netdev - Re: [RFC PATCH v2 3/8] net: phylink: call mac_an_restart for SGMII/QSGMII inband interfaces too

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Message-ID: <20191217232515.GR25745@shell.armlinux.org.uk>
Date:   Tue, 17 Dec 2019 23:25:15 +0000
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     davem@...emloft.net, jakub.kicinski@...ronome.com, andrew@...n.ch,
        f.fainelli@...il.com, vivien.didelot@...il.com,
        alexandru.marginean@....com, claudiu.manoil@....com,
        xiaoliang.yang_1@....com, yangbo.lu@....com,
        netdev@...r.kernel.org, alexandre.belloni@...tlin.com,
        horatiu.vultur@...rochip.com,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [RFC PATCH v2 3/8] net: phylink: call mac_an_restart for
 SGMII/QSGMII inband interfaces too

On Wed, Dec 18, 2019 at 12:18:26AM +0200, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> It doesn't quite make sense why restarting the AN process should be
> unique to 802.3z (1000Base-X) modes. It is valid to put an SGMII PCS in
> in-band AN mode, therefore also make PHYLINK re-trigger an
> auto-negotiation if needed.

The question I'd ask is how is that actually achieved on the link?

It makes sense for 1000base-X because either end can drop the ACK bit
to cause a renegotiation to occur, but it makes no sense for SGMII.

In SGMII:
1) there is no advertisement from the MAC to the PHY
2) the PHY is merely informing the MAC of the results of negotiation

Attempting to trigger a renegotiation at the MAC end does nothing
useful for SGMII, it doesn't cause the PHY to renegotiate with its
link partner.

The whole point of SGMII over 1000base-X is that the PHY informs the
MAC using in-band signalling what the results of negotiation were on
the media side of the PHY. SGMII provides no way to control the
advertisement.

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