Yes, ETH_PAD_SIZE is indeed a alignment fix.
Since the ethernet header is 14 bytes, the entire IP packet becomes
unaligned.
By adding a 2byte pad at the beginning of the ethernet packet,
everything becomes nicely aligned. This alignment boosts performance on
all 32bit processors, and is actually a requirement on processors which
do not allow unaligned memory accesses. The only disadvantage is that it
"wastes" 2 bytes.
Timmy Brolin
Christiaan Simons wrote:
Hi Frédéric,
Where is the best file to redefine ETH_PAD_SIZE ? I suppose that
lwipopts.h is the good location, but in ETH_PAD_SIZE is defined in
etharp.h, but before any include.
Yes, lwipopts.h is ok.
Isn't it better to move the
#define ETH_PAD_SIZE, after all #include in etharp.h? Or to add
#include "lwip/opt.h" before ?
Yes, at least after "lwip/opt.h" so your lwipopts define overrules the
etharp.h one.
I don't know what the purpose of this define is, maybe it is some alignment
fix or workaround.
Bye,
Christiaan Simons
Hardware Designer
Axon Digital Design
http://www.axon.tv
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