netdev - Re: [PATCH net-next v1 1/1] Documentation: networking: add detailed guide on Ethernet flow control configuration

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Message-ID: <aHn526JuMBpUB_T8@pengutronix.de>
Date: Fri, 18 Jul 2025 09:38:03 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Florian Fainelli <f.fainelli@...il.com>,
	Maxime Chevallier <maxime.chevallier@...tlin.com>,
	Kory Maincent <kory.maincent@...tlin.com>,
	Lukasz Majewski <lukma@...x.de>, Jonathan Corbet <corbet@....net>,
	kernel@...gutronix.de, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, Divya.Koppera@...rochip.com
Subject: Re: [PATCH net-next v1 1/1] Documentation: networking: add detailed
 guide on Ethernet flow control configuration

On Thu, Jul 17, 2025 at 06:29:53PM +0100, Russell King (Oracle) wrote:
> On Thu, Jul 17, 2025 at 12:37:02PM +0200, Oleksij Rempel wrote:
> > +Changing the Settings
> > +---------------------
> > +Use `ethtool -A <interface>` to change the settings.
> > +
> > +.. code-block:: bash
> > +
> > +  # Enable RX and TX pause, with autonegotiation
> > +  ethtool -A eth0 autoneg on rx on tx on
> > +
> > +  # Force RX pause on, TX pause off, without autonegotiation
> > +  ethtool -A eth0 autoneg off rx on tx off
> > +
> > +**Key Configuration Concepts**:
> > +
> > +* **Autonegotiation Mode**: The recommended mode. The driver programs the PHY
> > +    to *advertise* the `rx` and `tx` capabilities. The final active state is
> > +    determined by what both sides of the link agree on.
> 
> I'm not sure one cal call this "recommended mode", because it doesn't.
> If one specifies tx=0 rx=1, one would expect that the "recommend mode"
> would be tx=0 and rx=1, but if the link partner supports symmetric
> pause, you actually end up with tx=1 and rx=1. If the link partner
> supports only asymmetric, then you end up with tx=0 rx=1 as requested.
> 
> Perversely, if you specify tx=1 rx=1, then if the remote supports only
> asymmetric, you end up with everything disabled. Only tx=1 rx=1 is
> supported in this configuration, you can't end up with anything else.
> 
> Basically, I don't think calling it "recommended" works.

Ack, I'll drop "recommended".

Would it make sense to also add a short note about the limitations of
link-level flow control? For example, how pause frames can interfere
with traffic prioritization and QoS mechanisms.

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