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 11:42:58 +0000
From:	Saeed Mahameed <saeedm@...lanox.com>
To:	David Laight <David.Laight@...LAB.COM>
CC:	Amir Vadai <amirv@...lanox.com>,
	"David S. Miller" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Or Gerlitz <ogerlitz@...lanox.com>,
	Yevgeny Petrilin <yevgenyp@...lanox.com>
Subject: Re: [PATCH net-next V1] net/mlx4_en: ethtool force speed when asking
 for autoneg=off


> On Dec 8, 2014, at 12:57 PM, David Laight <David.Laight@...LAB.COM> wrote:
> 
> 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?
Hey David
I am not trying to fix the operating mode of the phy with this patch.
Here i am trying to give the user the ability To let the driver choose what to advertise when user want to force a specific speed using cmd->autoneg =off .

In the driver :
If (cmd->autoneg==off) advertise_according_speed(cmd->speed)
Else advertise(cmd->advertise)

Thanks
-Saeed
> 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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