[lwip-users] RE: [lwip] Anyone Using An Architecture That Isn't little or big endian?

[Earle Clubb]

Maybe I'm missing something, but it seems to me that all of the little/big
endian conversions in the stack can be eliminated if the required byte
swapping is done in the device driver just before the bytes are sent to the
network device.  Does this make sense, or am I way off?


Earle Clubb

Software Engineer
Valcom, Inc.

E-mail: address@hidden
Voice: (540) 563-2000 ext.253

-----Original Message-----
From: address@hidden [mailto:address@hidden Behalf Of John C.
Toman
Sent: Monday, August 19, 2002 3:04 PM
To: address@hidden
Subject: [lwip] Anyone Using An Architecture That Isn't little or big
endian?


Just checking. I'm considering writing and submitting code which would
eliminate many unnecessary byte swaps for little endian architectures,
specifically for code constants, without affecting big endian
architectures. I'm planning on making changes at least for my system and
will contribute them back to the lwIP tree if others are interested. For
instance, consider:

Currently in lwIP (one example from etharp.h):

#define ETHTYPE_ARP 0x0806
#define ETHTYPE_IP  0x0800

When these are used, they must be byte-swapped in the little endian case
(from ethernetif.c):

    switch(htons(ethhdr->type)) {
    case ETHTYPE_IP:
      arp_ip_input(netif, p);
      pbuf_header(p, -14);
      netif->input(p, netif);
      break;
    case ETHTYPE_ARP:
      p = arp_arp_input(netif, ethernetif->ethaddr, p);
      if(p != NULL) {
        low_level_output(ethernetif, p);
        pbuf_free(p);
      }
      break;

With a #ifdef for LITTLE_ENDIAN systems in the header files, this
example would become:

etharp.h:

#ifdef LITTLE_ENDIAN
#  define ETHTYPE_ARP 0x0608
#  define ETHTYPE_IP  0x0008
#else
#  define ETHTYPE_ARP 0x0806
#  define ETHTYPE_IP  0x0800
#endif

Then the code piece in ethernetif.c loses the htons() call, saving code
and execution time in the little endian case without harming the big
endian case:

    switch(ethhdr->type) {
    case ETHTYPE_IP:
      arp_ip_input(netif, p);
      pbuf_header(p, -14);
      netif->input(p, netif);
      break;
    case ETHTYPE_ARP:
      p = arp_arp_input(netif, ethernetif->ethaddr, p);
      if(p != NULL) {
        low_level_output(ethernetif, p);
        pbuf_free(p);
      }
      break;

My thinking was that dozens of htons, htonl, ntohs and ntohl calls in
lwIP could be eliminated this way or in similar ways, resulting in
cleaner code, as well as smaller code size and increased performance for
little endian systems.

The one drawback I can see to this approach is if there are other
architectures that store bytes differently than little or big endian,
they would need to come up with their own constants that represent the
network-storage format on their systems ...

Regards,

John


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