Power PC 440GX Jumbo Packets EMAC2/3

zDL Linux PPC linuxppc at 4bridgeworks.com
Thu Feb 24 22:46:26 EST 2005

I have been doing some experiments with Jumbo packets (MTU 9000) and have
noticed that once you exceed the 2KB fifo size, I end up with no response
from the box.

I can ping the box with no problem at all at sizes greater than 2KB, however
upon further investigation I noticed packets of size 2006 bytes causes no
further packet to be transmitted (adding debug prints to ibm_emac_core.c,
the driver believes the packet has been transmitted), after adding a further
42 bytes of header information this comes to the exact size of the Transmit
Fifo 2048 (2KB). I have also changed the Transmit Fifo Register to 1024
bytes, this solves the problem at 2006 bytes however now it has moved to 982
bytes of data (+42bytes of header = 1024)

Has anyone else experienced this problem while either using a smaller
transmit fifo (512 or 1024 bytes) and  MTU 1500 or a larger MTU (>2033) and
EMAC transmit fifo of 2048. I guess this will really apply to people with
larger MTU not many will decrease the Tx fifo, and I am further guessing
that not many are using 9000 MTU as not much hardware seems to support it, I
can only use it through a direct connection between PC's and not through a

Any suggestions on how to overcome this problem??? My only idea is to either
add data to the packet (don't think this will work) or forceably fragment
the packet (if it is 2K in size) to ensure that it never uses the exact size
of the tx fifo.....

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