linux-kernel - Re: [PATCH net-next v3 3/3] Documentation: net: add flow control guide and document ethtool API

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Message-ID: <aKheqhfqht1Cx31M@pengutronix.de>
Date: Fri, 22 Aug 2025 14:12:26 +0200
From: Oleksij Rempel <o.rempel@...gutronix.de>
To: Vladimir Oltean <vladimir.oltean@....com>
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>,
	Donald Hunter <donald.hunter@...il.com>,
	Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
	Jiri Pirko <jiri@...nulli.us>, Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>, kernel@...gutronix.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	Russell King <linux@...linux.org.uk>, Divya.Koppera@...rochip.com,
	Sabrina Dubroca <sd@...asysnail.net>,
	Stanislav Fomichev <sdf@...ichev.me>
Subject: Re: [PATCH net-next v3 3/3] Documentation: net: add flow control
 guide and document ethtool API

On Fri, Aug 22, 2025 at 02:35:19PM +0300, Vladimir Oltean wrote:
...
> > +
> > +* **Who uses it**: Any full-duplex link, from 10 Mbit/s to multi-gigabit speeds.
> > +
> > +The MAC (Media Access Controller)
> > +---------------------------------
> > +The MAC is the hardware component that actually sends and receives PAUSE
> > +frames. Its capabilities define the upper limit of what the driver can support.
> > +For link-wide PAUSE, MACs can vary in their support for symmetric (both
> > +directions) or asymmetric (independent TX/RX) flow control.
> > +
> > +For PFC, the MAC must be capable of generating and interpreting the
> > +priority-based PAUSE frames and managing separate pause states for each
> > +traffic class.
> > +
> > +Many MACs also implement automatic PAUSE frame transmission based on the fill
> > +level of their internal RX FIFO. This is typically configured with two
> > +thresholds:
> > +
> > +* **FLOW_ON (High Water Mark)**: When the RX FIFO usage reaches this
> > +  threshold, the MAC automatically transmits a PAUSE frame to stop the sender.
> > +
> > +* **FLOW_OFF (Low Water Mark)**: When the RX FIFO usage drops below this
> > +  threshold, the MAC transmits a PAUSE frame with a quanta of zero to tell
> > +  the sender it can resume transmission.
> > +
> > +The optimal values for these thresholds depend on the link's round-trip-time
> > +(RTT) and the peer's internal processing latency. The high water mark must be
> > +set low enough so that the MAC's RX FIFO does not overflow while waiting for
> > +the peer to react to the PAUSE frame. The driver is responsible for configuring
> > +sensible defaults according to the IEEE specification. User tuning should only
> > +be necessary in special cases, such as on links with unusually long cable
> > +lengths (e.g., long-haul fiber).
> 
> How would user tuning be achieved?

Do you mean how such tuning could be exposed to user space (e.g. via
ethtool/sysfs), or rather whether it makes sense to provide a user
interface for this at all, since drivers normally set safe defaults?

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