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RE: [lwip-users] Question on structure packing
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From: |
James Yates |
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Subject: |
RE: [lwip-users] Question on structure packing |
|
Date: |
Wed, 21 Apr 2004 10:37:20 +0100 |
I believe I also have a similar problem. I am currently porting LWIP to
run under RTEMS on a SH2 7145F with a CS8900 etherenet chip. During a
call to the init function for the cs8900if, the following line is called
which causes the whole app to bomb, currently running under GDB (serial)
via a stub:
cs8900if->ethaddr = (struct eth_addr *)&(netif->hwaddr[0]);
I have tried adding the padding words as you suggested but this line
still seems to bomb. I am pretty stuck. Does anyone have any suggestion?
Many Thanks
James
-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Robert Brown
Sent: 21 April 2004 00:31
To: Mailing list for lwIP users
Subject: RE: [lwip-users] Question on structure packing
I had similar problems with a buggy compiler. I made a quick and dirty
work around to avoid the need for packed structures. My target was an
ARM7 with a RTK8019AS ethernet device. There are a few changes
required:
/*----------------------------------------------------------------------
-------------*/
/*
* Explanation of the contortions performed on input and where
everything is.
*
* The lwIP stack uses C-structs to read/write protocol headers in an
* incoming/outgoing data segment. The definition of these structures
* relies on packing which allows header fields to be read/written by
name even
* when they are not word-aligned.
*
* For some reason the ARM compiler used for this project does not
* handle packed structures very well. We avoid this problem by
introducing
* pad fields into the data structures. The 2-octet pad field between
the
* Ethernet header and the IP header is added by the Ethernet device
driver for the
* network interface.
*
* This pad field forces the IP header to be word-aligned so packing is
not
* required.
*
*
*
**********************************
* Ethernet and IP headers with pading added in driver:
*
* src mac dest mac type pad
* | | | |
* -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
* | | | |
* 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
*
* v_hl ttl
* tos len id offst proto chksm src ip
* | | | | | | |
* -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
* | | | |
* 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
*
* dest ip IP payload (headers, data, etc.)
* |
* -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
* | | | |
* 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
*
*
*
**********************************
* Special case for ARP packets:
* For ARP packets additional pad fields are required after
* each Ethernet header field as illustrated below.
*
* src mac dest mac type pad
* | | | |
* -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
* | | | |
* 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
*
* proto
* hw len
* type proto hwlen opcode shwaddr pad1
* | | | | | |
* -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
* | | | |
* 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
*
* sipaddr dhwaddr pad2 dipaddr
* | | | |
* -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --
* | | | |
* 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
*
*/
1. For IP add two octets of "padding" between the ethernet header and
the IP header to ensure that the IP header is 4 byte aligned as follows:
PACK_STRUCT_BEGIN
struct eth_hdr {
PACK_STRUCT_FIELD(struct eth_addr dest);
PACK_STRUCT_FIELD(struct eth_addr src);
PACK_STRUCT_FIELD(u16_t type);
PACK_STRUCT_FIELD(u16_t pad); //<----added } PACK_STRUCT_STRUCT;
PACK_STRUCT_END
2. For ARP add two "pad" elements to word-align the IP addresses as
follows:
PACK_STRUCT_BEGIN
/** the ARP message */
struct etharp_hdr {
PACK_STRUCT_FIELD(struct eth_hdr ethhdr);
PACK_STRUCT_FIELD(u16_t hwtype);
PACK_STRUCT_FIELD(u16_t proto);
PACK_STRUCT_FIELD(u16_t _hwlen_protolen);
PACK_STRUCT_FIELD(u16_t opcode);
PACK_STRUCT_FIELD(struct eth_addr shwaddr);
PACK_STRUCT_FIELD(u16_t pad1); //<----added
PACK_STRUCT_FIELD(struct ip_addr sipaddr);
PACK_STRUCT_FIELD(struct eth_addr dhwaddr);
PACK_STRUCT_FIELD(u16_t pad2); //<----added
PACK_STRUCT_FIELD(struct ip_addr dipaddr); } PACK_STRUCT_STRUCT;
PACK_STRUCT_END
5. Modify etharp_arp_input() and etharp_query() in etharp.c to remove
padding from outgoing ARP replies and ARP queries, respectively.
Following is a snip of code that may serve the purpose.
/* Remove padding. It is valid to assume that for
* an ARP reply there is only one pbuf in action. */
{
unsigned char *payload;
unsigned char *packed_payload;
int i;
payload = (unsigned char *) &(hdr->sipaddr);
packed_payload = payload - 2;
/* Remove hdr->pad1 until we encounter hdr->pad2 */
for (i = 0; i < 10; i++)
{
*packed_payload++ = *payload++;
}
/* Reduce the length of the packet by 2 */
p->len -= 2;
p->tot_len -= 2;
/* Advance past hdr->pad2 */
payload += 2;
/* Remove hdr->pad2 and cover the last for bytes with
ethernet padding */
for (i = 0; i < 8; i++)
{
*packed_payload++ = *payload++;
}
/* Reduce the length of the packet by 2 */
p->len -= 2;
p->tot_len -= 2;
}
/* return ARP reply */
netif->linkoutput(netif, p);
4. Modify your ethernet driver to skip the padding octets when
reading/writing from/to the hardware
I think this is everything we changed to get it to work.
Regards,
Rob
-----Original Message-----
From: address@hidden
[mailto:address@hidden Behalf
Of John BODDIE
Sent: Tuesday, April 20, 2004 12:12 PM
To: 'Mailing list for lwIP users'
Subject: RE: [lwip-users] Question on structure packing
Hi Tony,
I'm currently working on an STi5516/17 Eval System with a DB558 ethernet
STEM module.
Would there be any chance of getting a copy of your modified sources?
Regards,
John
-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Mountifield, Tony
Sent: 20 April 2004 16:48
To: Mailing list for lwIP users
Subject: RE: [lwip-users] Question on structure packing
Hi John,
I am doing exactly the same exercise for the STi5516 evaluation system,
and found exactly the same problem. I have it working now, although
still some tidying to do, but had to rewrite the struture elements as
arrays of bytes instead of u16 and u32, and use macros to access them.
Since you're at ST, you probably know more about the issues than I do. I
found that if I did a longword access with a pointer that was not a
multiple of four, it fetched an aligned longword anyway. e.g.: if the
pointer value was NNN0, NNN1, NNN2 or NNN3, it would fetch the longword
consisting of those bytes, NNN0/1/2/3. So you can't even play clever
games with pointers, and have instead to fetch a byte at a time and
stick them together.
Cheers,
Tony
> -----Original Message-----
> From: John BODDIE [mailto:address@hidden
> Sent: 20 April 2004 16:30
> To: address@hidden
> Subject: [lwip-users] Question on structure packing
>
>
> Hi,
>
> I am currently porting lwIP 0.7.1 to run under OS20 on a 32-bit
> STMicro ST20. Unfortunately the ST20 compiler does not support
> structure packing leading to all sorts of data corruption problems.
>
> Does anyone know of anyway around the issue of structure packing?
>
> Regards,
>
> John
>
>
>
> _______________________________________________
> lwip-users mailing list
> address@hidden http://mail.gnu.org/mailman/listinfo/lwip-users
>
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