source: branches/minix3-book/kernel/clock.c@ 15

/* This file contains the clock task, which handles time related functions.
 * Important events that are handled by the CLOCK include setting and 
 * monitoring alarm timers and deciding when to (re)schedule processes. 
 * The CLOCK offers a direct interface to kernel processes. System services 
 * can access its services through system calls, such as sys_setalarm(). The
 * CLOCK task thus is hidden from the outside world.  
 *
 * Changes:
 *   Oct 08, 2005   reordering and comment editing (A. S. Woodhull)
 *   Mar 18, 2004   clock interface moved to SYSTEM task (Jorrit N. Herder) 
 *   Sep 30, 2004   source code documentation updated  (Jorrit N. Herder)
 *   Sep 24, 2004   redesigned alarm timers  (Jorrit N. Herder)
 *
 * The function do_clocktick() is triggered by the clock's interrupt 
 * handler when a watchdog timer has expired or a process must be scheduled. 
 *
 * In addition to the main clock_task() entry point, which starts the main 
 * loop, there are several other minor entry points:
 *   clock_stop:        called just before MINIX shutdown
 *   get_uptime:        get realtime since boot in clock ticks
 *   set_timer:         set a watchdog timer (+)
 *   reset_timer:       reset a watchdog timer (+)
 *   read_clock:        read the counter of channel 0 of the 8253A timer
 *
 * (+) The CLOCK task keeps tracks of watchdog timers for the entire kernel.
 * The watchdog functions of expired timers are executed in do_clocktick(). 
 * It is crucial that watchdog functions not block, or the CLOCK task may
 * be blocked. Do not send() a message when the receiver is not expecting it.
 * Instead, notify(), which always returns, should be used. 
 */

#include "kernel.h"
#include "proc.h"
#include <signal.h>
#include <minix/com.h>

/* Function prototype for PRIVATE functions. */ 
FORWARD _PROTOTYPE( void init_clock, (void) );
FORWARD _PROTOTYPE( int clock_handler, (irq_hook_t *hook) );
FORWARD _PROTOTYPE( int do_clocktick, (message *m_ptr) );

/* Clock parameters. */
#define COUNTER_FREQ (2*TIMER_FREQ) /* counter frequency using square wave */
#define LATCH_COUNT     0x00    /* cc00xxxx, c = channel, x = any */
#define SQUARE_WAVE     0x36    /* ccaammmb, a = access, m = mode, b = BCD */
                                /*   11x11, 11 = LSB then MSB, x11 = sq wave */
#define TIMER_COUNT ((unsigned) (TIMER_FREQ/HZ)) /* initial value for counter*/
#define TIMER_FREQ  1193182L    /* clock frequency for timer in PC and AT */

#define CLOCK_ACK_BIT   0x80    /* PS/2 clock interrupt acknowledge bit */

/* The CLOCK's timers queue. The functions in <timers.h> operate on this. 
 * Each system process possesses a single synchronous alarm timer. If other 
 * kernel parts want to use additional timers, they must declare their own 
 * persistent (static) timer structure, which can be passed to the clock
 * via (re)set_timer().
 * When a timer expires its watchdog function is run by the CLOCK task. 
 */
PRIVATE timer_t *clock_timers;          /* queue of CLOCK timers */
PRIVATE clock_t next_timeout;           /* realtime that next timer expires */

/* The time is incremented by the interrupt handler on each clock tick. */
PRIVATE clock_t realtime;               /* real time clock */
PRIVATE irq_hook_t clock_hook;          /* interrupt handler hook */

/*===========================================================================*
 *                              clock_task                                   *
 *===========================================================================*/
PUBLIC void clock_task()
{
/* Main program of clock task. If the call is not HARD_INT it is an error.
 */
  message m;                    /* message buffer for both input and output */
  int result;                   /* result returned by the handler */

  init_clock();                 /* initialize clock task */

  /* Main loop of the clock task.  Get work, process it. Never reply. */
  while (TRUE) {

      /* Go get a message. */
      receive(ANY, &m); 

      /* Handle the request. Only clock ticks are expected. */
      switch (m.m_type) {
      case HARD_INT:
          result = do_clocktick(&m);    /* handle clock tick */
          break;
      default:                          /* illegal request type */
          kprintf("CLOCK: illegal request %d from %d.\n", m.m_type,m.m_source);
      }
  }
}

/*===========================================================================*
 *                              do_clocktick                                 *
 *===========================================================================*/
PRIVATE int do_clocktick(m_ptr)
message *m_ptr;                         /* pointer to request message */
{
/* Despite its name, this routine is not called on every clock tick. It
 * is called on those clock ticks when a lot of work needs to be done.
 */

  /* A process used up a full quantum. The interrupt handler stored this
   * process in 'prev_ptr'.  First make sure that the process is not on the 
   * scheduling queues.  Then announce the process ready again. Since it has 
   * no more time left, it gets a new quantum and is inserted at the right 
   * place in the queues.  As a side-effect a new process will be scheduled.
   */ 
  if (prev_ptr->p_ticks_left <= 0 && priv(prev_ptr)->s_flags & PREEMPTIBLE) {
      lock_dequeue(prev_ptr);           /* take it off the queues */
      lock_enqueue(prev_ptr);           /* and reinsert it again */ 
  }

  /* Check if a clock timer expired and run its watchdog function. */
  if (next_timeout <= realtime) { 
        tmrs_exptimers(&clock_timers, realtime, NULL);
        next_timeout = clock_timers == NULL ? 
                TMR_NEVER : clock_timers->tmr_exp_time;
  }

  /* Inhibit sending a reply. */
  return(EDONTREPLY);
}

/*===========================================================================*
 *                              init_clock                                   *
 *===========================================================================*/
PRIVATE void init_clock()
{
  /* Initialize the CLOCK's interrupt hook. */
  clock_hook.proc_nr = CLOCK;

  /* Initialize channel 0 of the 8253A timer to, e.g., 60 Hz. */
  outb(TIMER_MODE, SQUARE_WAVE);        /* set timer to run continuously */
  outb(TIMER0, TIMER_COUNT);            /* load timer low byte */
  outb(TIMER0, TIMER_COUNT >> 8);       /* load timer high byte */
  put_irq_handler(&clock_hook, CLOCK_IRQ, clock_handler);/* register handler */
  enable_irq(&clock_hook);              /* ready for clock interrupts */
}

/*===========================================================================*
 *                              clock_stop                                   *
 *===========================================================================*/
PUBLIC void clock_stop()
{
/* Reset the clock to the BIOS rate. (For rebooting) */
  outb(TIMER_MODE, 0x36);
  outb(TIMER0, 0);
  outb(TIMER0, 0);
}

/*===========================================================================*
 *                              clock_handler                                *
 *===========================================================================*/
PRIVATE int clock_handler(hook)
irq_hook_t *hook;
{
/* This executes on each clock tick (i.e., every time the timer chip generates 
 * an interrupt). It does a little bit of work so the clock task does not have 
 * to be called on every tick.  The clock task is called when:
 *
 *      (1) the scheduling quantum of the running process has expired, or
 *      (2) a timer has expired and the watchdog function should be run.
 *
 * Many global global and static variables are accessed here.  The safety of
 * this must be justified. All scheduling and message passing code acquires a 
 * lock by temporarily disabling interrupts, so no conflicts with calls from 
 * the task level can occur. Furthermore, interrupts are not reentrant, the 
 * interrupt handler cannot be bothered by other interrupts.
 * 
 * Variables that are updated in the clock's interrupt handler:
 *      lost_ticks:
 *              Clock ticks counted outside the clock task. This for example
 *              is used when the boot monitor processes a real mode interrupt.
 *      realtime:
 *              The current uptime is incremented with all outstanding ticks.
 *      proc_ptr, bill_ptr:
 *              These are used for accounting.  It does not matter if proc.c
 *              is changing them, provided they are always valid pointers,
 *              since at worst the previous process would be billed.
 */
  register unsigned ticks;

  /* Acknowledge the PS/2 clock interrupt. */
  if (machine.ps_mca) outb(PORT_B, inb(PORT_B) | CLOCK_ACK_BIT);

  /* Get number of ticks and update realtime. */
  ticks = lost_ticks + 1;
  lost_ticks = 0;
  realtime += ticks;

  /* Update user and system accounting times. Charge the current process for
   * user time. If the current process is not billable, that is, if a non-user
   * process is running, charge the billable process for system time as well.
   * Thus the unbillable process' user time is the billable user's system time.
   */
  proc_ptr->p_user_time += ticks;
  if (priv(proc_ptr)->s_flags & PREEMPTIBLE) {
      proc_ptr->p_ticks_left -= ticks;
  }
  if (! (priv(proc_ptr)->s_flags & BILLABLE)) {
      bill_ptr->p_sys_time += ticks;
      bill_ptr->p_ticks_left -= ticks;
  }

  /* Check if do_clocktick() must be called. Done for alarms and scheduling.
   * Some processes, such as the kernel tasks, cannot be preempted. 
   */ 
  if ((next_timeout <= realtime) || (proc_ptr->p_ticks_left <= 0)) {
      prev_ptr = proc_ptr;                      /* store running process */
      lock_notify(HARDWARE, CLOCK);             /* send notification */
  } 
  return(1);                                    /* reenable interrupts */
}

/*===========================================================================*
 *                              get_uptime                                   *
 *===========================================================================*/
PUBLIC clock_t get_uptime()
{
/* Get and return the current clock uptime in ticks. */
  return(realtime);
}

/*===========================================================================*
 *                              set_timer                                    *
 *===========================================================================*/
PUBLIC void set_timer(tp, exp_time, watchdog)
struct timer *tp;               /* pointer to timer structure */
clock_t exp_time;               /* expiration realtime */
tmr_func_t watchdog;            /* watchdog to be called */
{
/* Insert the new timer in the active timers list. Always update the 
 * next timeout time by setting it to the front of the active list.
 */
  tmrs_settimer(&clock_timers, tp, exp_time, watchdog, NULL);
  next_timeout = clock_timers->tmr_exp_time;
}

/*===========================================================================*
 *                              reset_timer                                  *
 *===========================================================================*/
PUBLIC void reset_timer(tp)
struct timer *tp;               /* pointer to timer structure */
{
/* The timer pointed to by 'tp' is no longer needed. Remove it from both the
 * active and expired lists. Always update the next timeout time by setting
 * it to the front of the active list.
 */
  tmrs_clrtimer(&clock_timers, tp, NULL);
  next_timeout = (clock_timers == NULL) ? 
        TMR_NEVER : clock_timers->tmr_exp_time;
}

/*===========================================================================*
 *                              read_clock                                   *
 *===========================================================================*/
PUBLIC unsigned long read_clock()
{
/* Read the counter of channel 0 of the 8253A timer.  This counter counts
 * down at a rate of TIMER_FREQ and restarts at TIMER_COUNT-1 when it
 * reaches zero. A hardware interrupt (clock tick) occurs when the counter
 * gets to zero and restarts its cycle.  
 */
  unsigned count;

  outb(TIMER_MODE, LATCH_COUNT);
  count = inb(TIMER0);
  count |= (inb(TIMER0) << 8);
  
  return count;
}