Re: [lwip-users] ETH_PAD_SIZE

[Timmy Brolin]

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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