Re: [lwip-users] problem on ETH_PAD_SIZE!!!

[davide1984]

任强 wrote
> Hi,all!
> I can get packets in low_level_input. But after passed to
> ethernetif_input, they are discarded due to wrong ethhdr->type. I found
> out the wrong ethhdr->type is 2 bytes ahead of its proper position. I
> guess it's because of  ETH_PAD_SIZE( in lwipopts.h: #define ETH_PAD_SIZE 2
> ). I change it into 0, and ethhdr->type is right. But the packets I send
> are wrong at this time.
> This problem sticked me for a few days, so please give me some advice!
> Thanks！！

Hi Guys,
this post is old but I have exactly the same issue now.
I'm trying to make working lwip on an embedded system based on lm32
processor.
The system is already working now, but I have to increase the speed using
the emebedded DMA to read/write from the tx/rx fifo of the ethernet
interface with the phy.
Since those fifo are 4 bytes width, the payload address of tx/rx pbuf must
be aligned by 4, otherwise doesn't work.

The problem that i observe is this:

If I define ETH_PAD_SIZE=0 (with NO_SYS=1 and MEM_ALIGNMENT=4), the rx pbuf
allocated in the low_level_input function has the payload 4-byte aligned
(example: 0x...8) (correct). However, all pbufs that are used for
transmission are 2-bytes aligned (example: 0x...2) (not correct).

If I define ETH_PAD_SIZE=2, I observe that  the rx pbuf allocated in the
low_level_input function has the payload 2-bytes aligned. This cause an
issue in the function ethernetif_input, that cannot recognize the
eth_hdr->type.

I had this problem on lwip 1.2.0, so I decided to do an "upgrade" to v1.4.1,
but the issue is the same.

My low_level_input function is:

static struct pbuf *
low_level_input(struct netif *netif)
{
  /*struct ethernetif *ethernetif = netif->state;*/
  struct pbuf *p, *q;
  u16_t len;
  int i;
  unsigned int bytesToRead;

 
  /* Obtain the size of the packet and put it into the "len"
     variable. */
  bytesToRead = len = tsmac_rx_pkt_size(TS_MAC_CORE_BASE_ADDRESS);

  /* don't process further if there is no data to read */
  if(bytesToRead == 0)
    return(NULL);

#if ETH_PAD_SIZE
  len += ETH_PAD_SIZE; //allow room for Ethernet padding
#endif

  /* We allocate a pbuf chain of pbufs from the pool. */
  p = pbuf_alloc(PBUF_RAW, len, PBUF_POOL);

  if (p != NULL) {
#if ETH_PAD_SIZE
    pbuf_header(p, -ETH_PAD_SIZE); /* drop the padding word */
#endif
    /* We iterate over the pbuf chain until we have read the entire
     * packet into the pbuf. */
    for(q = p; q != NULL; q = q->next) {
       int size;

       /* Read enough bytes to fill this pbuf in the chain. The
        * available data in the pbuf is given by the q->len
        * variable.
          read data into(q->payload, q->len);*/
       size = (bytesToRead > q->len)?q->len:bytesToRead;
       bytesToRead-=size;

       //Read the data from ethernet interface
       tsmac_read(TS_MAC_CORE_BASE_ADDRESS,q->payload, size);
           
           
           
    }

#if ETH_PAD_SIZE
    pbuf_header(p, ETH_PAD_SIZE); /* reclaim the padding word */
#endif
 
  } else {
    tsmac_drop_rx_packet(TS_MAC_CORE_BASE_ADDRESS, bytesToRead);
  }

Any idea?
Thanks 



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