2.6.11内核CPU频率(khz)的计算

CPU频率的计算使用了两个函数init_cpu_khz()和calibrate_tsc()

 

 

----------------------------arch/i386/kernel/timers/Common.c--------------------

//tsc_quotient中保存通过calibrate_tsc()计算得出(CPU单个时钟周期的微秒数)*(2^32)的值，然后用1毫秒的微秒数即1000乘以2^32，再除以tsc_quotient，

//分子分母中的2^32抵消，最后的结果是1毫秒内的时钟周期数，即cpu频率的khz数

 

/* calculate cpu_khz */void __init init_cpu_khz(void){ if (cpu_has_tsc) {  unsigned long tsc_quotient = calibrate_tsc();  if (tsc_quotient) {   /* report CPU clock rate in Hz.    * The formula is (10^6 * 2^32) / (2^32 * 1 / (clocks/us)) =    * clock/second. Our precision is about 100 ppm.    */   { unsigned long eax=0, edx=1000;    __asm__("divl %2"           :"=a" (cpu_khz), "=d" (edx)                  :"r" (tsc_quotient),                  "0" (eax), "1" (edx));    printk("Detected %lu.%03lu MHz processor.\n", cpu_khz / 1000, cpu_khz % 1000);   }  } }}

 

----------------------------arch/i386/kernel/timers/Common.c--------------------

//计算并返回(CPU单个时钟周期的微秒数)*(2^32)的值

unsigned long __init calibrate_tsc(void){ mach_prepare_counter();//设置5ms的计数器 {  unsigned long startlow, starthigh;  unsigned long endlow, endhigh;  unsigned long count;  rdtsc(startlow,starthigh);//读取起始时间戳  mach_countup(&count);//等待5ms  rdtsc(endlow,endhigh);//读取结束时间戳  /* Error: ECTCNEVERSET */  if (count <= 1)   goto bad_ctc;//计算时间戳的计数差值，即5ms中CPU收到的时钟脉冲数  /* 64-bit subtract - gcc just messes up with long longs */  __asm__("subl %2,%0\n\t"   "sbbl %3,%1"   :"=a" (endlow), "=d" (endhigh)   :"g" (startlow), "g" (starthigh),    "0" (endlow), "1" (endhigh));  /* Error: ECPUTOOFAST */  if (endhigh)   goto bad_ctc;  /* Error: ECPUTOOSLOW */  if (endlow <= CALIBRATE_TIME)   goto bad_ctc; //计算出(每个clock的微秒数)*(2^32)的值并返回   __asm__("divl %2"   :"=a" (endlow), "=d" (endhigh)   :"r" (endlow), "0" (0), "1" (CALIBRATE_TIME));  return endlow; } /*  * The CTC wasn't reliable: we got a hit on the very first read,  * or the CPU was so fast/slow that the quotient wouldn't fit in  * 32 bits..  */bad_ctc: return 0;}