tx underrun ? (add entry into xl manpage)

[Paul Mather]

On Wed, 2005-11-30 at 04:37 -0800, Rob wrote:
> Vincent Blondel wrote:
> > Hello all,
> > 
> > When having a look at log files on my web servers, I
> regulary see next output on the 3COM ethernet
> interfaces :
> > 
> > 
> >     xl1: transmission error: 90
> >     xl1: tx underrun, increasing tx start threshold
> to 120 bytes
> >     xl1: transmission error: 90
> >     xl1: tx underrun, increasing tx start threshold
> to 180 bytes
> >     xl1: promiscuous mode enabled
> >     xl1: promiscuous mode disabled
> > 
> > Can somebody explain me what it is and if this
> situation is normal ?
> 
> Rumours are that these messages are harmless, but if
> someone
> can explain these messages properly, it would be nice
> to
> add an entry to the  DIAGNOSTICS  section of the xl
> manpage
> (hence, this mail also goes to doc mailinglist)
> 
> I see these messages too on my router/gateway:
> 
> xl1: transmission error: 90
> xl1: tx underrun, increasing tx start threshold to 120
> bytes
> xl1: transmission error: 90
> xl1: tx underrun, increasing tx start threshold to 180
> bytes
> xl0: transmission error: 90
> xl0: tx underrun, increasing tx start threshold to 120
> bytes
> xl1: transmission error: 90
> xl1: tx underrun, increasing tx start threshold to 240
> bytes

This is from the dc(4) man page:

     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.

I'm assuming the explanation also holds true for other drivers (xl,
etc.) that issue this warning.

Cheers,

Paul.
-- 
e-mail: paul at gromit.dlib.vt.edu

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