
129 /*
130 * If the test was successful set the number of runs to zero and
131 * stop. If not, decrement the number of runs an check if we can
132 * retry. In case of random warps no retry is attempted.
133 */
134 if (!nr_warps) {
135 atomic_set(&test_runs, 0);
136
137 pr_info("Counter synchronization [CPU#%d -> CPU#%u]: passed\n",
138 smp_processor_id(), cpu);
139 } else if (atomic_dec_and_test(&test_runs) || random_warps) {
140 /* Force it to 0 if random warps brought us here */
141 atomic_set(&test_runs, 0);
142
143 pr_info("Counter synchronization [CPU#%d -> CPU#%u]:\n",
144 smp_processor_id(), cpu);
145 pr_info("Measured %d cycles counter warp between CPUs", max_warp);
146 if (random_warps)
147 pr_warn("Counter warped randomly between CPUs\n");
148 }
149
150 /*
151 * Reset it - just in case we boot another CPU later:
152 */
153 atomic_set(&start_count, 0);
154 random_warps = 0;
155 nr_warps = 0;
156 max_warp = 0;
157 last_counter = 0;
158
159 /*
160 * Let the target continue with the bootup:
161 */
162 atomic_inc(&stop_count);
163
164 /*
165 * Retry, if there is a chance to do so.
166 */
167 if (atomic_read(&test_runs) > 0)
168 goto retry;
169}
170
171/*
172 * Freshly booted CPUs call into this:
173 */
174void synchronise_count_slave(int cpu)
175{
176 uint32_t cur_max_warp, gbl_max_warp, count;
177 int cpus = 2;
178
179 if (!cpu_has_counter || !mips_hpt_frequency)
180 return;
181
182 /* Kick the control CPU into the counter synchronization function */
183 smp_call_function_single(cpumask_first(cpu_online_mask),
184 check_counter_sync_source,
185 (unsigned long *)(unsigned long)cpu, 0);
186retry:
187 /*
188 * Register this CPU's participation and wait for the
189 * source CPU to start the measurement:
190 */
191 atomic_inc(&start_count);
192 while (atomic_read(&start_count) != cpus)
193 cpu_relax();
194
195 cur_max_warp = check_counter_warp();
196
197 /*
198 * Store the maximum observed warp value for a potential retry:
199 */
200 gbl_max_warp = max_warp;
201
202 /*
203 * Ok, we are done:
204 */
205 atomic_inc(&stop_count);
206
207 /*
208 * Wait for the source CPU to print stuff:
209 */
210 while (atomic_read(&stop_count) != cpus)
211 cpu_relax();
212
213 /*
214 * Reset it for the next sync test:
215 */
216 atomic_set(&stop_count, 0);
217
218 /*
219 * Check the number of remaining test runs. If not zero, the test
220 * failed and a retry with adjusted counter is possible. If zero the
221 * test was either successful or failed terminally.
222 */
223 if (!atomic_read(&test_runs)) {
224 /* Arrange for an interrupt in a short while */
225 write_c0_compare(read_c0_count() + COUNTON);
226 return;
227 }
228
229 /*
230 * If the warp value of this CPU is 0, then the other CPU
231 * observed time going backwards so this counter was ahead and
232 * needs to move backwards.
233 */
234 if (!cur_max_warp)
235 cur_max_warp = -gbl_max_warp;
236
237 count = read_c0_count();
238 count += cur_max_warp;
239 write_c0_count(count);
240
241 pr_debug("Counter compensate: CPU%u observed %d warp\n", cpu, cur_max_warp);
242
243 goto retry;
244
245}