LddUsingDelays < Training2

%SECTION0{name=LddUsingDelays}% How to delay execution

   * Linux timekeeping: =HZ=, jiffies

   * sometimes we need to delay execution, for example
      * short: device initialization, timeouts, ...
      * longer: need to do something at certain time

*bad: burning CPU cycles (don't use)*

<verbatim>
#define DELAY_MS 100
unsigned long j = jiffies + DELAY_MS * HZ / 1000;

while (time_before(jiffies, j1))
      cpu_relax();
</verbatim>

   * =cpu_relax= means _not doing much_. Calls architecture dependent
     code, actually only a memory barrier on powerpc.


*slightly better but still wasteful:*

<verbatim>
while (time_before(jiffies, j1))
      schedule();
</verbatim>

   * others can run, but system stays loaded (we stay in run queue of
     the scheduler)


*recommended for small (busy waiting) delays*

<verbatim>
void ndelay(unsigned long nsecs);
void udelay(unsigned long usecs);
void mdelay(unsigned long msecs);
</verbatim>

*better: sleep instead of busy waiting if possible:*

<verbatim>
void msleep(unsigned int ms);

/* 
 * interruptible version (for waitqueues)
 * return 0 or if interrupted remaining time
 */
unsigned long msleep_interruptible(unsigned int ms);

void ssleep(unsigned int sec);
</verbatim>

   * Interesting: =msleep(1)= can sleep for up to 20ms
    [[http://thread.gmane.org/gmane.linux.kernel/555287][lkml thread]]. 
    Proposed solution: implement msleep with hrtimers.
    

*longer delays: use waitqueues!:*

<verbatim>
wait_queue_head_t wait;
init_waitqueue_head(&wait);

/* condition is always false=0, delay in jiffies  */
wait_event_interruptible_timeout(wait, 0, delay);
</verbatim>
5.7.5. Allocating Memory		1. Denx Training Topics		5.7.7. Kernel library functions
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