netdev - Re: [PATCH net-next v2 02/13] net: phy: sfp: handle non-wired SFP connectors

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Date:   Sat, 5 May 2018 17:39:45 +0200
From:   Thomas Petazzoni <thomas.petazzoni@...tlin.com>
To:     Antoine Tenart <antoine.tenart@...tlin.com>
Cc:     Florian Fainelli <f.fainelli@...il.com>, davem@...emloft.net,
        kishon@...com, linux@...linux.org.uk, gregory.clement@...tlin.com,
        andrew@...n.ch, jason@...edaemon.net,
        sebastian.hesselbarth@...il.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, maxime.chevallier@...tlin.com,
        miquel.raynal@...tlin.com, nadavh@...vell.com, stefanc@...vell.com,
        ymarkman@...vell.com, mw@...ihalf.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v2 02/13] net: phy: sfp: handle non-wired SFP
 connectors

Hello,

On Fri, 4 May 2018 19:23:37 +0200, Antoine Tenart wrote:

> On Fri, May 04, 2018 at 10:04:48AM -0700, Florian Fainelli wrote:
> > On 05/04/2018 06:56 AM, Antoine Tenart wrote:  
> > > SFP connectors can be solder on a board without having any of their pins
> > > (LOS, i2c...) wired. In such cases the SFP link state cannot be guessed,
> > > and the overall link status reporting is left to other layers.
> > > 
> > > In order to achieve this, a new SFP_DEV status is added, named UNKNOWN.
> > > This mode is set when it is not possible for the SFP code to get the
> > > link status and as a result the link status is reported to be always UP
> > > from the SFP point of view.  
> > 
> > Why represent the SFP in Device Tree then? Why not just declare this is
> > a fixed link which would avoid having to introduce this "unknown" state.  
> 
> The other solution would have been to represent this as a fixed-link.
> But such a representation would report the link as being up all the
> time, which is something we wanted to avoid as the GoP in PPv2 can
> report some link status. This is achieved using SFP+phylink+PPv2.
> 
> And representing the SFP cage in the device tree, although it's a
> "dummy" one, helps describing the hardware.

Just to add to this: the board physically has a SFP cage, and a cable
can be connected to it, or not. So it is absolutely not a fixed link
(cable can be connected or not) and it really is a SFP cage.

Best regards,

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com