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 static void delay_loop(u64 __loops);
31
32 /*
33 * Calibration and selection of the delay mechanism happens only once
34 * during boot.
35 */
36 static void (*delay_fn)(u64) __ro_after_init = delay_loop;
37 static void (*delay_halt_fn)(u64 start, u64 cycles) __ro_after_init;
38
39 /* simple loop based delay: */
40 static void delay_loop(u64 __loops)
41 {
42 unsigned long loops = (unsigned long)__loops;
43
44 asm volatile(
45 " test %0,%0 \n"
46 " jz 3f \n"
47 " jmp 1f \n"
48
49 ".align 16 \n"
50 "1: jmp 2f \n"
51
52 ".align 16 \n"
53 "2: dec %0 \n"
54 " jnz 2b \n"
55 "3: dec %0 \n"
56
57 : "+a" (loops)
58 :
59 );
60 }
61
62 /* TSC based delay: */
63 static void delay_tsc(u64 cycles)
64 {
65 u64 bclock, now;
66 int cpu;
67
68 preempt_disable();
69 cpu = smp_processor_id();
70 bclock = rdtsc_ordered();
71 for (;;) {
72 now = rdtsc_ordered();
73 if ((now - bclock) >= cycles)
74 break;
75
76 /* Allow RT tasks to run */
77 preempt_enable();
78 rep_nop();
79 preempt_disable();
80
81 /*
82 * It is possible that we moved to another CPU, and
83 * since TSC's are per-cpu we need to calculate
84 * that. The delay must guarantee that we wait "at
85 * least" the amount of time. Being moved to another
86 * CPU could make the wait longer but we just need to
87 * make sure we waited long enough. Rebalance the
88 * counter for this CPU.
89 */
90 if (unlikely(cpu != smp_processor_id())) {
91 cycles -= (now - bclock);
92 cpu = smp_processor_id();
93 bclock = rdtsc_ordered();
94 }
95 }
96 preempt_enable();
97 }
98
99 /*
100 * On Intel the TPAUSE instruction waits until any of:
101 * 1) the TSC counter exceeds the value provided in EDX:EAX
102 * 2) global timeout in IA32_UMWAIT_CONTROL is exceeded
103 * 3) an external interrupt occurs
104 */
105 static void delay_halt_tpause(u64 start, u64 cycles)
106 {
107 u64 until = start + cycles;
108 u32 eax, edx;
109
110 eax = lower_32_bits(until);
111 edx = upper_32_bits(until);
112
113 /*
114 * Hard code the deeper (C0.2) sleep state because exit latency is
115 * small compared to the "microseconds" that usleep() will delay.
116 */
117 __tpause(TPAUSE_C02_STATE, edx, eax);
118 }
119
120 /*
121 * On some AMD platforms, MWAITX has a configurable 32-bit timer, that
122 * counts with TSC frequency. The input value is the number of TSC cycles
123 * to wait. MWAITX will also exit when the timer expires.
124 */
125 static void delay_halt_mwaitx(u64 unused, u64 cycles)
126 {
127 u64 delay;
128
129 delay = min_t(u64, MWAITX_MAX_WAIT_CYCLES, cycles);
130 /*
131 * Use cpu_tss_rw as a cacheline-aligned, seldom accessed per-cpu
132 * variable as the monitor target.
133 */
134 __monitorx(raw_cpu_ptr(&cpu_tss_rw), 0, 0);
135
136 /*
137 * AMD, like Intel, supports the EAX hint and EAX=0xf means, do not
138 * enter any deep C-state and we use it here in delay() to minimize
139 * wakeup latency.
140 */
141 __mwaitx(MWAITX_DISABLE_CSTATES, delay, MWAITX_ECX_TIMER_ENABLE);
142 }
143
144 /*
145 * Call a vendor specific function to delay for a given amount of time. Because
146 * these functions may return earlier than requested, check for actual elapsed
147 * time and call again until done.
148 */
149 static void delay_halt(u64 __cycles)
150 {
151 u64 start, end, cycles = __cycles;
152
153 /*
154 * Timer value of 0 causes MWAITX to wait indefinitely, unless there
155 * is a store on the memory monitored by MONITORX.
156 */
157 if (!cycles)
158 return;
159
160 start = rdtsc_ordered();
161
162 for (;;) {
163 delay_halt_fn(start, cycles);
164 end = rdtsc_ordered();
165
166 if (cycles <= end - start)
167 break;
168
169 cycles -= end - start;
170 start = end;
171 }
172 }
173
174 void __init use_tsc_delay(void)
175 {
176 if (delay_fn == delay_loop)
177 delay_fn = delay_tsc;
178 }
179
180 void __init use_tpause_delay(void)
181 {
182 delay_halt_fn = delay_halt_tpause;
183 delay_fn = delay_halt;
184 }
185
186 void use_mwaitx_delay(void)
187 {
188 delay_halt_fn = delay_halt_mwaitx;
189 delay_fn = delay_halt;
190 }
191
192 int read_current_timer(unsigned long *timer_val)
193 {
194 if (delay_fn == delay_tsc) {
195 *timer_val = rdtsc();
196 return 0;
197 }
198 return -1;
199 }
200
201 void __delay(unsigned long loops)
202 {
203 delay_fn(loops);
204 }
205 EXPORT_SYMBOL(__delay);
206
207 noinline void __const_udelay(unsigned long xloops)
208 {
209 unsigned long lpj = this_cpu_read(cpu_info.loops_per_jiffy) ? : loops_per_jiffy;
210 int d0;
211
212 xloops *= 4;
213 asm("mull %%edx"
214 :"=d" (xloops), "=&a" (d0)
215 :"1" (xloops), "" (lpj * (HZ / 4)));
216
217 __delay(++xloops);
218 }
219 EXPORT_SYMBOL(__const_udelay);
220
221 void __udelay(unsigned long usecs)
222 {
223 __const_udelay(usecs * 0x000010c7); /* 2**32 / 1000000 (rounded up) */
224 }
225 EXPORT_SYMBOL(__udelay);
226
227 void __ndelay(unsigned long nsecs)
228 {
229 __const_udelay(nsecs * 0x00005); /* 2**32 / 1000000000 (rounded up) */
230 }
231 EXPORT_SYMBOL(__ndelay);
232
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.