netdev - Re: [PATCH net v2] ethtool: reset #lanes when lanes is omitted

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Date:   Tue, 4 Apr 2023 09:01:13 +0300
From:   Ido Schimmel <idosch@...dia.com>
To:     Andy Roulin <aroulin@...dia.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        jiri@...dia.com, andrew@...n.ch, f.fainelli@...il.com,
        mkubecek@...e.cz, mlxsw@...dia.com,
        Danielle Ratson <danieller@...dia.com>
Subject: Re: [PATCH net v2] ethtool: reset #lanes when lanes is omitted

On Mon, Apr 03, 2023 at 02:20:53PM -0700, Andy Roulin wrote:
> If the number of lanes was forced and then subsequently the user
> omits this parameter, the ksettings->lanes is reset. The driver
> should then reset the number of lanes to the device's default
> for the specified speed.
> 
> However, although the ksettings->lanes is set to 0, the mod variable
> is not set to true to indicate the driver and userspace should be
> notified of the changes.
> 
> The consequence is that the same ethtool operation will produce
> different results based on the initial state.
> 
> If the initial state is:
> $ ethtool swp1 | grep -A 3 'Speed: '
>         Speed: 500000Mb/s
>         Lanes: 2
>         Duplex: Full
>         Auto-negotiation: on
> 
> then executing 'ethtool -s swp1 speed 50000 autoneg off' will yield:
> $ ethtool swp1 | grep -A 3 'Speed: '
>         Speed: 500000Mb/s
>         Lanes: 2
>         Duplex: Full
>         Auto-negotiation: off
> 
> While if the initial state is:
> $ ethtool swp1 | grep -A 3 'Speed: '
>         Speed: 500000Mb/s
>         Lanes: 1
>         Duplex: Full
>         Auto-negotiation: off
> 
> executing the same 'ethtool -s swp1 speed 50000 autoneg off' results in:
> $ ethtool swp1 | grep -A 3 'Speed: '
>         Speed: 500000Mb/s
>         Lanes: 1
>         Duplex: Full
>         Auto-negotiation: off
> 
> This patch fixes this behavior. Omitting lanes will always results in
> the driver choosing the default lane width for the chosen speed. In this
> scenario, regardless of the initial state, the end state will be, e.g.,
> 
> $ ethtool swp1 | grep -A 3 'Speed: '
>         Speed: 500000Mb/s
>         Lanes: 2
>         Duplex: Full
>         Auto-negotiation: off
> 
> Fixes: 012ce4dd3102 ("ethtool: Extend link modes settings uAPI with lanes")
> Signed-off-by: Andy Roulin <aroulin@...dia.com>
> Reviewed-by: Danielle Ratson <danieller@...dia.com>

Reviewed-by: Ido Schimmel <idosch@...dia.com>