arch/x86/lib/delay.c

   1 /*
   2  *      Precise Delay Loops for i386
   3  *
   4  *      Copyright (C) 1993 Linus Torvalds
   5  *      Copyright (C) 1997 Martin Mares <mj@atrey.karlin.mff.cuni.cz>
   6  *      Copyright (C) 2008 Jiri Hladky <hladky _dot_ jiri _at_ gmail _dot_ com>
   7  *
   8  *      The __delay function must _NOT_ be inlined as its execution time
   9  *      depends wildly on alignment on many x86 processors. The additional
  10  *      jump magic is needed to get the timing stable on all the CPU's
  11  *      we have to worry about.
  12  */
  13
  14 #include <linux/module.h>
  15 #include <linux/sched.h>
  16 #include <linux/timex.h>
  17 #include <linux/preempt.h>
  18 #include <linux/delay.h>
  19
  20 #include <asm/processor.h>
  21 #include <asm/delay.h>
  22 #include <asm/timer.h>
  23
  24 #ifdef CONFIG_SMP
  25 # include <asm/smp.h>
  26 #endif
  27
  28 /* simple loop based delay: */
  29 static void delay_loop(unsigned long loops)
  30 {
  31         asm volatile(
  32                 "       test %0,%0      \n"
  33                 "       jz 3f           \n"
  34                 "       jmp 1f          \n"
  35
  36                 ".align 16              \n"
  37                 "1:     jmp 2f          \n"
  38
  39                 ".align 16              \n"
  40                 "2:     dec %0          \n"
  41                 "       jnz 2b          \n"
  42                 "3:     dec %0          \n"
  43
  44                 : /* we don't need output */
  45                 :"a" (loops)
  46         );
  47 }
  48
  49 /* TSC based delay: */
  50 static void delay_tsc(unsigned long __loops)
  51 {
  52         u64 bclock, now, loops = __loops;
  53         int cpu;
  54
  55         preempt_disable();
  56         cpu = smp_processor_id();
  57         bclock = rdtsc_ordered();
  58         for (;;) {
  59                 now = rdtsc_ordered();
  60                 if ((now - bclock) >= loops)
  61                         break;
  62
  63                 /* Allow RT tasks to run */
  64                 preempt_enable();
  65                 rep_nop();
  66                 preempt_disable();
  67
  68                 /*
  69                  * It is possible that we moved to another CPU, and
  70                  * since TSC's are per-cpu we need to calculate
  71                  * that. The delay must guarantee that we wait "at
  72                  * least" the amount of time. Being moved to another
  73                  * CPU could make the wait longer but we just need to
  74                  * make sure we waited long enough. Rebalance the
  75                  * counter for this CPU.
  76                  */
  77                 if (unlikely(cpu != smp_processor_id())) {
  78                         loops -= (now - bclock);
  79                         cpu = smp_processor_id();
  80                         bclock = rdtsc_ordered();
  81                 }
  82         }
  83         preempt_enable();
  84 }
  85
  86 /*
  87  * Since we calibrate only once at boot, this
  88  * function should be set once at boot and not changed
  89  */
  90 static void (*delay_fn)(unsigned long) = delay_loop;
  91
  92 void use_tsc_delay(void)
  93 {
  94         delay_fn = delay_tsc;
  95 }
  96
  97 int read_current_timer(unsigned long *timer_val)
  98 {
  99         if (delay_fn == delay_tsc) {
 100                 *timer_val = rdtsc();
 101                 return 0;
 102         }
 103         return -1;
 104 }
 105
 106 void __delay(unsigned long loops)
 107 {
 108         delay_fn(loops);
 109 }
 110 EXPORT_SYMBOL(__delay);
 111
 112 inline void __const_udelay(unsigned long xloops)
 113 {
 114         int d0;
 115
 116         xloops *= 4;
 117         asm("mull %%edx"
 118                 :"=d" (xloops), "=&a" (d0)
 119                 :"1" (xloops), "0"
 120                 (this_cpu_read(cpu_info.loops_per_jiffy) * (HZ/4)));
 121
 122         __delay(++xloops);
 123 }
 124 EXPORT_SYMBOL(__const_udelay);
 125
 126 void __udelay(unsigned long usecs)
 127 {
 128         __const_udelay(usecs * 0x000010c7); /* 2**32 / 1000000 (rounded up) */
 129 }
 130 EXPORT_SYMBOL(__udelay);
 131
 132 void __ndelay(unsigned long nsecs)
 133 {
 134         __const_udelay(nsecs * 0x00005); /* 2**32 / 1000000000 (rounded up) */
 135 }
 136 EXPORT_SYMBOL(__ndelay);