[lwip-devel] [task #7896] Support zero-copy drivers

[Simon Goldschmidt]

Follow-up Comment #1, task #7896 (project lwip):

Ok, seems like this is one of the last tasks left before releasing 1.4.0
(together with bug #29361, which is kind of related to this one).

I think there is not much coding involved here but we rather need to know
what we need to support zero copy. For that, we would need input from many
developers/users telling us what they need. Personally, the only requirements
for my implementation are:

- delayed freeing of TX pbufs (i.e. transmit might take some time / pbuf is
*not* transmitted when netif->linkoutput returns - this is already taken care
of by adjusting the docs to say "applications may not reuse pbufs after
passing them to send/output functions")
- RX pbufs should be properly aligned (depending on the cache line size -
this currently doesn't work as there is only one MEM_ALIGNMENT)

As Jonathan pointed out some more requirements below, I think the most
portable solution would be to allow the driver to allocate pbufs in its own
memory and to let pbuf_free() call a port-defined function to free those pbufs
- we have plenty numbers left in the 'enum pbuf_type'.

This would also help to solve bug #29361. However, the downside is that the
whole stack would have to be reviewed in order to remove assumptions made on
different pbuf types (i.e. pbuf_header fails on PBUF_REF in some cases, etc.).

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