1 // SPDX-License-Identifier: GPL-2.0
2 /*
3 * Precise Delay Loops for i386
4 *
5 * Copyright (C) 1993 Linus Torvalds
6 * Copyright (C) 1997 Martin Mares <mj@atrey.karlin.mff.cuni.cz>
7 * Copyright (C) 2008 Jiri Hladky <hladky _dot_ jiri _at_ gmail _dot_ com>
8 *
9 * The __delay function must _NOT_ be inlined as its execution time
10 * depends wildly on alignment on many x86 processors. The additional
11 * jump magic is needed to get the timing stable on all the CPU's
12 * we have to worry about.
13 */
14
15 #include <linux/export.h>
16 #include <linux/sched.h>
17 #include <linux/timex.h>
18 #include <linux/preempt.h>
19 #include <linux/delay.h>
20
21 #include <asm/processor.h>
22 #include <asm/delay.h>
23 #include <asm/timer.h>
24 #include <asm/mwait.h>
25
26 #ifdef CONFIG_SMP
27 # include <asm/smp.h>
28 #endif
29
30 /* simple loop based delay: */
31 static void delay_loop(unsigned long loops)
32 {
33 asm volatile(
34 " test %0,%0 \n"
35 " jz 3f \n"
36 " jmp 1f \n"
37
38 ".align 16 \n"
39 "1: jmp 2f \n"
40
41 ".align 16 \n"
42 "2: dec %0 \n"
43 " jnz 2b \n"
44 "3: dec %0 \n"
45
46 : /* we don't need output */
47 :"a" (loops)
48 );
49 }
50
51 /* TSC based delay: */
52 static void delay_tsc(unsigned long __loops)
53 {
54 u64 bclock, now, loops = __loops;
55 int cpu;
56
57 preempt_disable();
58 cpu = smp_processor_id();
59 bclock = rdtsc_ordered();
60 for (;;) {
61 now = rdtsc_ordered();
62 if ((now - bclock) >= loops)
63 break;
64
65 /* Allow RT tasks to run */
66 preempt_enable();
67 rep_nop();
68 preempt_disable();
69
70 /*
71 * It is possible that we moved to another CPU, and
72 * since TSC's are per-cpu we need to calculate
73 * that. The delay must guarantee that we wait "at
74 * least" the amount of time. Being moved to another
75 * CPU could make the wait longer but we just need to
76 * make sure we waited long enough. Rebalance the
77 * counter for this CPU.
78 */
79 if (unlikely(cpu != smp_processor_id())) {
80 loops -= (now - bclock);
81 cpu = smp_processor_id();
82 bclock = rdtsc_ordered();
83 }
84 }
85 preempt_enable();
86 }
87
88 /*
89 * On some AMD platforms, MWAITX has a configurable 32-bit timer, that
90 * counts with TSC frequency. The input value is the loop of the
91 * counter, it will exit when the timer expires.
92 */
93 static void delay_mwaitx(unsigned long __loops)
94 {
95 u64 start, end, delay, loops = __loops;
96
97 /*
98 * Timer value of 0 causes MWAITX to wait indefinitely, unless there
99 * is a store on the memory monitored by MONITORX.
100 */
101 if (loops == 0)
102 return;
103
104 start = rdtsc_ordered();
105
106 for (;;) {
107 delay = min_t(u64, MWAITX_MAX_LOOPS, loops);
108
109 /*
110 * Use cpu_tss_rw as a cacheline-aligned, seldomly
111 * accessed per-cpu variable as the monitor target.
112 */
113 __monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0);
114
115 /*
116 * AMD, like Intel's MWAIT version, supports the EAX hint and
117 * EAX=0xf0 means, do not enter any deep C-state and we use it
118 * here in delay() to minimize wakeup latency.
119 */
120 __mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);
121
122 end = rdtsc_ordered();
123
124 if (loops <= end - start)
125 break;
126
127 loops -= end - start;
128
129 start = end;
130 }
131 }
132
133 /*
134 * Since we calibrate only once at boot, this
135 * function should be set once at boot and not changed
136 */
137 static void (*delay_fn)(unsigned long) = delay_loop;
138
139 void use_tsc_delay(void)
140 {
141 if (delay_fn == delay_loop)
142 delay_fn = delay_tsc;
143 }
144
145 void use_mwaitx_delay(void)
146 {
147 delay_fn = delay_mwaitx;
148 }
149
150 int read_current_timer(unsigned long *timer_val)
151 {
152 if (delay_fn == delay_tsc) {
153 *timer_val = rdtsc();
154 return 0;
155 }
156 return -1;
157 }
158
159 void __delay(unsigned long loops)
160 {
161 delay_fn(loops);
162 }
163 EXPORT_SYMBOL(__delay);
164
165 noinline void __const_udelay(unsigned long xloops)
166 {
167 unsigned long lpj = this_cpu_read(cpu_info.loops_per_jiffy) ? : loops_per_jiffy;
168 int d0;
169
170 xloops *= 4;
171 asm("mull %%edx"
172 :"=d" (xloops), "=&a" (d0)
173 :"1" (xloops), "0" (lpj * (HZ / 4)));
174
175 __delay(++xloops);
176 }
177 EXPORT_SYMBOL(__const_udelay);
178
179 void __udelay(unsigned long usecs)
180 {
181 __const_udelay(usecs * 0x000010c7); /* 2**32 / 1000000 (rounded up) */
182 }
183 EXPORT_SYMBOL(__udelay);
184
185 void __ndelay(unsigned long nsecs)
186 {
187 __const_udelay(nsecs * 0x00005); /* 2**32 / 1000000000 (rounded up) */
188 }
189 EXPORT_SYMBOL(__ndelay);