linux-kernel - Re: [PATCH net-next 1/2] net: phy: sfp: make the i2c-bus property really optional

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Message-ID: <20180517150155.GB17671@n2100.armlinux.org.uk>
Date:   Thu, 17 May 2018 16:01:55 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Antoine Tenart <antoine.tenart@...tlin.com>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        thomas.petazzoni@...tlin.com, maxime.chevallier@...tlin.com,
        gregory.clement@...tlin.com, miquel.raynal@...tlin.com,
        nadavh@...vell.com, stefanc@...vell.com, ymarkman@...vell.com,
        mw@...ihalf.com
Subject: Re: [PATCH net-next 1/2] net: phy: sfp: make the i2c-bus property
 really optional

On Thu, May 17, 2018 at 02:41:28PM +0200, Andrew Lunn wrote:
> On Thu, May 17, 2018 at 10:29:06AM +0200, Antoine Tenart wrote:
> > The SFF,SFP documentation is clear about making all the DT properties,
> > with the exception of the compatible, optional. In practice this is not
> > the case and without an i2c-bus property provided the SFP code will
> > throw NULL pointer exceptions.
> > 
> > This patch is an attempt to fix this.
> 
> Hi Antoine, Russell
> 
> How usable is an SFF/SFP module without access to the i2c EEPROM? I
> guess this comes down to link speed. Can it be manually configured?

While we can support forcing the speed/duplex through ethtool, there
is no obvious facility via ethtool to set the protocol currently (eg,
SGMII vs 1000base-X).  There is, however, the possibility to use the
advertise mask to determine which mode we should be using, but that's
awkward to deal with via ethtool as ethtool wants a hex mask, which
really isn't user friendly.

The protocol matters - for example, a copper SFP module will typically
want to use SGMII, and if it encounters 1000base-X on the other side,
the PHY won't pass data because it will believe that the link to the
MAC is down.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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