netdev - Re: [PATCH net-next 1/6] ethtool: Extend link modes settings uAPI with lanes

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Message-ID: <20201013084320.40283ba6@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Tue, 13 Oct 2020 08:43:20 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Danielle Ratson <danieller@...dia.com>
Cc:     Ido Schimmel <idosch@...sch.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "Jiri Pirko" <jiri@...dia.com>, "andrew@...n.ch" <andrew@...n.ch>,
        "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "mkubecek@...e.cz" <mkubecek@...e.cz>, mlxsw <mlxsw@...dia.com>,
        Ido Schimmel <idosch@...dia.com>,
        "johannes@...solutions.net" <johannes@...solutions.net>
Subject: Re: [PATCH net-next 1/6] ethtool: Extend link modes settings uAPI
 with lanes

On Tue, 13 Oct 2020 14:29:29 +0000 Danielle Ratson wrote:
> > On Mon, 12 Oct 2020 15:33:45 +0000 Danielle Ratson wrote:  
> > > > What's the use for this in practical terms? Isn't the lane count
> > > > basically implied by the module that gets plugged in?  
> > >
> > > The use is to enable the user to decide how to achieve a certain speed.
> > > For example, if he wants to get 100G and the port has 4 lanes, the
> > > speed can be achieved it using both 2 lanes of 50G and 4 lanes of 25G,
> > > as a port with 4 lanes width can work in 2 lanes mode with double
> > > speed each. So, by specifying "lanes 2" he will achieve 100G using 2
> > > lanes of 50G.  
> > 
> > Can you give a concrete example of serdes capabilities of the port, what SFP
> > gets plugged in and what configuration user wants to select?  
> 
> Example:
> - swp1 is a 200G port with 4 lanes.
> - QSFP28 is plugged in.
> - The user wants to select configuration of 100G speed using 2 lanes, 50G each.

I mean.. sounds quite contrived to me.

But I'm not opposed if others are fine with the change.