netdev - Re: e1000 driver problems

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Date:	Tue, 04 Dec 2007 10:05:13 -0800
From:	Rick Jones <rick.jones2@...com>
To:	Lukas Hejtmanek <xhejtman@....muni.cz>
CC:	"Kok, Auke" <auke-jan.h.kok@...el.com>,
	Matt Mathis <mathis@....edu>, NetDev <netdev@...r.kernel.org>
Subject: Re: e1000 driver problems

Here is some boilerplate on autoneg which I've been using in other 
forums for a number of years when questions about autoneg vs hardcoding 
and duplex-mismatch arise:

How 100Base-T Autoneg is supposed to work:

When both sides of the link are set to autoneg, they will "negotiate"
the duplex setting and select full-duplex if both sides can do
full-duplex.

If one side is hardcoded and not using autoneg, the autoneg process
will "fail" and the side trying to autoneg is required by spec to use
half-duplex mode.

If one side is using half-duplex, and the other is using full-duplex,
sorrow and woe is the usual result.

So, the following table shows what will happen given various settings
on each side:

                  Auto       Half       Full

    Auto        Happiness   Lucky      Sorrow

    Half        Lucky       Happiness  Sorrow

    Full        Sorrow      Sorrow     Happiness

Happiness means that there is a good shot of everything going well.
Lucky means that things will likely go well, but not because you did
anything correctly :) Sorrow means that there _will_ be a duplex
mis-match.

When there is a duplex mismatch, on the side running half-duplex you
will see various errors and probably a number of _LATE_ collisions
("normal" collisions don't count here).  On the side running
full-duplex you will see things like FCS errors.  Note that those
errors are not necessarily conclusive, they are simply indicators.

Further, it is important to keep in mind that a "clean" ping (or the
like - eg "linkloop" or default netperf TCP_RR) test result is
inconclusive here - a duplex mismatch causes lost traffic _only_ when
both sides of the link try to speak at the same time. A typical ping
test, being synchronous, one at a time request/response, never tries
to have both sides talking at the same time.

Finally, when/if you migrate to 1000Base-T, everything has to be set
to auto-neg anyway.

rick jones
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