netdev - Re: [PATCH 1/1] phy: Add 2.5G SGMII interface mode

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Message-ID: <20171130182621.GC28828@lunn.ch>
Date:   Thu, 30 Nov 2017 19:26:21 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Bhaskar Upadhaya <bhaskar.upadhaya@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/1] phy: Add 2.5G SGMII interface mode

On Thu, Nov 30, 2017 at 06:15:20PM +0000, Russell King - ARM Linux wrote:
> On Thu, Nov 30, 2017 at 06:41:27PM +0100, Andrew Lunn wrote:
> > SGMII supports passing auto-negotiation results from the PHY to the
> > MAC. 1000BASE-X does not.
> > 
> > SGMII supports the PHY running at 10, 100, and 1000 Mbps. But to
> > support this, the MAC needs to replicate the bits 100, or 10 times
> > when the PHY is running in 10 or 100Mbps mode.
> > 
> > So with your 2.5G SGMII, you need to replicate the bits 250, 25, or
> > 2.5 times if they PHY is running at lower speeds. This last one is
> > interesting.
> 
> That's not what I've read so far - but I don't know about the PHY
> in this exact case because the docs are only available under NDA
> (which makes it incredibly difficult to have this discussion.)
> 
> However, from what I can ascertain from a Xilinx document, 2.5G is
> 1G SGMII clocked 2.5x faster.  When in 2.5G mode, the other modes
> are unavailable.

Hi Russell

Thanks for looking into the details.

So you need the PHY driver to see what it has negotiated, and when it
calls the adjust_link callback, the MAC needs look at the
phydev->interface and set the MAC to 2.5G SGMII or SGMII.

Same as the Marvell 10G PHY driver flips between
PHY_INTERFACE_MODE_10GKR and PHY_INTERFACE_MODE_SGMII depending on
what it has negotiated.

     Andrew