arch/x86/lib/delay.c

   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);