Re: [lwip-devel] Re: [task #7040] Work on tcp_enqueue

[address@hidden]

bill wrote:
> The changes in the driver are not substantial to checksum at that level.
> There's a little parsing and extra setup to load registers and to start the
> DMA-checksumming.  We're going down this path - I will let you know the gain
> for a NIOS II platform for a DMA-checksum driver.
>   

I don't think I fully understand you: we didn't have a MAC that supports 
HW-checksum, we had a DMA-less MAC with a custom DMA engine writing the 
packets to/from the MAC (of course all handled by the MAC driver). The 
goal was to use a custom DMA-copy HW that supports checksum calculation 
while copying, but we only had a very small FPGA, so it didn't fit.

Of course the fastest solution is HW-checksum, but the goal is for lwIP 
to be as fast as possible without that (while still being as small as 
possible).

I think trying to create full packets complies with the LW goal (given 
RAM size and execution overhead, not code size), since we need less 
segments. The savings of one-pbuf-per-segment are even bigger since we 
save a lot of pbuf structs.

> I think the less than MSS full pufs hits performance less than
> avoiding it by searching to the end of the chain to split a pbuf and 
add to
> the last one, especially when the chains are long.

We do that for small writes, so why not do it for big writes? I'd favour 
to get rid of the long pbuf chains altogether since they waste RAM (the 
pbuf structs) and are inefficient (when calculating checksum).

Simon