text: /*
   * arch/x86_64/kernel/tsc_sync.c: check TSC synchronization.
   *
   * Copyright (C) 2006, Red Hat, Inc., Ingo Molnar
   *
   * We check whether all boot CPUs have their TSC's synchronized,
   * print a warning if not and turn off the TSC clock-source.
   *
   * The warp-check is point-to-point between two CPUs, the CPU
   * initiating the bootup is the 'source CPU', the freshly booting
   * CPU is the 'target CPU'.
   *
   * Only two CPUs may participate - they can enter in any order.
   * ( The serial nature of the boot logic and the CPU hotplug lock
   *   protects against more than 2 CPUs entering this code. )
   */
  #include <linux/spinlock.h>
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/smp.h>
  #include <linux/nmi.h>
  #include <asm/tsc.h>
  
  /*
   * Entry/exit counters that make sure that both CPUs
   * run the measurement code at once:
   */
  static __cpuinitdata atomic_t start_count;
  static __cpuinitdata atomic_t stop_count;
  
  /*
   * We use a raw spinlock in this exceptional case, because
   * we want to have the fastest, inlined, non-debug version
   * of a critical section, to be able to prove TSC time-warps:
   */
  static __cpuinitdata raw_spinlock_t sync_lock = __RAW_SPIN_LOCK_UNLOCKED;
  static __cpuinitdata cycles_t last_tsc;
  static __cpuinitdata cycles_t max_warp;
  static __cpuinitdata int nr_warps;
  
  /*
   * TSC-warp measurement loop running on both CPUs:
   */
  static __cpuinit void check_tsc_warp(void)
  {
  	cycles_t start, now, prev, end;
  	int i;
  
  	start = get_cycles_sync();
  	/*
  	 * The measurement runs for 20 msecs:
  	 */
  	end = start + tsc_khz * 20ULL;
  	now = start;
  
  	for (i = 0; ; i++) {
  		/*
  		 * We take the global lock, measure TSC, save the
  		 * previous TSC that was measured (possibly on
  		 * another CPU) and update the previous TSC timestamp.
  		 */
  		__raw_spin_lock(&sync_lock);
  		prev = last_tsc;
  		now = get_cycles_sync();
  		last_tsc = now;
  		__raw_spin_unlock(&sync_lock);
  
  		/*
  		 * Be nice every now and then (and also check whether
  		 * measurement is done [we also insert a 100 million
  		 * loops safety exit, so we dont lock up in case the
  		 * TSC readout is totally broken]):
  		 */
  		if (unlikely(!(i & 7))) {
  			if (now > end || i > 100000000)
  				break;
  			cpu_relax();
  			touch_nmi_watchdog();
  		}
  		/*
  		 * Outside the critical section we can now see whether
  		 * we saw a time-warp of the TSC going backwards:
  		 */
  		if (unlikely(prev > now)) {
  			__raw_spin_lock(&sync_lock);
  			max_warp = max(max_warp, prev - now);
  			nr_warps++;
  			__raw_spin_unlock(&sync_lock);
  		}
  
  	}
  }
  
  /*
   * Source CPU calls into this - it waits for the freshly booted
   * target CPU to arrive and then starts the measurement:
   */
  void __cpuinit check_tsc_sync_source(int cpu)
  {
  	int cpus = 2;
  
  	/*
  	 * No need to check if we already know that the TSC is not
  	 * synchronized:
  	 */
  	if (unsynchronized_tsc())
  		return;
  
  	printk(KERN_INFO "checking TSC synchronization [CPU#%d -> CPU#%d]:",
  			  smp_processor_id(), cpu);
  
  	/*
  	 * Reset it - in case this is a second bootup:
  	 */
  	atomic_set(&stop_count, 0);
  
  	/*
  	 * Wait for the target to arrive:
  	 */
  	while (atomic_read(&start_count) != cpus-1)
  		cpu_relax();
  	/*
  	 * Trigger the target to continue into the measurement too:
  	 */
  	atomic_inc(&start_count);
  
  	check_tsc_warp();
  
  	while (atomic_read(&stop_count) != cpus-1)
  		cpu_relax();
  
  	/*
  	 * Reset it - just in case we boot another CPU later:
  	 */
  	atomic_set(&start_count, 0);
  
  	if (nr_warps) {
  		printk("\n");
  		printk(KERN_WARNING "Measured %Ld cycles TSC warp between CPUs,"
  				    " turning off TSC clock.\n", max_warp);
  		mark_tsc_unstable("check_tsc_sync_source failed");
  		nr_warps = 0;
  		max_warp = 0;
  		last_tsc = 0;
  	} else {
  		printk(" passed.\n");
  	}
  
  	/*
  	 * Let the target continue with the bootup:
  	 */
  	atomic_inc(&stop_count);
  }
  
  /*
   * Freshly booted CPUs call into this:
   */
  void __cpuinit check_tsc_sync_target(void)
  {
  	int cpus = 2;
  
  	if (unsynchronized_tsc())
  		return;
  
  	/*
  	 * Register this CPU's participation and wait for the
  	 * source CPU to start the measurement:
  	 */
  	atomic_inc(&start_count);
  	while (atomic_read(&start_count) != cpus)
  		cpu_relax();
  
  	check_tsc_warp();
  
  	/*
  	 * Ok, we are done:
  	 */
  	atomic_inc(&stop_count);
  
  	/*
  	 * Wait for the source CPU to print stuff:
  	 */
  	while (atomic_read(&stop_count) != cpus)
  		cpu_relax();
  }
  #undef NR_LOOPS
  
  