Re: [lwip-users] udp (RFC!)

[Leon Woestenberg]

Hello David,

>
> I suppose I could fix this at a low level and copy all PBUF_ROM buffers
> into PBUF_RAM buffers before queuing onto the ARP queue or queuing in
> the output driver. I could write a pbuf_copy_if_rom() function in pbuf.c
> to do it.
>
Yup, this is the prefered approach for the current lwIP stack.

However, take in mind that a packet can consist of a chain of differently
typed PBUF_ types.

The trick would be to replace the PBUF_ROM in this chain by corresponding
copies in PBUF_RAM. Also, the same goes for PBUF_REF types (pointing
to application-RAM).

Thinking of it, maybe it would be possible to start implementing PBUF_REF
types in the current source anyway. They should not break anything else.

I will think about this a little more. Your feedback is great so far!

---

I understand your particular implementation of the ring buffer that defers
actual
sending using a thread.

Yes, it would be hard to implement a pass-by-reference socket layer,
implementing
zero-copy all the way. A concession (i.e. copying) must be made _somewhere_.

Still, my preference would be to defer a copy to the lower levels (like your
DMA
system).

Implementing copying at the high level will induce this everyone using lwIP,
removing the current zero-copy feature for everyone.

Regards,

Leon.