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

[Joel Cunningham]

Follow-up Comment #9, task #7896 (project lwip):

For zero-copy TX into the driver, is the goal here to support a synchronous
zero-copy output through the netif_output_fn()?  

Meaning that once the output function returns, the transmit has been complete
and the pbuf memory is no longer referenced by the driver

I ask because in my products, the underlying driver operates in an
asynchronous manner where the packet is enqueued during the netif_output_fn
and then is transmitted at a later time.

Asynchronous zero-copy TX is more complicated.  I'm thinking it would work by
the driver queuing a reference to the pbuf (incrementing pbuf->ref) and then
calling pbuf_free() once it has completed the transmit.  Any modifications to
the pbuf when pbuf->ref > 1 (by LwIP on TCP rexmit or driver) would be
required making a copy of the packet.  Also, any PBUF_REF would need to be
copied since that memory may not be in scope after LwIP is done with the
netif_output_fn (UDP fits this case)

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