chip revisions. (A buggy taiwanese chip? I'm just shocked; shocked I tell
you.) So far I have only observed the anomalous behavior on board with
PCI revision 33 chips. At the moment, this seems to include only the
Netgear FA310-TX rev D1 boards with chips labeled NGMC169B. (Possibly this
means it's an 82c169B part from Lite-On.)
The bug only manifests itself in promiscuous mode, and usually only at
10Mbps half-duplex. (I have not observed the problem in full-duplex mode,
and I don't think it ever happens at 100Mbps.) The bug appears to be in
the receiver DMA engine. Normally, the chip is programmed with a linked
list of receiver descriptors, each with a receive buffer capable of holding
a complete full-sized ethernet frame. During periods of heavy traffic
(i.e. ping -c 100 -f 8100 <otherhost>), the receiver will sometimes appear
to upload its entire FIFO memory contents instead of just uploading the
desired received frame. The uploaded data will span several receive
buffers, in spite of the fact that the chip has been told to only use
one descriptor per frame, and appears to consist of previously transmitted
frames with the correct received frame appended to the end.
Unfortunately, there is no way to determine exactly how much data is
uploaded when this happens; the chip doesn't tell you anything except the
size of the desired received frame, and the amount of bogus data varies.
Sometimes, the desired frame is also split across multiple buffers.
The workaround is ugly and nasty. The driver assembles all of the data
from the bogus frames into a single buffer. The receive buffers are always
zeroed out, and we program the chip to always include the receive CRC
at the end of each frame. We therefore know that we can start from the
end of the buffer and scan back until we encounter a non-zero data byte,
and say conclusively that this is the end of the desired frame. We can
then subtract the frame length from this address to determine the real
start of the frame, and copy it into an mbuf and pass it on.
This is kludgy and time consuming, but it's better than dropping frames.
It's not too bad since the problem only happens at 10Mbps.
The workaround is only enabled for chips with PCI revision == 33. The
LinkSys LNE100TX and Matrox FastNIC 10/100 cards use a revision 32 chip
and work fine in promiscuous mode. Netgear support has confirmed that
they "have some previous knowledge of problems in promiscuous mode" but
didn't have a workaround. The people at Lite-On who would be able to
suggest a possible fix are on vacation. So, I decided to implement a
workaround of my own until I hear from them. I suppose this problem made
it through Netgear's QA department since Windows doesn't normally use
promiscuous mode, and if Windows doesn't need the feature than it can't
possibly be important, right? Grrr.
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