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.
Isn't that normal BD behaviour? Anyway, the opencores MAC does it that
way, too. What you have to do is allocate one PBUF_POOL that has the
size of the maximum received ETH frame for each BD. When one BD changes
to 'received', use the address stored in the BD to get back the pbuf
pointer (will be ~16 bytes below for PBUF_POOLs), allocate a fresh
PBUF_POOL, store its payload address into the BD and set it as available
again. At this point, you have the BD available for receiving again and
can pass the previous PBUF_POOL into the stack (without copying at all)!
Simon
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