linux-kernel - Re: [PATCH] net: phy: mscc: enable MAC SerDes autonegotiation

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Message-ID: <20220208155306.GA3003@localhost>
Date:   Tue, 8 Feb 2022 21:23:06 +0530
From:   Raag Jadav <raagjadav@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Steen Hegelund <steen.hegelund@...rochip.com>,
        Bjarni Jonasson <bjarni.jonasson@...rochip.com>,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: phy: mscc: enable MAC SerDes autonegotiation

On Tue, Feb 08, 2022 at 09:45:13AM +0000, Russell King (Oracle) wrote:
> On Mon, Feb 07, 2022 at 11:19:48PM +0530, Raag Jadav wrote:
> > On Sun, Feb 06, 2022 at 07:01:41PM +0100, Andrew Lunn wrote:
> > > On Sun, Feb 06, 2022 at 10:42:34PM +0530, Raag Jadav wrote:
> > > > On Sat, Feb 05, 2022 at 03:57:49PM +0100, Andrew Lunn wrote:
> > > > > On Sat, Feb 05, 2022 at 12:14:52PM +0530, Raag Jadav wrote:
> > > > > > Enable MAC SerDes autonegotiation to distinguish between
> > > > > > 1000BASE-X, SGMII and QSGMII MAC.
> > > > > 
> > > > > How does autoneg help you here? It just tells you about duplex, pause
> > > > > etc. It does not indicate 1000BaseX, SGMII etc. The PHY should be
> > > > > using whatever mode it was passed in phydev->interface, which the MAC
> > > > > sets when it calls the connection function. If the PHY dynamically
> > > > > changes its host side mode as a result of what that line side is
> > > > > doing, it should also change phydev->interface. However, as far as i
> > > > > can see, the mscc does not do this.
> > > > >
> > > > 
> > > > Once the PHY auto-negotiates parameters such as speed and duplex mode
> > > > with its link partner over the copper link as per IEEE 802.3 Clause 27,
> > > > the link partner’s capabilities are then transferred by PHY to MAC
> > > > over 1000BASE-X or SGMII link using the auto-negotiation functionality
> > > > defined in IEEE 802.3z Clause 37.
> > > 
> > > None of this allows you to distinguish between 1000BASE-X, SGMII and
> > > QSGMII, which is what the commit message says.
> > > 
> > 
> > I agree, the current commit message is misleading.
> > 
> > > It does allow you to get duplex, pause, and maybe speed via in band
> > > signalling. But you should also be getting the same information out of
> > > band, via the phylib callback.
> > > 
> > > There are some MACs which don't seem to work correctly without the in
> > > band signalling, so maybe that is your problem? Please could you give
> > > more background about your problem, what MAC and PHY combination are
> > > you using, what problem you are seeing, etc.
> > > 
> > 
> > MAC implementation[1] in a lot of NXP SoCs comes with in-band aneg enabled
> > by default, and it does expect Clause 37 auto-negotiation to complete
> > between MAC and PHY before the actual data transfer happens.
> > 
> > [1] https://community.nxp.com/pwmxy87654/attachments/pwmxy87654/t-series/3241/1/AN3869(1).pdf
> > 
> > I faced such issue while integrating VSC85xx PHY
> > with one of the recent NXP SoC having similar MAC implementation.
> > Not sure if this is a problem on MAC side or PHY side,
> > But having Clause 37 support should help in most cases I believe.
> 
> Clause 37 is 1000BASE-X negotiation, which is different from SGMII - a
> point which is even made in your PDF above in section 1.1.
> 
> You will need both ends to be operating in SGMII mode for 10M and 100M
> to work. If one end is in 1000BASE-X mdoe and the other is in SGMII,
> it can appear to work, but it won't be working correctly.
> 
> Please get the terminology correct here when talking about SGMII or
> 1000BASE-X.
> 

Thank you for the clarification.
Really appreciate it.

Cheers,
Raag

> -- 
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