text: /* calibrate.c: default delay calibration
   *
   * Excised from init/main.c
   *  Copyright (C) 1991, 1992  Linus Torvalds
   */
  
  #include <linux/jiffies.h>
  #include <linux/delay.h>
  #include <linux/init.h>
  #include <linux/timex.h>
  #include <linux/smp.h>
  
  unsigned long lpj_fine;
  unsigned long preset_lpj;
  static int __init lpj_setup(char *str)
  {
  	preset_lpj = simple_strtoul(str,NULL,0);
  	return 1;
  }
  
  __setup("lpj=", lpj_setup);
  
  #ifdef ARCH_HAS_READ_CURRENT_TIMER
  
  /* This routine uses the read_current_timer() routine and gets the
   * loops per jiffy directly, instead of guessing it using delay().
   * Also, this code tries to handle non-maskable asynchronous events
   * (like SMIs)
   */
  #define DELAY_CALIBRATION_TICKS			((HZ < 100) ? 1 : (HZ/100))
  #define MAX_DIRECT_CALIBRATION_RETRIES		5
  
  static unsigned long __cpuinit calibrate_delay_direct(void)
  {
  	unsigned long pre_start, start, post_start;
  	unsigned long pre_end, end, post_end;
  	unsigned long start_jiffies;
  	unsigned long timer_rate_min, timer_rate_max;
  	unsigned long good_timer_sum = 0;
  	unsigned long good_timer_count = 0;
  	int i;
  
  	if (read_current_timer(&pre_start) < 0 )
  		return 0;
  
  	/*
  	 * A simple loop like
  	 *	while ( jiffies < start_jiffies+1)
  	 *		start = read_current_timer();
  	 * will not do. As we don't really know whether jiffy switch
  	 * happened first or timer_value was read first. And some asynchronous
  	 * event can happen between these two events introducing errors in lpj.
  	 *
  	 * So, we do
  	 * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  	 * 2. check jiffy switch
  	 * 3. start <- timer value before or after jiffy switch
  	 * 4. post_start <- When we are sure that jiffy switch has happened
  	 *
  	 * Note, we don't know anything about order of 2 and 3.
  	 * Now, by looking at post_start and pre_start difference, we can
  	 * check whether any asynchronous event happened or not
  	 */
  
  	for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  		pre_start = 0;
  		read_current_timer(&start);
  		start_jiffies = jiffies;
  		while (jiffies <= (start_jiffies + 1)) {
  			pre_start = start;
  			read_current_timer(&start);
  		}
  		read_current_timer(&post_start);
  
  		pre_end = 0;
  		end = post_start;
  		while (jiffies <=
  		       (start_jiffies + 1 + DELAY_CALIBRATION_TICKS)) {
  			pre_end = end;
  			read_current_timer(&end);
  		}
  		read_current_timer(&post_end);
  
  		timer_rate_max = (post_end - pre_start) /
  					DELAY_CALIBRATION_TICKS;
  		timer_rate_min = (pre_end - post_start) /
  					DELAY_CALIBRATION_TICKS;
  
  		/*
  		 * If the upper limit and lower limit of the timer_rate is
  		 * >= 12.5% apart, redo calibration.
  		 */
  		if (pre_start != 0 && pre_end != 0 &&
  		    (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
  			good_timer_count++;
  			good_timer_sum += timer_rate_max;
  		}
  	}
  
  	if (good_timer_count)
  		return (good_timer_sum/good_timer_count);
  
  	printk(KERN_WARNING "calibrate_delay_direct() failed to get a good "
  	       "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n");
  	return 0;
  }
  #else
  static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
  #endif
  
  /*
   * This is the number of bits of precision for the loops_per_jiffy.  Each
   * bit takes on average 1.5/HZ seconds.  This (like the original) is a little
   * better than 1%
   * For the boot cpu we can skip the delay calibration and assign it a value
   * calculated based on the timer frequency.
   * For the rest of the CPUs we cannot assume that the timer frequency is same as
   * the cpu frequency, hence do the calibration for those.
   */
  #define LPS_PREC 8
  
  void __cpuinit calibrate_delay(void)
  {
  	unsigned long ticks, loopbit;
  	int lps_precision = LPS_PREC;
  	static bool printed;
  
  	if (preset_lpj) {
  		loops_per_jiffy = preset_lpj;
  		if (!printed)
  			pr_info("Calibrating delay loop (skipped) "
  				"preset value.. ");
  	} else if ((!printed) && lpj_fine) {
  		loops_per_jiffy = lpj_fine;
  		pr_info("Calibrating delay loop (skipped), "
  			"value calculated using timer frequency.. ");
  	} else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
  		if (!printed)
  			pr_info("Calibrating delay using timer "
  				"specific routine.. ");
  	} else {
  		loops_per_jiffy = (1<<12);
  
  		if (!printed)
  			pr_info("Calibrating delay loop... ");
  		while ((loops_per_jiffy <<= 1) != 0) {
  			/* wait for "start of" clock tick */
  			ticks = jiffies;
  			while (ticks == jiffies)
  				/* nothing */;
  			/* Go .. */
  			ticks = jiffies;
  			__delay(loops_per_jiffy);
  			ticks = jiffies - ticks;
  			if (ticks)
  				break;
  		}
  
  		/*
  		 * Do a binary approximation to get loops_per_jiffy set to
  		 * equal one clock (up to lps_precision bits)
  		 */
  		loops_per_jiffy >>= 1;
  		loopbit = loops_per_jiffy;
  		while (lps_precision-- && (loopbit >>= 1)) {
  			loops_per_jiffy |= loopbit;
  			ticks = jiffies;
  			while (ticks == jiffies)
  				/* nothing */;
  			ticks = jiffies;
  			__delay(loops_per_jiffy);
  			if (jiffies != ticks)	/* longer than 1 tick */
  				loops_per_jiffy &= ~loopbit;
  		}
  	}
  	if (!printed)
  		pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
  			loops_per_jiffy/(500000/HZ),
  			(loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);
  
  	printed = true;
  }
  