lwIP port status

[Simon Kallweit <simon.kallweit@intefo.ch>]

Hi

It's time for a little status update on the lwIP 1.3.x port. Let me give 
you a little list of what has been done lately:

* CDL should include every configuration option offered by lwIP now, was 
reviewed by John Dallaway and got restructured and cleaned up afterwards.
* Implemented 'Sequential' mode.
* Updated to the latest lwIP sources from CVS again.
* Reviewed test cases.
* Cleanup, cleanup, cleanup.

What's left to do is lots of testing and writing documentation etc. I 
think it would be great to finish the port when lwIP hits the 1.3.1 
release which should not be too far from now. If time permits, I may 
also work a bit on PPP.

For now I would like to shortly discuss my 'Sequential' implementation. 
In 'Sequential' mode, lwIP is configured with NO_SYS == 0 and allows for 
multiple threads to use the stack (netcomm, sockets). The main thread 
(tcpip) is basically used to synchronize all threads accessing the stack 
by sequentially processing incoming messages on a message queue. When a 
packet arrives on an interface, a message pointing to the packet is sent 
to the message queue. This is what is done with the SLIP interface for 
example. SLIP is running in it's own thread, reading from the serial 
device (blocked). When a packet is received, it is passed to the tcpip 
thread by posting a message to the message queue. This way the packet is 
processed in sync with the tcpip thread. For ethernet devices I choose 
another path. The old port implemented a separate ethernet delivery 
thread which would wait on a semaphore for packets to arrive. It would 
wake up when a packet arrives and read it from the interface and send a 
message to the tcpip thread. In my port I got rid of the delivery thread 
and implemented sending/receiving as follows:

When a packet is sent, lwIP assumes that after the call to output the 
packet, the packet is no longer access by the driver and can be 
released. This means that the packet should be sent before the call 
returns -> blocking, or packets would have to be copied and put in a 
queue, which I think does not make much sense. I therefore implemented 
sending to be blocking.

Receiving packets could be done in parallel to the tcpip thread, but 
packets still have to be processed in sync to the tcpip thread. I choose 
a really simple route. When a packet arrives, a callback is registered 
with the tcpip thread, which is called back as soon as ongoing 
processing in the tcpip thread is finished. The callback then reads all 
the pending packets from the ethernet devices and directly processes them.

What do people think of this approach? Is it feasible or completely 
broken? :)

Anybody cares to do some testing of the new lwIP port?

Simon