Re: [lwip-devel] Packet queues (was Re: [bug #11400] ARP multi-packet-queue modifies TCP ...)

[Kieran Mansley]

On Thu, 2007-04-05 at 16:44 +0100, Jonathan Larmour wrote:

> 
> A new pbuf type seems the most backwardly compatible approach as no 
> external APIs change, and drivers don't even have to support it: the user 
> can ensure they only use it if they know the driver supports it. The notion 
> of whether that pbuf needs copying or not stays with the pbuf all the way 
> through the stack with no further changes. This approach is also amenable 
> to being a configurable option since it should be well contained. Pretty 
> much the only special treatment for PBUF_RAM_NOCOPY would be in the low 
> level drivers themselves, which would call pbuf_free() on tx completion for 
> this type of pbuf.
> 
> Note that tcp_write is already asymmetric with udp_send: the former takes a 
> character array and length, the latter takes a pbuf instead of those args. 
> This change would only mean the pbuf also subsumes the 'copy' property as 
> well.

You make a compelling case!  I find the change in ownership of the pbuf
depending on its type a little disconcerting though - I prefer APIs to
have few special cases like this, unless they are really obvious.  Would
we also need to introduce ROM_NOCOPY and POOL_NOCOPY equivalents to
protect against the pbuf structure being reused?  i.e. Is it just the
payload region we're trying to protect, or the whole pbuf structure?

Kieran