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2.6.12内核在\mach-s3c2410\time.c中,有这样几个函数
#define TIMER_USEC_SHIFT 16
static inline unsigned long
timer_mask_usec_ticks(unsigned long scaler, unsigned long pclk)
{
unsigned long den = pclk / 1000;
return ((1000 << TIMER_USEC_SHIFT) * scaler + (den >> 1)) / den;
}
static inline unsigned long timer_ticks_to_usec(unsigned long ticks)
{
unsigned long res;
res = ticks * timer_usec_ticks; //timer_usec_ticks = timer_mask_usec_ticks(6, pclk)
res += 1 << (TIMER_USEC_SHIFT - 4); /* round up slightly */
return res >> TIMER_USEC_SHIFT;
}
其中我重点要用到的是 timer_ticks_to_usec() 函数,我不明白这个函数中的最后两句起什么作用,特别是TIMER_USEC_SHIFT 这个宏的作用,请大侠赐教,谢谢! |
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