Want to understand one patch Post: 302921569

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Operating Systems Linux Want to understand one patch Post 302921569 by shivnandan kuma on Saturday 18th of October 2014 02:03:07 PM

10-18-2014

Registered User

Want to understand one patch

I just wanted to understand the timer patch in timer.c.
It has already been merged in linux mainline kernel

It says that

Quote:

On architectures with sizeof(int) < sizeof (long), the
computation of mask inside apply_slack() can be undefined if the
computed bit is > 32.

I did not understand how it calcuates expires_limit as 0x20000000e from given value of expires=0xffffe6f5 and slack=25.

Any help will be apreciable.

E.g. with: expires = 0xffffe6f5 and slack = 25, we get:

Code:

   expires_limit = 0x20000000e
   bit = 33
   mask = (1 << 33) - 1  /* undefined */

It calculates expires_limit from the function apply_stack.

Code:

   static inline`enter code here`
     unsigned long apply_slack(struct timer_list *timer, unsigned long expires)
    {
         unsigned long expires_limit, mask;
         int bit;
 
         if (timer->slack >= 0) {`enter code here`
                 expires_limit = expires + timer->slack;
         } else {
                 long delta = expires - jiffies;
 
                 if (delta < 256)
                         return expires;
 
                expires_limit = expires + delta / 256;
         }
         mask = expires ^ expires_limit;
         if (mask == 0)
                 return expires;
 
         bit = find_last_bit(&mask, BITS_PER_LONG);
 
         mask = (1 << bit) - 1;

          expires_limit = expires_limit & ~(mask);
         return expires_limit;
    }

I am unable to understand how expires_limit = 0x20000000e??
Any help will be appreciated .

Thanks,

Last edited by Scrutinizer; 10-18-2014 at 03:08 PM.. Reason: Quote tags and code tags

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LEARN ABOUT CENTOS

sdl_settimer

SDL_SetTimer(3) 						 SDL API Reference						   SDL_SetTimer(3)

NAME

       SDL_SetTimer - Set a callback to run after the specified number of milliseconds has elapsed.

SYNOPSIS

       #include "SDL.h"

       int SDL_SetTimer(Uint32 interval, SDL_TimerCallback callback);

CALLBACK

       /* Function prototype for the timer callback function */ typedef Uint32 (*SDL_TimerCallback)(Uint32 interval);

DESCRIPTION

       Set  a  callback  to run after the specified number of milliseconds has elapsed. The callback function is passed the current timer interval
       and returns the next timer interval. If the returned value is the same as the one passed in, the periodic alarm continues, otherwise a  new
       alarm is scheduled.

       To cancel a currently running timer, call SDL_SetTimer(0, NULL);

       The timer callback function may run in a different thread than your main constant, and so shouldn't call any functions from within itself.

       The  maximum  resolution  of this timer is 10 ms, which means that if you request a 16 ms timer, your callback will run approximately 20 ms
       later on an unloaded system. If you wanted to set a flag signaling a frame update at 30 frames per second (every 33 ms), you  might  set  a
       timer for 30 ms (see example below).

       If you use this function, you need to pass SDL_INIT_TIMER to SDL_Init().

	      Note:

	      This  function  is  kept for compatibility but has been superseded by the new timer functions SDL_AddTimer and SDL_RemoveTimer which
	      support multiple timers.

EXAMPLES

       SDL_SetTimer((33/10)*10, my_callback);

SEE ALSO

       SDL_AddTimer

SDL
							      Tue 11 Sep 2001, 23:01						   SDL_SetTimer(3)