Editing BogoMips (section)

== Computation of BogoMips ==
With kernel 2.6.x, BogoMips are implemented in the <code>/usr/src/linux/init/calibrate.c</code> kernel source file. It computes the Linux kernel timing parameter <code>loops_per_jiffy</code> (see [[jiffy (time)|jiffy]]) value. The explanation from source code:
<pre><nowiki>
  /*
   * A simple loop like
   *  while ( jiffies < start_jiffies+1)
   *    start = read_current_timer();
   * will not do. As we don't really know whether jiffy switch
   * happened first or timer_value was read first. And some asynchronous
   * event can happen between these two events introducing errors in lpj.
   *
   * So, we do
   * 1. pre_start <- When we are sure that jiffy switch hasn't happened
   * 2. check jiffy switch
   * 3. start <- timer value before or after jiffy switch
   * 4. post_start <- When we are sure that jiffy switch has happened
   *
   * Note, we don't know anything about order of 2 and 3.
   * Now, by looking at post_start and pre_start difference, we can
   * check whether any asynchronous event happened or not
   */
</nowiki></pre>

<code>loops_per_jiffy</code> is used to implement <code>udelay</code> (delay in microseconds) and <code>ndelay</code> (delay in nanoseconds) functions. These functions are needed by some drivers to wait for hardware. Note that a [[busy waiting]] technique is used, so the kernel is effectively blocked when executing <code>ndelay</code>/<code>udelay</code> functions. For i386 architecture <code>delay_loop</code> is implemented in <code>/usr/src/linux/arch/i386/lib/delay.c</code> as:
<syntaxhighlight lang="c">
/* simple loop based delay: */
static void delay_loop(unsigned long loops)
{
  int d0;

  __asm__ __volatile__(
    "\tjmp 1f\n"
    ".align 16\n"
    "1:\tjmp 2f\n"
    ".align 16\n"
    "2:\tdecl %0\n\tjns 2b"
    :"=&a" (d0)
    :"0" (loops));
}
</syntaxhighlight>
equivalent to the following assembler code
<syntaxhighlight lang="asm">
;  input: eax = d0
; output: eax = 0
       jmp  start
.align 16
start: jmp  body
.align 16
body:  decl eax
       jns  body
</syntaxhighlight>
which can be rewritten to C-pseudocode
<syntaxhighlight lang="c">
static void delay_loop(long loops)
{
  long d0 = loops;
  do {
    --d0;
  } while (d0 >= 0);
}
</syntaxhighlight>

Full and complete information and details about BogoMips, and hundreds of reference entries can be found in the (outdated) BogoMips mini-Howto.<ref name=howto/>