netdev - RE: [PATCH net-next V1] net/mlx4_en: ethtool force speed when asking for autoneg=off

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Date:	Mon, 8 Dec 2014 10:56:30 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Amir Vadai' <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>,
	"Saeed Mahameed" <saeedm@...lanox.com>
Subject: RE: [PATCH net-next V1] net/mlx4_en: ethtool force speed when
 asking for autoneg=off

From: Amir Vadai
> From: Saeed Mahameed <saeedm@...lanox.com>
> 
> Use cmd->autoneg == AUTONEG_DISABLE as a user hint to force specific speed.
> We don't want to rely on ethtool to calculate advertised link modes when
> forcing specific speed, a user can request a specific speed and specify
> "autoneg off" in ethtool command to give a hint for forcing this speed.

I'm not 100% sure what you are trying to achieve?

By far the safest way to 'force' a specific speed is to set the
advertised modes to contain only the desired speed.
Doing anything else on links that are capable of auto-negotiation
is a complete recipe for disaster.

Even if you fix the operating mode of the PHY and MAC you almost
certainly want to advertise that mode to the remote system.

Yes, I know this is made all the more complicated by 10/100M autodetect.

	David

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