netdev - Re: [PATCH net-next 5/7] net: dsa: felix: delete .phylink_mac_an

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Date:   Thu, 25 Jun 2020 17:53:49 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     davem@...emloft.net, netdev@...r.kernel.org, andrew@...n.ch,
        f.fainelli@...il.com, vivien.didelot@...il.com,
        claudiu.manoil@....com, alexandru.marginean@....com,
        ioana.ciornei@....com
Subject: Re: [PATCH net-next 5/7] net: dsa: felix: delete
 .phylink_mac_an_restart code

On Thu, Jun 25, 2020 at 06:23:29PM +0300, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> In hardware, the AN_RESTART field for these SerDes protocols (SGMII,
> USXGMII) clears the resolved configuration from the PCS's
> auto-negotiation state machine.
> 
> But PHYLINK has a different interpretation of "AN restart". It assumes
> that this Linux system is capable of re-triggering an auto-negotiation
> sequence, something which is only possible with 1000Base-X and
> 2500Base-X, where the auto-negotiation is symmetrical. In SGMII and
> USXGMII, there's an AN master and an AN slave, and it isn't so much of
> "auto-negotiation" as it is "PHY passing the resolved link state on to
> the MAC".

This is not "a different interpretation".

The LX2160A documentation for this PHY says:

  9             Restart Auto Negotiation
 Restart_Auto_N Self-clearing Read / Write command bit, set to '1' to
                restart an auto negotiation sequence. Set to '0'
		(Reset value) in normal operation mode. Note: Controls
		the Clause 37 1000Base-X Auto-negotiation.

It doesn't say anything about clearing anything for SGMII.

Also, the Cisco SGMII specification does not indicate that it is
possible to restart the "autonegotiation" - the PHY is the controlling
end of the SGMII link.  There is no clause in the SGMII specification
that indicates that changing the MAC's tx_config word to the PHY will
have any effect on the PHY once the data path has been established.

Finally, when a restart of negotiation is requested, and we have a PHY
attached in SGMII mode, we will talk to that PHY to cause a restart of
negotiation on the media side, which will implicitly cause the link
to drop and re-establish, causing the SGMII side to indicate link down
and subsequently re-establish according to the media side results.

So, please, lay off your phylink bashing in your commit messages.

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