/* Copyright (C) 2004 David Decotigny

   This program is free software; you can redistribute it and/or
   modify it under the terms of the GNU General Public License
   as published by the Free Software Foundation; either version 2
   of the License, or (at your option) any later version.
   
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
   
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307,
   USA. 
*/

#include <sos/errno.h>
#include <sos/klibc.h>
#include <sos/assert.h>
#include <sos/list.h>

#include "sched.h"


/**
 * The definition of the scheduler queue. We could have used a normal
 * kwaitq here, it would have had the same properties (regarding
 * priority ordering mainly). But we don't bother with size
 * considerations here (in kwaitq, we had better make the kwaitq
 * structure as small as possible because there are a lot of kwaitq in
 * the system: at least 1 per opened file), so that we can implement a
 * much faster way of handling the prioritized jobs.
 */
struct sos_sched_queue
{
  unsigned int nr_threads;
  struct sos_thread *thread_list[SOS_SCHED_NUM_PRIO];
};


/**
 * We manage 2 queues: a queue being scanned for ready threads
 * (active_queue) and a queue to store the threads the threads having
 * expired their time quantuum.
 */
static struct sos_sched_queue *active_queue, *expired_queue;


/**
 * The instances for the active/expired queues
 */
static struct sos_sched_queue sched_queue[2];


sos_ret_t sos_sched_subsystem_setup()
{
  memset(sched_queue, 0x0, sizeof(sched_queue));
  active_queue  = & sched_queue[0];
  expired_queue = & sched_queue[1];

  return SOS_OK;
}


/**
 * Helper function to add a thread in a ready queue AND to change the
 * state of the given thread to "READY".
 *
 * @param insert_at_tail TRUE to tell to add the thread at the end of
 * the ready list. Otherwise it is added at the head of it.
 */
static sos_ret_t add_in_ready_queue(struct sos_sched_queue *q,
				    struct sos_thread *thr,
				    sos_bool_t insert_at_tail)
{
  sos_sched_priority_t prio;

  SOS_ASSERT_FATAL( (SOS_THR_CREATED == thr->state)
		    || (SOS_THR_RUNNING == thr->state) /* Yield */
		    || (SOS_THR_BLOCKED == thr->state) );

  /* Add the thread to the CPU queue */
  prio = sos_thread_get_priority(thr);
  if (insert_at_tail)
    list_add_tail_named(q->thread_list[prio], thr,
			ready.rdy_prev, ready.rdy_next);
  else
    list_add_head_named(q->thread_list[prio], thr,
			ready.rdy_prev, ready.rdy_next);
  thr->ready.rdy_queue = q;
  q->nr_threads ++;

  /* Ok, thread is now really ready to be (re)started */
  thr->state = SOS_THR_READY;

  return SOS_OK;
}


sos_ret_t sos_sched_set_ready(struct sos_thread *thr)
{
  sos_ret_t retval;

  /* Don't do anything for already ready threads */
  if (SOS_THR_READY == thr->state)
    return SOS_OK;

  if (SOS_SCHED_PRIO_IS_RT(sos_thread_get_priority(thr)))
    {
      /* Real-time thread: schedule it for the present turn */
      retval = add_in_ready_queue(active_queue, thr, TRUE);
    }
  else
    {
      /* Non real-time thread: schedule it for next turn */
      retval = add_in_ready_queue(expired_queue, thr, TRUE);
    }

  return retval;
}


sos_ret_t sos_sched_change_priority(struct sos_thread *thr,
				    sos_sched_priority_t priority)
{
  struct sos_thread *thread_list;
  SOS_ASSERT_FATAL(SOS_THR_READY == thr->state);

  /* Temp variable */
  thread_list
    = thr->ready.rdy_queue->thread_list[sos_thread_get_priority(thr)];

  list_delete_named(thread_list, thr, ready.rdy_prev, ready.rdy_next);

  /* Update lists */
  thread_list = thr->ready.rdy_queue->thread_list[priority];
  list_add_tail_named(thread_list, thr, ready.rdy_prev, ready.rdy_next);
  thr->ready.rdy_queue->thread_list[priority] = thread_list;

  return SOS_OK;
}


struct sos_thread * sos_reschedule(struct sos_thread *current_thread,
				   sos_bool_t do_yield)
{
  sos_sched_priority_t prio;

  if (SOS_THR_ZOMBIE == current_thread->state)
    {
      /* Don't think of returning to this thread since it is
	 terminated */
      /* Nop */
    }
  else if (SOS_THR_BLOCKED != current_thread->state)
    {
      /* Take into account the current executing thread unless it is
	 marked blocked */
      if (do_yield)
	{
	  /* Ok, reserve it for next turn */
	  if (SOS_SCHED_PRIO_IS_RT(sos_thread_get_priority(current_thread)))
	    add_in_ready_queue(active_queue, current_thread, TRUE);
	  else
	    add_in_ready_queue(expired_queue, current_thread, TRUE);
	}
      else
	{
	  /* Put it at the head of the active list */
	  add_in_ready_queue(active_queue, current_thread, FALSE);
	}
    }


  /* Active queue is empty ? */
  if (active_queue->nr_threads <= 0)
    {
      /* Yes: Exchange it with the expired queue */
      struct sos_sched_queue *q;
      q = active_queue;
      active_queue = expired_queue;
      expired_queue = q;
    }

  /* Now loop over the priorities in the active queue, looking for a
     non-empty queue */
  for (prio = SOS_SCHED_PRIO_HIGHEST ; prio <= SOS_SCHED_PRIO_LOWEST ; prio ++)
    {
      struct sos_thread *next_thr;

      if (list_is_empty_named(active_queue->thread_list[prio],
			      ready.rdy_prev, ready.rdy_next))
	continue;
      
      /* Queue is not empty: take the thread at its head */
      next_thr = list_pop_head_named(active_queue->thread_list[prio],
				     ready.rdy_prev, ready.rdy_next);
      active_queue->nr_threads --;

      return next_thr;
    }


  SOS_FATAL_ERROR("No kernel thread ready ?!");
  return NULL;
}