I have never seen a 100Mbit MAC with linked DMA buffers which did not
support buffers of different sizes...
Usually you have some kind of "buffer descriptor" for each buffer, and
in that descriptor you enter the size and address of the actual buffer,
as well as a reference to the next buffer descriptor in the chain.
This usually maps very well to the linked PBUFs of lwip.
But I guess there could be a MAC or two out there with buffer size
limitations.
/Timmy
Muhamad Ikhwan Ismail wrote:
Hi Timmy,
It did cross my mind but unfortunately I could only set single value
as the FEC buffer size (unless I am misinformed by the user guide).
Greetings,
Ikhwan
------------------------------------------------------------------------
Date: Sun, 25 May 2008 17:47:36 +0200
From: [EMAIL PROTECTED]
To: [email protected]
Subject: Re: [lwip-users] Misalignment problems on PowerPC (little
endian mode)
If your FEC does not allow storing of packets at an 2 byte offset,
then you could try setting up the FEC to store the Ethernet header
in one small PBUF, and the rest of the packet in a large 1500 byte
PBUF. That way your IP headers will be aligned.
The downside is of course that for this to work all incomming
packets regardless of size will use the full 1500 bytes.
You would need to setup every other pbuf in the FEC Rx DMA chain
to 14 bytes and 1500 bytes respectively.
Regards,
Timmy Brolin
Muhamad Ikhwan Ismail wrote:
Hi Timmy,
If you read my prev post, you would know why the padding is
useless to me.
To Simon,
Understandable. Well if someone has the same problem, and
checks this thread I guess he can always just bring it back up.
PowerPC is more robust in big endian mode, it even corrects
the misalignment in big endian mode internallly (hardware).
Why and how
to be honest I dont know (barely had any training on PowerPC,
learned all through books.)
There are certain PowerPC assembler intructions that cause
problems in little endian mode, I dont remember them by hard, need
to check the book.
Though you are correct about my method of correcting the
problem would make lwip slower :
1. My driver didnt have to copy the received buffer at all. So
I spare a lot there.
2. Fixing alignment in little endian mode is a very2 long
process/routine.
3. And I dont have to copy the header everytime. E.g in TCP I
only had to copy the header once for the multiple
header processing.
So I hope this brings the end to the thread, at least till
someone might need something out of it. I am thankful that so
many people
had tried to help.
Greetings,
Ikhwan
------------------------------------------------------------------------
> Date: Sat, 24 May 2008 22:28:38 +0200
> From: [EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
> To: [email protected] <mailto:[email protected]>
> Subject: Re: [lwip-users] Misalignment problems on PowerPC
(little endian mode)
>
>
> >> I have the source which can be compiled if the option
> >> LWIP_MISALIGNMENT is set to 1. If anyone interested and
have the same
> >> problem, I am more than willing to share and help. If you
guys want
> >> to incorporate it to lwip as alternative solution to
structure
> >> packing I am glad I can help. This lowers the stack
performance a bit
> >> though.
> >
> >
> > Not to offend you, but personally, I see this as a bug in
the compiler
> > you are using and we don't support broken compilers:
structure packing
> > _is_ a must for lwIP!
> >
> > Hope you are still having fun with lwIP!
> >
> > Simon
> >
> Actually, no. Structure packing is not necessarily a must
for lwip.
> In many cases lwip should work just fine without structure
packing. I
> know that Texas Instruments DSP compilers will work with
lwip without
> structure packing.
> Patches made to lwip several years ago ensured that if
ETH_PAD_SIZE is
> set to 2, then no variable will ever be unaligned. This
solves most of
> the problems which would otherwise require structure packing.
>
> There are two things a compiler may do which will still mess
up the lwip
> structs:
> 1: Some compilers pad the size of all structs so that the
size of a
> struct becomes a multiple of 4.
> 2: Some comiplers put all structs on an address boundary of 4,
> regardless of the actual boundary requirements of the
members of the struct.
> Compilers which does any of the above would still require
packed
> structs. Compilers which does neither of the above should
work without
> packed structs, if ETH_PAD_SIZE is set to 2.
>
> Regards,
> Timmy Brolin
>
>
> _______________________________________________
> lwip-users mailing list
> [email protected] <mailto:[email protected]>
> http://lists.nongnu.org/mailman/listinfo/lwip-users
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