Re: [lwip-users] Packet queuing to an ARP table entry

[address@hidden]

Amena El Homsi wrote:
> I am trying to send ping request using Lwip.
> The ICMP header and ping data is in my system frame memory referenced 
> by a PBUF_ROM 'p' (p->payload points to the ICMP hdr in the frame memory.

Is this correct? How long is the contents of this buffer stable? 
PBUF_ROM means ROM - i.e. stable for very long, not just until you think 
normal transmission is done. This is *not* very common!

> The PBUF_ROM is chained by a PBUF_RAM 'q' in which the ETHERNET and IP 
> headers are saved
> [..]
> When I tried to send the first ping, LwIP sends an ARP request and 
> queue the packet. The problem is that LwIP creates a new pbuf RAM 'r' 
> and copies 'q' of type PBUF RAM and 'p' of type PBUF_ROM to the new 
> PBUF RAM!

I guess it's like that since your case is just very uncommon. Noone has 
ever bothered to improve for this. Normally, even if you have PBUF_ROM 
pbufs at some point, they are not the first frames sent to some devices. 
Plus it's normally not worth to improve for not copying here.

> Indeed I don't want to copy PBUF_ROM payload to the new PBUF_RAM, is 
> there a way to avoid such copy? why LwIP doesn't copy 'q' to 'r' while 
> preserving the chain between the PBUF_RAM and PBUF_ROM?

Now this might work for the special case where the last pbuf(s) in a 
chain are ROM (remember we need to copy all non-ROM pbufs). We could of 
course create a new ROM pbuf referencing the same data, which would 
prevent copying.

May I ask why it's worth making this change? I.e. does it really hurt 
performance or memory constraints in your case?


Simon