Date: Thu, 08 Nov 2001 14:01:19 -0800 From: Michael Loftis <mike@activemessage.com> To: setantae <setantae@submonkey.net> Cc: questions@FreeBSD.ORG Subject: Re: dc(4) TX underrun error Message-ID: <3BEB00AF.9B80BF75@activemessage.com> References: <20011108121547.GA3260@rhadamanth>
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What it means (specifically) (taken from the man page verbatim) is:
dc%d: TX underrun -- increasing TX threshold The device generated a
transmit underrun error while attempting to DMA and transmit a packet.
This happens if the host is not able to DMA the packet data into the
NIC's FIFO fast enough. The driver will dynamically increase the trans-
mit start threshold so that more data must be DMAed into the FIFO before
the NIC will start transmitting it onto the wire.
basically the NIC will start to transmit after a cerain number of bytes are in it's
input area through DMA, usually this is significantly less tahn an ethernet frame
(1500). So what happens is the NIC starts to assemble and transmit the packet
before it has all of it which is a good idea if you're in a hurry, and the host can
keep up with the DMA transfer to give you the rest of the packet before you reach
the end of your DMA stream. Unfortunately sometimes this isn't the case, on
older/slower hardware the DMA controller may get sidetracked and not finish off the
transfer quite quickly enough so the transmitter on the NIC starves for a
millisecond or so too long, it then makes a note and complains to the software
driver.
The driver is informing you that it's increasing the low water mark that must be in
the cards FIFO buffer before it starts transmitting (it's a quick PIO to the card
that changes the setting in the hardware).
setantae wrote:
> Yesterday, as I was FTPing down the stuff for the linux base port, this
> popped up in /var/adm/messages :
>
> Nov 7 21:38:25 rhadamanth /kernel: dc0: TX underrun -- increasing TX threshold
>
> I'm not particularly worried about the message, as it looks totally
> harmless to me, but was just wondering if anyone could explain what it
> actually means ??
>
> Thanks,
>
> Ceri
>
> --
> keep a mild groove on
>
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