netdev - Re: [PATCH] net: phylink: Pass state to pcs

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Date:   Wed, 15 Dec 2021 00:49:14 +0000
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Sean Anderson <sean.anderson@...o.com>
Cc:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Marcin Wojtas <mw@...ihalf.com>, UNGLinuxDriver@...rochip.com,
        "David S . Miller" <davem@...emloft.net>,
        Claudiu Beznea <claudiu.beznea@...rochip.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>,
        Jose Abreu <Jose.Abreu@...opsys.com>,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH] net: phylink: Pass state to pcs_config

On Tue, Dec 14, 2021 at 07:16:53PM -0500, Sean Anderson wrote:
> Ok, so let me clarify my understanding. Perhaps this can be eliminated
> through a different approach.
> 
> When I read the datasheet for mvneta (which hopefully has the same
> logic here, since I could not find a datasheet for an mvpp2 device), I
> noticed that the Pause_Adv bit said
> 
> > It is valid only if flow control mode is defined by Auto-Negotiation
> > (as defined by the <AnFcEn> bit).
> 
> Which I interpreted to mean that if AnFcEn was clear, then no flow
> control was advertised. But perhaps it instead means that the logic is
> something like
> 
> if (AnFcEn)
> 	Config_Reg.PAUSE = Pause_Adv;
> else
> 	Config_Reg.PAUSE = SetFcEn;
> 
> which would mean that we can just clear AnFcEn in link_up if the
> autonegotiated pause settings are different from the configured pause
> settings.

Having actually played with this hardware quite a bit and observed what
it sends, what it implements for advertising is:

	Config_Reg.PAUSE = Pause_Adv;

Config_Reg gets sent over the 1000BASE-X link to the link partner, and
we receive Remote_Reg from the link partner.

Then, the hardware implements:

	if (AnFcEn)
		MAC_PAUSE = Config_Reg.PAUSE & Remote_Reg.PAUSE;
	else
		MAC_PAUSE = SetFcEn;

In otherwords, AnFcEn controls whether the result of autonegotiation
or the value of SetFcEn controls whether the MAC enables symmetric
pause mode.

Pause_Adv comes from the advertisement, and this is controlled from
ethtool -s and somewhat by ethtool -A.

AnFcEn is controlled purely and only by ethtool -A ... autoneg on|off.
You can't derive this from "state".

SetFcEn comes from ethtool -A ... tx and rx parameters (which must
both be on or both be off.)

Since we have no knowledge what Remote_Reg contains (it's not made
accessible by the hardware), it's impossible to derive whether the
autonegotiated pause settings are different from the configured pause
settings.

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