Re: [lwip-users] Transferring large data fast and pointing pbufs directly to Ethernet receive buffers

[Jonathan Larmour]

address@hidden wrote:
> > Secondly, the PowerPC loads packets directly into buffer descriptor 
> > memory. Is looks possible (best) at low_level_input to point pbufs 
> > right into the received packet without copying. If I’m correct that 
> > this can be done, the question I have is, where can I know the data 
> > was read by the upper layer to be able free the pbuf and at the same 
> > time to be able to free memory for the Ethernet controller?
> Normally, you would allocate a pbuf including data buffer: type 
> PBUF_POOL and make sure the data buffer is in one piece (p->next == 
> NULL; for that, PBUF_POOL_BUFSIZE has to be set to 1516 for ethernet). 

Just being pedantic I know, but that isn't strictly true - I'm working on 
one board which can scatter/gather pbufs and where the memory pointed to 
by the buffer descriptors is fixed at 128 bytes.

However if you do have the memory it is more efficient to use the full 
1516. The question then is whether you have the memory because _every_ 
packet received will occupy the full 1516 bytes, and if you're keeping 
them around till you've got the lot, that can add up. If you are receiving 
your 4MB image in 256 byte chunks, you'd need at least 24MB of RAM ;).

NB the value of 1516 bytes can depend on your hardware, e.g. if your 
hardware also transfers the CRCs. See 
http://sd.wareonearth.com/~phil/net/jumbo/ for example.

> Then you set the ETH DMA to receive to p->payload. When a packet is 
> received, substract the offset from p->payload so you get the pbuf 
> pointer, which you can pass into the stack.

Just to clarify what Simon is saying, pool pbufs consist of a 'struct 
pbuf' immediately followed in memory by the payload - so if you have the 
payload pointer, you know where the struct pbuf is.

> Of course, this only works 
> if your ETH DMA can receive to the memory where the memp pools are 
> allocated.
>
> Unfortunately, using a PBUF_REF for receiving input packets doesn't work 
> so good I think...

Indeed. I've certainly only been working in terms of (inherently 
preallocated) pool pbufs.

Jifl
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