kernel/lib/stdio/delay.c

   1 /*
   2  *  ZeX/OS
   3  *  Copyright (C) 2007  Tomas 'ZeXx86' Jedrzejek (zexx86@zexos.org)
   4  *  Copyright (C) 2008  Tomas 'ZeXx86' Jedrzejek (zexx86@zexos.org)
   5  *
   6  *  This program is free software: you can redistribute it and/or modify
   7  *  it under the terms of the GNU General Public License as published by
   8  *  the Free Software Foundation, either version 3 of the License, or
   9  *  (at your option) any later version.
  10  *
  11  *  This program is distributed in the hope that it will be useful,
  12  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  13  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14  *  GNU General Public License for more details.
  15  *
  16  *  You should have received a copy of the GNU General Public License
  17  *  along with this program.  If not, see <http://www.gnu.org/licenses/>.
  18  */
  19
  20
  21 /*
  22  * Allright, let's review how we're going to do this.
  23  *
  24  * Goal:
  25  * The goal is to create a reasonably accurate version of the delay()
  26  * function, which creates a delay in multiples of one millisecond.
  27  *
  28  * Restrictions:
  29  * We don't want to use the PIT for the delay.  In your average
  30  * bare-hardware system (ie OS) you'll be using the PIT for quite a few
  31  * other things already (in my OS code, channel 0 and 1 are used by the
  32  * scheduler, and channel 2 would probably be needed to control the
  33  * speaker.)
  34  *
  35  * Method:
  36  * A simple loop would delay the computer wonderfully.  Ie, take something
  37  * like:
  38  *
  39  *         for(i=0;i<BIGNUMBER;i++);  // Delay loop
  40  *
  41  * That kills time wonderfully.  All we need to do is to find out what
  42  * BIGNUMBER we need to use to delay one millisecond.  Finding the correct
  43  * BIGNUMBER for your machine is called *delay loop calibration*.
  44  *
  45  * We can calibrate the delay loop using the PIT.  In the initialisation
  46  * phase of the OS, when the PIT is not yet hooked to the scheduler, it
  47  * can be freely used to do this.  When the delay loop has been calibrated
  48  * against the PIT, the PIT is free to be used for other purposes.
  49  *
  50  * There are many ways to calibrate a delay loop with the PIT.  This is
  51  * one way (used in Linux.)  More documentation in the code.
  52  */
  53
  54 /**************************************************************************/
  55
  56 /*
  57  * When I was testing the delay code, I notice that the calibration we use
  58  * is very delicate.  Originally, I had inlined the delay for() loops into
  59  * the delay() and calibrateDelayLoop() functions, but this doesn't work.
  60  * Due to different register allocation or something similar the compiler
  61  * might have generated different code in the two situations (I didn't
  62  * check this.)  Or alignment differences might have caused the problems.
  63  * Anyway, to solve the problem, we always use the __delay() function for
  64  * the delay for() loop, because it always is the same code with the same
  65  * alignment. __delay() is called from delay() as well as from
  66  * calibrateDelayLoop().  By using __delay() we can fine-tune our
  67  * calibration without it losing its finesse afterwards.
  68  */
  69
  70 void __delay(unsigned long loops)
  71 {
  72     unsigned long c;
  73     for (c = 0; c < loops; c ++);
  74 }
  75
  76 extern unsigned long timer_swticks;
  77
  78 void delay (unsigned long milliseconds)
  79 {
  80     __delay (milliseconds * timer_swticks);     /* Delay milliseconds ms */
  81 }
  82
  83 /* The end */