/****************************************************************************
 * mm/iob/iob_alloc_qentry.c
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.  The
 * ASF licenses this file to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance with the
 * License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
 * License for the specific language governing permissions and limitations
 * under the License.
 *
 ****************************************************************************/

/****************************************************************************
 * Included Files
 ****************************************************************************/

#include <nuttx/config.h>

#include <assert.h>
#include <errno.h>

#include <nuttx/irq.h>
#include <nuttx/arch.h>
#include <nuttx/mm/iob.h>

#include "iob.h"

#if CONFIG_IOB_NCHAINS > 0

/****************************************************************************
 * Private Functions
 ****************************************************************************/

/****************************************************************************
 * Name: iob_alloc_qcommitted
 *
 * Description:
 *   Allocate an I/O buffer by taking the buffer at the head of the committed
 *   list.
 *
 ****************************************************************************/

static FAR struct iob_qentry_s *iob_alloc_qcommitted(void)
{
  FAR struct iob_qentry_s *iobq = NULL;
  irqstate_t flags;

  /* We don't know what context we are called from so we use extreme measures
   * to protect the committed list:  We disable interrupts very briefly.
   */

  flags = spin_lock_irqsave(&g_iob_lock);

  /* Take the I/O buffer from the head of the committed list */

  iobq = g_iob_qcommitted;
  if (iobq != NULL)
    {
      /* Remove the I/O buffer from the committed list */

      g_iob_qcommitted = iobq->qe_flink;

      /* Put the I/O buffer in a known state */

      iobq->qe_head = NULL; /* Nothing is contained */
    }

  spin_unlock_irqrestore(&g_iob_lock, flags);
  return iobq;
}

/****************************************************************************
 * Name: iob_allocwait_qentry
 *
 * Description:
 *   Allocate an I/O buffer chain container by taking the buffer at the head
 *   of the free list.  This function is intended only for internal use by
 *   the IOB module.
 *
 ****************************************************************************/

static FAR struct iob_qentry_s *iob_allocwait_qentry(void)
{
  FAR struct iob_qentry_s *qentry;
  irqstate_t flags;
  int ret = OK;

  /* The following must be atomic; interrupt must be disabled so that there
   * is no conflict with interrupt level I/O buffer chain container
   * allocations.  This is not as bad as it sounds because interrupts will be
   * re-enabled while we are waiting for I/O buffers to become free.
   */

  flags = enter_critical_section();

  /* Try to get an I/O buffer chain container.  If successful, the semaphore
   * count will bedecremented atomically.
   */

  qentry = iob_tryalloc_qentry();
  while (ret == OK && qentry == NULL)
    {
      /* If not successful, then the semaphore count was less than or equal
       * to zero (meaning that there are no free buffers).  We need to wait
       * for an I/O buffer chain container to be released when the
       * semaphore count will be incremented.
       */

      ret = nxsem_wait_uninterruptible(&g_qentry_sem);
      if (ret >= 0)
        {
          /* When we wake up from wait successfully, an I/O buffer chain
           * container was freed and we hold a count for one IOB.  Unless
           * something failed, we should have an IOB waiting for us in the
           * committed list.
           */

          qentry = iob_alloc_qcommitted();
          DEBUGASSERT(qentry != NULL);

          if (qentry == NULL)
            {
              /* This should not fail, but we allow for that possibility to
               * handle any potential, non-obvious race condition.  Perhaps
               * the free IOB ended up in the g_iob_free list?
               *
               * We need release our count so that it is available to
               * iob_tryalloc(), perhaps allowing another thread to take our
               * count.  In that event, iob_tryalloc() will fail above and
               * we will have to wait again.
               */

              nxsem_post(&g_qentry_sem);
              qentry = iob_tryalloc_qentry();
            }
        }
    }

  leave_critical_section(flags);
  return qentry;
}

/****************************************************************************
 * Public Functions
 ****************************************************************************/

/****************************************************************************
 * Name: iob_alloc_qentry
 *
 * Description:
 *   Allocate an I/O buffer chain container by taking the buffer at the head
 *   of the free list. This function is intended only for internal use by
 *   the IOB module.
 *
 ****************************************************************************/

FAR struct iob_qentry_s *iob_alloc_qentry(void)
{
  /* Were we called from the interrupt level? */

  if (up_interrupt_context() || sched_idletask())
    {
      /* Yes, then try to allocate an I/O buffer without waiting */

      return iob_tryalloc_qentry();
    }
  else
    {
      /* Then allocate an I/O buffer, waiting as necessary */

      return iob_allocwait_qentry();
    }
}

/****************************************************************************
 * Name: iob_tryalloc_qentry
 *
 * Description:
 *   Try to allocate an I/O buffer chain container by taking the buffer at
 *   the head of the free list without waiting for the container to become
 *   free. This function is intended only for internal use by the IOB module.
 *
 ****************************************************************************/

FAR struct iob_qentry_s *iob_tryalloc_qentry(void)
{
  FAR struct iob_qentry_s *iobq;
  irqstate_t flags;

  /* We don't know what context we are called from so we use extreme measures
   * to protect the free list:  We disable interrupts very briefly.
   */

  flags = spin_lock_irqsave(&g_iob_lock);
  iobq  = g_iob_freeqlist;
  if (iobq)
    {
      /* Remove the I/O buffer chain container from the free list and
       * decrement the counting semaphore that tracks the number of free
       * containers.
       */

      g_iob_freeqlist = iobq->qe_flink;

      /* Take a semaphore count.  Note that we cannot do this in
       * in the orthodox way by calling nxsem_wait() or nxsem_trywait()
       * because this function may be called from an interrupt
       * handler. Fortunately we know at at least one free buffer
       * so a simple decrement is all that is needed.
       */

      g_qentry_sem.semcount--;
      DEBUGASSERT(g_qentry_sem.semcount >= 0);

      /* Put the I/O buffer in a known state */

      iobq->qe_head = NULL; /* Nothing is contained */
    }

  spin_unlock_irqrestore(&g_iob_lock, flags);
  return iobq;
}

#endif /* CONFIG_IOB_NCHAINS > 0 */