[lwip-users] LWIP LPCWare Example: Questions on Transmit PBUF Usage

[Justin Reinhart]

Hello,

I'm using a version of lwip 1.4.1 ported to an NXP LPC18xx, the demo code of which can be had here. I'm essentially replicating their 'sa_tcpecho' project with my own project/board. So far, everything is working great except for transmission, and I had somewhat of a general question on transmit pbuf usage. This chip/example uses a DMA to fill pre-allocated pbufs with data.

The chip I'm using has a fast section of memory where receive pbufs are preallocated in lwipopts and my driver, and the example code above runs well executing a TCP echo. However, for the transmit function they simply pass it the receive pbuf (duh, echo). I'm wondering, if I have an array somewhere in slow memory that I want to transmit in response to the reception (call it Xmt.Data[100] with a length of Xmt.Length), how do I go about doing this? Do I use pbuf_alloc(PBUF_RAW, (u16_t) EMAC_ETH_MAX_FLEN, PBUF_POOL) and copy the data into this new pbuf from slow memory? If I've already got a pool, do I need to allocate/deallocate anything? Any demo code using a transmit pbuf or any personal experience would help me out a great deal in understanding transmit pbuf usage. Thanks in advance, and sorry for the text-full question.

enum echo_states
{
  ES_NONE = 0,
  ES_ACCEPTED,
  ES_RECEIVED,
  ES_CLOSING
};

struct echo_state
{
  u8_t state;
  u8_t retries;
  struct tcp_pcb *pcb;
  /* pbuf (chain) to recycle */
  struct pbuf *p;
};


err_t
echo_recv(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err)
{
  struct echo_state *es;
  err_t ret_err;

  LWIP_ASSERT("arg != NULL",arg != NULL);
  es = (struct echo_state *)arg;
  if (p == NULL)
  {
    /* remote host closed connection */
    es->state = ES_CLOSING;
    if(es->p == NULL)
    {
       /* we're done sending, close it */
       echo_close(tpcb, es);
    }
    else
    {
      /* we're not done yet */
      tcp_sent(tpcb, echo_sent);
      echo_send(tpcb, es);
    }
    ret_err = ERR_OK;
  }
  else if(err != ERR_OK)
  {
    /* cleanup, for unkown reason */
    if (p != NULL)
    {
      es->p = NULL;
      pbuf_free(p);
    }
    ret_err = err;
  }
  else if(es->state == ES_ACCEPTED)
  {
    /* first data chunk in p->payload */
    es->state = ES_RECEIVED;
    /* store reference to incoming pbuf (chain) */
    es->p = p;
    /* install send completion notifier */
    tcp_sent(tpcb, echo_sent);

/* *******NEED TO BUILD DIFFERENT XMT MESSAGE HERE AND PASS CORRECT PBUF TO echo_send() ***** */

    echo_send(tpcb, es);
    ret_err = ERR_OK;
  }
  else if (es->state == ES_RECEIVED)
  {
    /* read some more data */
    if(es->p == NULL)
    {
      es->p = p;
      tcp_sent(tpcb, echo_sent);
      echo_send(tpcb, es);
    }
    else
    {
      struct pbuf *ptr;

      /* chain pbufs to the end of what we recv'ed previously  */
      ptr = es->p;
      pbuf_chain(ptr,p);
    }
    ret_err = ERR_OK;
  }
  else if(es->state == ES_CLOSING)
  {
    /* odd case, remote side closing twice, trash data */
    tcp_recved(tpcb, p->tot_len);
    es->p = NULL;
    pbuf_free(p);
    ret_err = ERR_OK;
  }
  else
  {
    /* unkown es->state, trash data  */
    tcp_recved(tpcb, p->tot_len);
    es->p = NULL;
    pbuf_free(p);
    ret_err = ERR_OK;
  }
  return ret_err;
}


void
echo_send(struct tcp_pcb *tpcb, struct echo_state *es)
{
  struct pbuf *ptr;
  err_t wr_err = ERR_OK;
 
  while ((wr_err == ERR_OK) &&
         (es->p != NULL) &&
         (es->p->len <= tcp_sndbuf(tpcb)))
  {
  ptr = es->p;

  /* enqueue data for transmission */
  wr_err = tcp_write(tpcb, ptr->payload, ptr->len, 1);
  if (wr_err == ERR_OK)
  {
     u16_t plen;
      u8_t freed;

     plen = ptr->len;
     /* continue with next pbuf in chain (if any) */
     es->p = ptr->next;
     if(es->p != NULL)
     {
       /* new reference! */
       pbuf_ref(es->p);
     }
     /* chop first pbuf from chain */
      do
      {
        /* try hard to free pbuf */
        freed = pbuf_free(ptr);
      }
      while(freed == 0);
     /* we can read more data now */
     tcp_recved(tpcb, plen);
   }
   else if(wr_err == ERR_MEM)
   {
      /* we are low on memory, try later / harder, defer to poll */
     es->p = ptr;
   }
   else
   {
     /* other problem ?? */
   }
  }
}

- Justin