/****************************************************************************
 * net/iob/iob_alloc.c
 *
 *   Copyright (C) 2014 Gregory Nutt. All rights reserved.
 *   Author: Gregory Nutt <gnutt@nuttx.org>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 * 3. Neither the name NuttX nor the names of its contributors may be
 *    used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
 * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
 * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
 * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
 * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
 * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 * POSSIBILITY OF SUCH DAMAGE.
 *
 ****************************************************************************/

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

#include <nuttx/config.h>

#include <semaphore.h>
#include <assert.h>

#include <nuttx/arch.h>
#include <nuttx/net/iob.h>

#include "iob.h"

/****************************************************************************
 * Pre-processor Definitions
 ****************************************************************************/

/****************************************************************************
 * Private Types
 ****************************************************************************/

/****************************************************************************
 * Private Data
 ****************************************************************************/

/****************************************************************************
 * Public Data
 ****************************************************************************/

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

/****************************************************************************
 * Name: iob_tryalloc
 *
 * Description:
 *   Try to allocate an I/O buffer by taking the buffer at the head of the
 *   free list.
 *
 ****************************************************************************/

static FAR struct iob_s *iob_tryalloc(void)
{
  FAR struct iob_s *iob;
  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 = irqsave();
  iob   = g_iob_freelist;
  if (iob)
    {
      /* Remove the I/O buffer from the free list and decrement the counting
       * semaphore that tracks the number of free IOBs.
       */

      g_iob_freelist = iob->io_flink;
      DEBUGVERIFY(sem_trywait(&g_iob_sem));
      irqrestore(flags);

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

      iob->io_flink  = NULL; /* Not in a chain */
      iob->io_len    = 0;    /* Length of the data in the entry */
      iob->io_offset = 0;    /* Offset to the beginning of data */
      iob->io_pktlen = 0;    /* Total length of the packet */
      return iob;
    }

  irqrestore(flags);
  return NULL;
}

/****************************************************************************
 * Name: iob_allocwait
 *
 * Description:
 *   Allocate an I/O buffer, waiting if necessary.  This function cannot be
 *   called from any interrupt level logic.
 *
 ****************************************************************************/

static FAR struct iob_s *iob_allocwait(void)
{
  FAR struct iob_s *iob;
  irqstate_t flags;
  int ret;

  /* The following must be atomic; interrupt must be disabled so that there
   * is no conflict with interrupt level I/O buffer 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 = irqsave();
  do
    {
      /* Try to get an I/O buffer.  If successful, the semaphore count
       * will be decremented atomically.
       */

      iob = iob_tryalloc();
      if (!iob)
        {
          /* 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 to be released when the semaphore
           * count will be incremented.
           */

          ret = sem_wait(&g_iob_sem);

          /* When we wake up from wait, an I/O buffer was returned to
           * the free list.  However, if there are concurrent allocations
           * from interrupt handling, then I suspect that there is a
           * race condition.  But no harm, we will just wait again in
           * that case.
           */
        }
    }
  while (ret == OK && !iob);

  irqrestore(flags);
  return iob;
}

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

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

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

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

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

      return iob_allocwait();
    }
}