2.6.11内核CPU频率(khz)的计算

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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

1. /* calculate cpu_khz */
   
2. void __init init_cpu_khz(void)
   
3. {
   
4. if (cpu_has_tsc) {
   
5.   unsigned long tsc_quotient = calibrate_tsc();
   
6.   if (tsc_quotient) {
   
7.    /* report CPU clock rate in Hz.
   
8.     * The formula is (10^6 * 2^32) / (2^32 * 1 / (clocks/us)) =
   
9.     * clock/second. Our precision is about 100 ppm.
   
10.     */
    
11.    { unsigned long eax=0, edx=1000;
    
12.     __asm__("divl %2"
    
13.            :"=a" (cpu_khz), "=d" (edx)
    
14.                   :"r" (tsc_quotient),
    
15.                   "0" (eax), "1" (edx));
    
16.     printk("Detected %lu.%03lu MHz processor.\n", cpu_khz / 1000, cpu_khz % 1000);
    
17.    }
    
18.   }
    
19. }
    
20. }
    

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----------------------------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

1. unsigned long __init calibrate_tsc(void)
   
2. {
   
3. mach_prepare_counter();//设置5ms的计数器
   
4. 
   
5. {
   
6.   unsigned long startlow, starthigh;
   
7.   unsigned long endlow, endhigh;
   
8.   unsigned long count;
   
9. 
   
10.   rdtsc(startlow,starthigh);//读取起始时间戳
    
11.   mach_countup(&count);//等待5ms
    
12.   rdtsc(endlow,endhigh);//读取结束时间戳
    
13. 
    
14. 
    
15.   /* Error: ECTCNEVERSET */
    
16.   if (count <= 1)
    
17.    goto bad_ctc;
    
18. 
    
19. //计算时间戳的计数差值，即5ms中CPU收到的时钟脉冲数
    
20. 
    
21.   /* 64-bit subtract - gcc just messes up with long longs */
    
22.   __asm__("subl %2,%0\n\t"
    
23.    "sbbl %3,%1"
    
24.    :"=a" (endlow), "=d" (endhigh)
    
25.    :"g" (startlow), "g" (starthigh),
    
26.     "0" (endlow), "1" (endhigh));
    
27. 
    
28.   /* Error: ECPUTOOFAST */
    
29.   if (endhigh)
    
30.    goto bad_ctc;
    
31. 
    
32.   /* Error: ECPUTOOSLOW */
    
33.   if (endlow <= CALIBRATE_TIME)
    
34.    goto bad_ctc;
    
35. 
    
36. 
    
37. 
    
38. //计算出(每个clock的微秒数)*(2^32)的值并返回
    
39. 
    
40. 
    
41. 
    
42.   __asm__("divl %2"
    
43.    :"=a" (endlow), "=d" (endhigh)
    
44.    :"r" (endlow), "0" (0), "1" (CALIBRATE_TIME));
    
45. 
    
46.   return endlow;
    
47. }
    
48. 
    
49. /*
    
50.   * The CTC wasn't reliable: we got a hit on the very first read,
    
51.   * or the CPU was so fast/slow that the quotient wouldn't fit in
    
52.   * 32 bits..
    
53.   */
    
54. bad_ctc:
    
55. return 0;
    
56. }
    

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