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

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Wed, Dec 18, 2019 at 12:18:26AM +0200, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@nxp.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.

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