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

[Bill Auerbach]

> Bill,
> 
> If you can describe the way the buffer descriptors and memory allocation
> works, maybe there is a way to use 'custom memory pools' to simulate
> what you need.
> 
> Jared

BDs (Buffer Descriptors) are a sequential list of 2-word (8 byte) entries
that contain status that the program and hardware update, the buffer length,
and a pointer to the buffer.  The last BD has a bit marking the last and the
PowerPC has a register to know the first. It can cycle through the BDs using
buffers as needed.  It checks each BD status for the next free one and
stores the packet into memory pointed to by the BD.  The program clears a
bit when it's done with a buffer and the PPC sets the bit when it's filled a
buffer.  It's a pretty cool mechanism I've not seen before.  If I point the
pbuf to the buffer and can mark the "done" bit when lwIP has processed it, I
remove one level of copying.  Maybe no copying if the image can be processed
sequentially (this looks to be possible).

Bill