/* 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>
  #include <linux/percpu.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 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;
  	unsigned long measured_times[MAX_DIRECT_CALIBRATION_RETRIES];
  	int max = -1; /* index of measured_times with max/min values or not set */
  	int min = -1;
  	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 (time_before_eq(jiffies, start_jiffies + 1)) {
  			pre_start = start;
  			read_current_timer(&start);
  		}
  		read_current_timer(&post_start);
  
  		pre_end = 0;
  		end = post_start;
  		while (time_before_eq(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 (start >= post_end)
  			printk(KERN_NOTICE "calibrate_delay_direct() ignoring "
  					"timer_rate as we had a TSC wrap around"
  					" start=%lu >=post_end=%lu
  ",
  				start, post_end);
  		if (start < post_end && 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;
  			measured_times[i] = timer_rate_max;
  			if (max < 0 || timer_rate_max > measured_times[max])
  				max = i;
  			if (min < 0 || timer_rate_max < measured_times[min])
  				min = i;
  		} else
  			measured_times[i] = 0;
  
  	}
  
  	/*
  	 * Find the maximum & minimum - if they differ too much throw out the
  	 * one with the largest difference from the mean and try again...
  	 */
  	while (good_timer_count > 1) {
  		unsigned long estimate;
  		unsigned long maxdiff;
  
  		/* compute the estimate */
  		estimate = (good_timer_sum/good_timer_count);
  		maxdiff = estimate >> 3;
  
  		/* if range is within 12% let's take it */
  		if ((measured_times[max] - measured_times[min]) < maxdiff)
  			return estimate;
  
  		/* ok - drop the worse value and try again... */
  		good_timer_sum = 0;
  		good_timer_count = 0;
  		if ((measured_times[max] - estimate) <
  				(estimate - measured_times[min])) {
  			printk(KERN_NOTICE "calibrate_delay_direct() dropping "
  					"min bogoMips estimate %d = %lu
  ",
  				min, measured_times[min]);
  			measured_times[min] = 0;
  			min = max;
  		} else {
  			printk(KERN_NOTICE "calibrate_delay_direct() dropping "
  					"max bogoMips estimate %d = %lu
  ",
  				max, measured_times[max]);
  			measured_times[max] = 0;
  			max = min;
  		}
  
  		for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  			if (measured_times[i] == 0)
  				continue;
  			good_timer_count++;
  			good_timer_sum += measured_times[i];
  			if (measured_times[i] < measured_times[min])
  				min = i;
  			if (measured_times[i] > measured_times[max])
  				max = i;
  		}
  
  	}
  
  	printk(KERN_NOTICE "calibrate_delay_direct() failed to get a good "
  	       "estimate for loops_per_jiffy.
  Probably due to long platform "
  		"interrupts. Consider using \"lpj=\" boot option.
  ");
  	return 0;
  }
  #else
  static unsigned long calibrate_delay_direct(void)
  {
  	return 0;
  }
  #endif
  
  /*
   * This is the number of bits of precision for the loops_per_jiffy.  Each
   * time we refine our estimate after the first takes 1.5/HZ seconds, so try
   * to start with a good estimate.
   * 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
  
  static unsigned long calibrate_delay_converge(void)
  {
  	/* First stage - slowly accelerate to find initial bounds */
  	unsigned long lpj, lpj_base, ticks, loopadd, loopadd_base, chop_limit;
  	int trials = 0, band = 0, trial_in_band = 0;
  
  	lpj = (1<<12);
  
  	/* wait for "start of" clock tick */
  	ticks = jiffies;
  	while (ticks == jiffies)
  		; /* nothing */
  	/* Go .. */
  	ticks = jiffies;
  	do {
  		if (++trial_in_band == (1<<band)) {
  			++band;
  			trial_in_band = 0;
  		}
  		__delay(lpj * band);
  		trials += band;
  	} while (ticks == jiffies);
  	/*
  	 * We overshot, so retreat to a clear underestimate. Then estimate
  	 * the largest likely undershoot. This defines our chop bounds.
  	 */
  	trials -= band;
  	loopadd_base = lpj * band;
  	lpj_base = lpj * trials;
  
  recalibrate:
  	lpj = lpj_base;
  	loopadd = loopadd_base;
  
  	/*
  	 * Do a binary approximation to get lpj set to
  	 * equal one clock (up to LPS_PREC bits)
  	 */
  	chop_limit = lpj >> LPS_PREC;
  	while (loopadd > chop_limit) {
  		lpj += loopadd;
  		ticks = jiffies;
  		while (ticks == jiffies)
  			; /* nothing */
  		ticks = jiffies;
  		__delay(lpj);
  		if (jiffies != ticks)	/* longer than 1 tick */
  			lpj -= loopadd;
  		loopadd >>= 1;
  	}
  	/*
  	 * If we incremented every single time possible, presume we've
  	 * massively underestimated initially, and retry with a higher
  	 * start, and larger range. (Only seen on x86_64, due to SMIs)
  	 */
  	if (lpj + loopadd * 2 == lpj_base + loopadd_base * 2) {
  		lpj_base = lpj;
  		loopadd_base <<= 2;
  		goto recalibrate;
  	}
  
  	return lpj;
  }
  
  static DEFINE_PER_CPU(unsigned long, cpu_loops_per_jiffy) = { 0 };
  
  /*
   * Check if cpu calibration delay is already known. For example,
   * some processors with multi-core sockets may have all cores
   * with the same calibration delay.
   *
   * Architectures should override this function if a faster calibration
   * method is available.
   */
  unsigned long __attribute__((weak)) calibrate_delay_is_known(void)
  {
  	return 0;
  }
  
  void calibrate_delay(void)
  {
  	unsigned long lpj;
  	static bool printed;
  	int this_cpu = smp_processor_id();
  
  	if (per_cpu(cpu_loops_per_jiffy, this_cpu)) {
  		lpj = per_cpu(cpu_loops_per_jiffy, this_cpu);
  		if (!printed)
  			pr_info("Calibrating delay loop (skipped) "
  				"already calibrated this CPU");
  	} else if (preset_lpj) {
  		lpj = preset_lpj;
  		if (!printed)
  			pr_info("Calibrating delay loop (skipped) "
  				"preset value.. ");
  	} else if ((!printed) && lpj_fine) {
  		lpj = lpj_fine;
  		pr_info("Calibrating delay loop (skipped), "
  			"value calculated using timer frequency.. ");
  	} else if ((lpj = calibrate_delay_is_known())) {
  		;
  	} else if ((lpj = calibrate_delay_direct()) != 0) {
  		if (!printed)
  			pr_info("Calibrating delay using timer "
  				"specific routine.. ");
  	} else {
  		if (!printed)
  			pr_info("Calibrating delay loop... ");
  		lpj = calibrate_delay_converge();
  	}
  	per_cpu(cpu_loops_per_jiffy, this_cpu) = lpj;
  	if (!printed)
  		pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)
  ",
  			lpj/(500000/HZ),
  			(lpj/(5000/HZ)) % 100, lpj);
  
  	loops_per_jiffy = lpj;
  	printed = true;
  }