Re: half-/full-duplex ethernet and ifconfig

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

On Wed, Nov 14, 2012 at 07:51:39AM +0000, David Laight wrote:
> On Wed, Nov 14, 2012 at 11:57:11AM +0900, Masanobu SAITOH wrote:
> > 
> >  Currently, we have no API to specify an advertise setting for auto
> > negotiation particularly. I think it's useful on some cases.
> 
> When I was playing with ethernet PHY/MAC setup many years ago I
> basically decided that the best way to 'force' the mode was to
> restrict the bits in the ANAR (AutoNegotiate Advertise Reg) rather
> than forcing the MAC itself into a specific mode.
> That way you are much less likely to get painful duplex mismatches.

Yes.  I firmly believe that allowing "hardwired" setting of duplex and
speed is insane.  If it ever solves a problem, the problem should be
solved at the other end of the link, instead.

I note that both Apple and most Windows ethernet driver vendors
implement exactly what you suggest above.  On a Mac with Apple's Ethernet
drivers there is no way to force the link to a particular speed and duplex;
setting the media and mediaopt settings simply restricts the autonegotiation
as you describe above.

However, to David's original question: if you're going to use the media
and mediaopt settings to control speed/duplex, then yes, you get half-duplex
unless you expressly specify "mediaopt full-duplex".  This was the original
design (you can check with Jason) and drivers that do something else are
nonconformant.  Changing it at this point will break things for many users,
specifically by causing setups intended to be half-duplex to not be so.

Thor