netdev - Re: [PATCH v3 net-next 5/6] net: dsa: felix: delete .phylink_mac_an

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Date:   Sun, 5 Jul 2020 14:13:20 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Vladimir Oltean <olteanv@...il.com>, davem@...emloft.net,
        netdev@...r.kernel.org
Cc:     andrew@...n.ch, vivien.didelot@...il.com, claudiu.manoil@....com,
        alexandru.marginean@....com, ioana.ciornei@....com,
        linux@...linux.org.uk
Subject: Re: [PATCH v3 net-next 5/6] net: dsa: felix: delete
 .phylink_mac_an_restart code



On 7/5/2020 9:16 AM, Vladimir Oltean wrote:
> From: Vladimir Oltean <vladimir.oltean@....com>
> 
> Phylink uses the .mac_an_restart method to offer the user an
> implementation of the "ethtool -r" behavior, when the media-side auto
> negotiation can be restarted by the local MAC PCS. This is the case for
> fiber modes 1000Base-X and 2500Base-X (IEEE clause 37) that don't have
> an Ethernet PHY connected locally, and the media is connected to the MAC
> PCS directly.
> 
> On the other hand, the Cisco SGMII and USXGMII standards also have an
> auto negotiation mechanism based on IEEE 802.3 clause 37 (their
> respective specs require a MAC PCS and a PHY PCS to implement the same
> state machine, which is described in IEEE 802.3 "Auto-Negotiation Figure
> 37-6"), so the ability to restart auto-negotiation is intrinsically
> symmetrical (the MAC PCS can do it too).
> 
> However, it appears that not all SGMII/USXGMII PHYs have logic to
> restart the MDI-side auto-negotiation process when they detect a
> transition of the SGMII link from data mode to configuration mode.
> Some do (VSC8234) and some don't (AR8033, MV88E1111). IEEE and/or Cisco
> specification wordings to not help to prove whether propagating the "AN
> restart" event from MII side ("mr_restart_an") to MDI side
> ("mr_restart_negotiation") is required behavior - neither of them
> specifies any mandatory interaction between the clause 37 AN state
> machine from Figure 37-6 and the clause 28 AN state machine from Figure
> 28-18.
> 
> Therefore, even if a certain behavior could be proven as being required,
> real-life SGMII/USXGMII PHYs are inconsistent enough that a clause 37 AN
> restart cannot be used by phylink to reliably trigger a media-side
> renegotiation, when the user requests it via ethtool.
> 
> The only remaining use that the .mac_an_restart callback might possibly
> have, given what we know now, is to implement some silicon quirks, but
> so far that has proven to not be necessary.
> 
> So remove this code for now, since it never gets called and we don't
> foresee any circumstance in which it might be, either.
> 
> Signed-off-by: Vladimir Oltean <vladimir.oltean@....com>

Reviewed-by: Florian Fainelli <f.fainelli@...il.com>
-- 
Florian