Re: Fix for kern/20700

[Thor Lancelot Simon <tls%panix.com@localhost>]

On Tue, Dec 06, 2011 at 10:41:01AM +0100, Manuel Bouyer wrote:
> 
> But with old devices, hard setting both ends is often the only
> way to get the desired media.

I think this is largely a myth.  I believe it is caused by the unpleasant
fact that there's a race condition in the interaction between Nway and
parallel autodetection, such that when one side is set to Nway and
the other's set to no autonegotiation, the Nway side can fall back to
parallel autodetection and end up with the wrong duplex setting.

The result is that people will set one side to hardwired speed and
duplex (no autonegotiation) and *that* will then cause the link partner
to "autonegotiate" the wrong duplex setting as it fails at Nway and tries
to do parallel detection.

This problem has become less common as vendors have stopped making it
possible to turn off Nway, since turning off Nway is what causes it.

Using the user's selections to restrict the set of what modes will be
offered is a good thing.  Turning off Nway entirely just because the
user has picked a particular mode is a very bad thing.

At the very most, it should perhaps be possible to turn off autonegotiation
by specifying an additional "force" media option but I think even that is
a bad idea.  Apple, for example, doesn't make it possible...