[lwip-users] byte alignment

[Tyrel Newton]

For the system I'm using, when an Ethernet frame is transmitted, it has 
to be copied into a 32-bit aligned contiguous buffer within the mac. 
This means that ideally, the beginning of the Ethernet frame (i.e. the 
destination address) will be aligned on a 32-bit boundary so that it can 
be copied verbatim with aligned reads/writes. Now, I realize this makes 
the contained IP/TCP frames unaligned and I've read that setting the 
ETH_PAD_SIZE=2 improves performance on 32-bit machines by (presumably) 
making the contained IP/TCP frames 32-bit aligned (assuming 
MEM_ALIGNMENT=4).

For my case, it would seem that un-aligning the contained IP/TCP frames 
would be better so that the final copy to the mac is aligned at the 
start of the Ethernet frame. I'm wondering about the validity of this 
statement and if it has any merit performances wise. If the Ethernet 
frame being sent is not 32-bit aligned, I basically have to do byte 
reads to a temporary buffer which is then written to the mac.

My other question is about actually getting the pbuf->payload to be 
32-bit aligned at the start of the generated Ethernet frame. I would 
think that setting ETH_PAD_SIZE=0 and MEM_ALIGNMENT=4 would produce 
32-bit aligned Ethernet frames in the resulting PBUF_RAM-type pbufs. 
However, this doesn't seem to be the case in my testing as the 
p->payload in PBUF_RAM-type pbufs appear to always be 16-bit aligned 
with the aforementioned settings. What settings do I need to create 
32-bit aligned Ethernet frames for output? Btw, I'm using malloc/free as 
provided by my libc build.

Thanks,
Tyrel