#include <linux/fs.h>
#include <linux/notifier.h>
#include <linux/cpufreq.h>

static int my_cpufreq_notifier( struct notifier_block *nb, unsigned long val, void *data )
{
   struct cpufreq_freqs *freq = data;

   switch( val ) {
   case CPUFREQ_PRECHANGE:
      printk("CPUFREQ_PRECHANGE ");
      break;
   case CPUFREQ_POSTCHANGE:
      printk("CPUFREQ_POSTCHANGE ");
      break;
   case CPUFREQ_RESUMECHANGE:
      printk("CPUFREQ_RESUMECHANGE ");
      break;
   case CPUFREQ_SUSPENDCHANGE:
      printk("CPUFREQ_SUSPENDCHANGE ");
      break;
   default:
      printk("unknown val\n");
      break;
   }
   printk("cpu: %d, old: %d, new: %d\n", freq->cpu, freq->old, freq->new );
   return 0;
}

static struct notifier_block nb={
   .notifier_call = my_cpufreq_notifier,
};

static int __init mod_init(void)
{
   if( cpufreq_register_notifier(&nb,CPUFREQ_TRANSITION_NOTIFIER) ) {
      printk("cpufreq_register_notifer failed\n");
      return -EIO;
   }
   printk("mod_init: cpufreq_get: %d\n", cpufreq_get(0));// CPU 0
   return 0;
}

static void __exit mod_exit(void)
{
   cpufreq_unregister_notifier( &nb, CPUFREQ_TRANSITION_NOTIFIER );
}

module_init( mod_init );
module_exit( mod_exit );
MODULE_LICENSE("GPL");