title: How usecs_to_jiffies transforms usecs to jiffies if jiffies are in resolution of msecs?
body: <p>from <a href="http://www.kernel.org/doc/man-pages/online/pages/man7/time.7.html" rel="nofollow">here</a>:</p>
  
  <blockquote>
    <p>The value of HZ varies across kernel versions and
    hardware platforms.  On i386
           the situation is as follows: on kernels up to and including 2.4.x, HZ was 100,
           giving a jiffy value of 0.01 seconds; starting with 2.6.0, HZ was raised to
           1000, giving a jiffy of 0.001 seconds.  Since kernel 2.6.13, the HZ value is a
           kernel configuration parameter and can be 100, 250 (the default) or 1000,
           yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds.
           Since kernel 2.6.20, a further frequency is available: 300, a number that
           divides evenly for the common video frame rates (PAL, 25 HZ; NTSC, 30 HZ).</p>
  </blockquote>
  
  <p>So how can I transform <code>5usec</code> to <code>jiffies</code>? </p>
  
  <pre><code>extern unsigned long usecs_to_jiffies(const unsigned int u);
  </code></pre>
  
  <p>it seems to be useless since jiffies resolution is not high enough to measure useconds. </p>