netdev - Re: [PATCH net-next v5 1/1] Documentation: net: add flow control guide and document ethtool API

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Message-ID: <aMvF8yNJbPSqqypY@archie.me>
Date: Thu, 18 Sep 2025 15:42:27 +0700
From: Bagas Sanjaya <bagasdotme@...il.com>
To: Oleksij Rempel <o.rempel@...gutronix.de>, 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>,
	Vladimir Oltean <vladimir.oltean@....com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Jesper Dangaard Brouer <hawk@...nel.org>,
	John Fastabend <john.fastabend@...il.com>
Cc: 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 v5 1/1] Documentation: net: add flow control
 guide and document ethtool API

On Thu, Sep 18, 2025 at 07:15:38AM +0200, Oleksij Rempel wrote:
> +* **How it works**: To inhibit incoming data, a receiving device can force a
> +    collision on the line. When the sending station detects this collision, it
> +    terminates its transmission, sends a "jam" signal, and then executes the
> +    "Collision backoff and retransmission" procedure as defined in IEEE 802.3,
> +    Section 4.2.3.2.5. This algorithm makes the sender wait for a random
> +    period before attempting to retransmit. By repeatedly forcing collisions,
> +    the receiver can effectively throttle the sender's transmission rate.

Please align the bullet list text. I see hanging indent instead there in
htmldocs output.

> +* **What it is**: A standard Ethernet frame with a globally reserved
> +    destination MAC address (``01-80-C2-00-00-01``). This address is in a range
> +    that standard IEEE 802.1D-compliant bridges do not forward. However, some
> +    unmanaged or misconfigured bridges have been reported to forward these
> +    frames, which can disrupt flow control across a network.
> +
> +* **How it works**: The frame contains a MAC Control opcode for PAUSE
> +    (``0x0001``) and a ``pause_time`` value, telling the sender how long to
> +    wait before sending more data frames. This time is specified in units of
> +    "pause quantum", where one quantum is the time it takes to transmit 512 bits.
> +    For example, one pause quantum is 51.2 microseconds on a 10 Mbit/s link,
> +    and 512 nanoseconds on a 1 Gbit/s link. A ``pause_time`` of zero indicates
> +    that the transmitter can resume transmission, even if a previous non-zero
> +    pause time has not yet elapsed.

Same here.

> +* **What it is**: PFC allows a receiver to pause traffic for one or more of the
> +    8 standard priority levels without stopping traffic for other priorities.
> +    This is critical in data center environments for protocols that cannot
> +    tolerate packet loss due to congestion (e.g., Fibre Channel over Ethernet
> +    or RoCE).
> +
> +* **How it works**: PFC uses a specific PAUSE frame format. It shares the same
> +    globally reserved destination MAC address (``01-80-C2-00-00-01``) as legacy
> +    PAUSE frames but uses a unique opcode (``0x0101``). The frame payload
> +    contains two key fields:
> +
> +    - **``priority_enable_vector``**: An 8-bit mask where each bit corresponds to
> +      one of the 8 priorities. If a bit is set to 1, it means the pause time
> +      for that priority is active.
> +    - **``time_vector``**: A list of eight 2-octet fields, one for each priority.
> +      Each field specifies the ``pause_time`` for its corresponding priority,
> +      measured in units of ``pause_quanta`` (the time to transmit 512 bits).

Ditto.

Thanks.

-- 
An old man doll... just what I always wanted! - Clara

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