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Revert "Documentation: net: add flow control guide and document ethtool API"
This reverts commit 7bd80ed89d72285515db673803b021469ba71ee8. I should not have merged it to begin with due to pending review and changes to be addressed. Link: https://patch.msgid.link/c6f3af12df9b7998920a02027fc8893ce82afc4c.1759239721.git.pabeni@redhat.com Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Paolo Abeni
parent
3cfb33f92d
commit
1a98f5699b
@ -864,9 +864,7 @@ attribute-sets:
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-
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name: pause-stat
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doc: Statistics counters for link-wide PAUSE frames (IEEE 802.3 Annex 31B).
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attr-cnt-name: __ethtool-a-pause-stat-cnt
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enum-name: ethtool-a-pause-stat
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attributes:
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-
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name: unspec
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@ -877,17 +875,13 @@ attribute-sets:
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type: pad
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-
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name: tx-frames
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doc: Number of PAUSE frames transmitted.
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type: u64
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-
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name: rx-frames
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doc: Number of PAUSE frames received.
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type: u64
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-
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name: pause
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doc: Parameters for link-wide PAUSE (IEEE 802.3 Annex 31B).
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attr-cnt-name: __ethtool-a-pause-cnt
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enum-name: ethtool-a-pause
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attributes:
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-
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name: unspec
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@ -899,40 +893,19 @@ attribute-sets:
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nested-attributes: header
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-
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name: autoneg
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doc: |
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Acts as a mode selector for the driver.
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On GET: indicates the driver's behavior. If true, the driver will
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respect the negotiated outcome; if false, the driver will use a
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forced configuration.
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On SET: if true, the driver configures the PHY's advertisement based
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on the rx and tx attributes. If false, the driver forces the MAC
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into the state defined by the rx and tx attributes.
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type: u8
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-
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name: rx
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doc: |
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Enable receiving PAUSE frames (pausing local TX).
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On GET: reflects the currently preferred configuration state.
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type: u8
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-
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name: tx
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doc: |
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Enable transmitting PAUSE frames (pausing peer TX).
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On GET: reflects the currently preferred configuration state.
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type: u8
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-
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name: stats
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doc: |
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Contains the pause statistics counters. The source of these
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statistics is determined by stats-src.
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type: nest
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nested-attributes: pause-stat
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-
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name: stats-src
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doc: |
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Selects the source of the MAC statistics, values from
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enum ethtool_mac_stats_src. This allows requesting statistics
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from the individual components of the MAC Merge layer.
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type: u32
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-
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name: eee
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@ -1,373 +0,0 @@
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.. SPDX-License-Identifier: GPL-2.0
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.. _ethernet-flow-control:
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=====================
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Ethernet Flow Control
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=====================
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This document is a practical guide to Ethernet Flow Control in Linux, covering
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what it is, how it works, and how to configure it.
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What is Flow Control?
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=====================
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Flow control is a mechanism to prevent a fast sender from overwhelming a
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slow receiver with data, which would cause buffer overruns and dropped packets.
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The receiver can signal the sender to temporarily stop transmitting, giving it
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time to process its backlog.
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Standards references
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====================
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Ethernet flow control mechanisms are specified across consolidated IEEE base
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standards; some originated as amendments:
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- Collision-based flow control is part of CSMA/CD in **IEEE 802.3**
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(half-duplex).
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- Link-wide PAUSE is defined in **IEEE 802.3 Annex 31B**
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(originally **802.3x**).
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- Priority-based Flow Control (PFC) is defined in **IEEE 802.1Q Clause 36**
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(originally **802.1Qbb**).
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In the remainder of this document, the consolidated clause numbers are used.
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How It Works: The Mechanisms
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============================
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The method used for flow control depends on the link's duplex mode.
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.. note::
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The user-visible ``ethtool`` pause API described in this document controls
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**link-wide PAUSE** (IEEE 802.3 Annex 31B) only. It does not control the
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collision-based behavior that exists on half-duplex links.
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1. Half-Duplex: Collision-Based Flow Control
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--------------------------------------------
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On half-duplex links, a device cannot send and receive simultaneously, so PAUSE
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frames are not used. Flow control is achieved by leveraging the CSMA/CD
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(Carrier Sense Multiple Access with Collision Detection) protocol itself.
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* **How it works**: To inhibit incoming data, a receiving device can force a
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collision on the line. When the sending station detects this collision, it
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terminates its transmission, sends a "jam" signal, and then executes the
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"Collision backoff and retransmission" procedure as defined in IEEE 802.3,
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Section 4.2.3.2.5. This algorithm makes the sender wait for a random
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period before attempting to retransmit. By repeatedly forcing collisions,
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the receiver can effectively throttle the sender's transmission rate.
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.. note::
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While this mechanism is part of the IEEE standard, there is currently no
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generic kernel API to configure or control it. Drivers should not enable
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this feature until a standardized interface is available.
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.. warning::
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On shared-medium networks (e.g. 10BASE2, or twisted-pair networks using a
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hub rather than a switch) forcing collisions inhibits traffic **across the
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entire shared segment**, not just a single point-to-point link. Enabling
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such behavior is generally undesirable.
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2. Full-Duplex: Link-wide PAUSE (IEEE 802.3 Annex 31B)
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------------------------------------------------------
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On full-duplex links, devices can send and receive at the same time. Flow
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control is achieved by sending a special **PAUSE frame**, defined by IEEE
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802.3 Annex 31B. This mechanism pauses all traffic on the link and is therefore
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called *link-wide PAUSE*.
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* **What it is**: A standard Ethernet frame with a globally reserved
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destination MAC address (``01-80-C2-00-00-01``). This address is in a range
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that standard IEEE 802.1D-compliant bridges do not forward. However, some
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unmanaged or misconfigured bridges have been reported to forward these
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frames, which can disrupt flow control across a network.
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* **How it works**: The frame contains a MAC Control opcode for PAUSE
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(``0x0001``) and a ``pause_time`` value, telling the sender how long to
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wait before sending more data frames. This time is specified in units of
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"pause quantum", where one quantum is the time it takes to transmit 512 bits.
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For example, one pause quantum is 51.2 microseconds on a 10 Mbit/s link,
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and 512 nanoseconds on a 1 Gbit/s link. A ``pause_time`` of zero indicates
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that the transmitter can resume transmission, even if a previous non-zero
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pause time has not yet elapsed.
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* **Who uses it**: Any full-duplex link, from 10 Mbit/s to multi-gigabit speeds.
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3. Full-Duplex: Priority-based Flow Control (PFC) (IEEE 802.1Q Clause 36)
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-------------------------------------------------------------------------
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Priority-based Flow Control is an enhancement to the standard PAUSE mechanism
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that allows flow control to be applied independently to different classes of
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traffic, identified by their priority level.
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* **What it is**: PFC allows a receiver to pause traffic for one or more of the
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8 standard priority levels without stopping traffic for other priorities.
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This is critical in data center environments for protocols that cannot
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tolerate packet loss due to congestion (e.g., Fibre Channel over Ethernet
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or RoCE).
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* **How it works**: PFC uses a specific PAUSE frame format. It shares the same
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globally reserved destination MAC address (``01-80-C2-00-00-01``) as legacy
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PAUSE frames but uses a unique opcode (``0x0101``). The frame payload
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contains two key fields:
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- **``priority_enable_vector``**: An 8-bit mask where each bit corresponds to
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one of the 8 priorities. If a bit is set to 1, it means the pause time
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for that priority is active.
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- **``time_vector``**: A list of eight 2-octet fields, one for each priority.
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Each field specifies the ``pause_time`` for its corresponding priority,
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measured in units of ``pause_quanta`` (the time to transmit 512 bits).
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.. note::
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When PFC is enabled for at least one priority on a port, the standard
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**link-wide PAUSE** (IEEE 802.3 Annex 31B) must be disabled for that port.
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The two mechanisms are mutually exclusive (IEEE 802.1Q Clause 36).
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Configuring Flow Control
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========================
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Link-wide PAUSE and Priority-based Flow Control are configured with different
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tools.
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Configuring Link-wide PAUSE with ``ethtool`` (IEEE 802.3 Annex 31B)
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-------------------------------------------------------------------
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Use ``ethtool -a <interface>`` to view and ``ethtool -A <interface>`` to change
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the link-wide PAUSE settings.
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.. code-block:: bash
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# View current link-wide PAUSE settings
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ethtool -a eth0
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# Enable RX and TX pause, with autonegotiation
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ethtool -A eth0 autoneg on rx on tx on
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**Key Configuration Concepts**:
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* **Pause Autoneg vs Generic Autoneg**: ``ethtool -A ... autoneg {on,off}``
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controls **Pause Autoneg** (Annex 31B) only. It is independent from the
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**Generic link autonegotiation** configured with ``ethtool -s``. A device can
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have Generic autoneg **on** while Pause Autoneg is **off**, and vice versa.
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* **If Pause Autoneg is off** (``-A ... autoneg off``): the device will **not**
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advertise pause in the PHY. The MAC PAUSE state is **forced** according to
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``rx``/``tx`` and does not depend on partner capabilities or resolution.
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Ensure the peer is configured complementarily for PAUSE to be effective.
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* **If generic autoneg is off** but **Pause Autoneg is on**, the pause policy
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is **remembered** by the kernel and applied later when Generic autoneg is
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enabled again.
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* **Autonegotiation Mode**: The PHY will *advertise* the ``rx`` and ``tx``
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capabilities. The final active state is determined by what both sides of the
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link agree on. See the "PHY (Physical Layer Transceiver)" section below,
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especially the *Resolution* subsection, for details of the negotiation rules.
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* **Forced Mode**: This mode is necessary when autonegotiation is not used or
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not possible. This includes links where one or both partners have
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autonegotiation disabled, or in setups without a PHY (e.g., direct
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MAC-to-MAC connections). The driver bypasses PHY advertisement and
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directly forces the MAC into the specified ``rx``/``tx`` state. The
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configuration on both sides of the link must be complementary. For
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example, if one side is set to ``tx on`` ``rx off``, the link partner must be
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set to ``tx off`` ``rx on`` for flow control to function correctly.
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Configuring PFC with ``dcb`` (IEEE 802.1Q Clause 36)
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----------------------------------------------------
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PFC is part of the Data Center Bridging (DCB) subsystem and is managed with the
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``dcb`` tool (iproute2). Some deployments use ``dcbtool`` (lldpad) instead; this
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document shows ``dcb(8)`` examples.
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**Viewing PFC Settings**:
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.. code-block:: text
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$ dcb pfc show dev eth0
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pfc-cap 8 macsec-bypass off delay 4096
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prio-pfc 0:off 1:off 2:off 3:off 4:off 5:off 6:on 7:on
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This shows the PFC state (on/off) for each priority (0-7).
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**Changing PFC Settings**:
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.. code-block:: bash
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# Enable PFC on priorities 6 and 7, leaving others as they are
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$ dcb pfc set dev eth0 prio-pfc 6:on 7:on
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# Disable PFC for all priorities except 6 and 7
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$ dcb pfc set dev eth0 prio-pfc all:off 6:on 7:on
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Monitoring Flow Control
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=======================
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The standard way to check if flow control is actively being used is to view the
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pause-related statistics.
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**Monitoring Link-wide PAUSE**:
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Use ``ethtool --include-statistics -a <interface>``.
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.. code-block:: text
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$ ethtool --include-statistics -a eth0
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Pause parameters for eth0:
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...
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Statistics:
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tx_pause_frames: 0
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rx_pause_frames: 0
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**Monitoring PFC**:
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PFC statistics (sent and received frames per priority) are available
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through the ``dcb`` tool.
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.. code-block:: text
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$ dcb pfc show dev eth0 requests indications
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requests 0:0 1:0 2:0 3:1024 4:2048 5:0 6:0 7:0
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indications 0:0 1:0 2:0 3:512 4:4096 5:0 6:0 7:0
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The ``requests`` counters track transmitted PFC frames (TX), and the
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``indications`` counters track received PFC frames (RX).
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Link-wide PAUSE Autonegotiation Details
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=======================================
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The autonegotiation process for link-wide PAUSE is managed by the PHY and
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involves advertising capabilities and resolving the outcome.
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* Terminology (link-wide PAUSE):
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- **Symmetric pause**: both directions are paused when requested (TX+RX
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enabled).
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- **Asymmetric pause**: only one direction is paused (e.g., RX-only or
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TX-only).
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In IEEE 802.3 advertisement/resolution, symmetric/asymmetric are encoded
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using two bits (Pause/Asym) and resolved per the standard truth tables
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below.
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* **Advertisement**: The PHY advertises the MAC's flow control capabilities.
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This is done using two bits in the advertisement register: "Symmetric
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Pause" (Pause) and "Asymmetric Pause" (Asym). These bits should be
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interpreted as a combined value, not as independent flags. The kernel
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converts the user's ``rx`` and ``tx`` settings into this two-bit value as
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follows:
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.. code-block:: text
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tx rx | Pause Asym
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-------+-------------
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0 0 | 0 0
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0 1 | 1 1
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1 0 | 0 1
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1 1 | 1 0
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* **Resolution**: After negotiation, the PHY reports the link partner's
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advertised Pause and Asym bits. The final flow control mode is determined
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by the combination of the local and partner advertisements, according to
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the IEEE 802.3 standard:
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.. code-block:: text
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Local Device | Link Partner | Result
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Pause Asym | Pause Asym |
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-------------------+--------------------+---------
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0 X | 0 X | Disabled
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0 1 | 1 0 | Disabled
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0 1 | 1 1 | TX only
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1 0 | 0 X | Disabled
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1 X | 1 X | TX + RX
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1 1 | 0 1 | RX only
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It is important to note that the advertised bits reflect the *current
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configuration* of the MAC, which may not represent its full hardware
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capabilities.
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Kernel Policy: "Set and Trust"
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==============================
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The ethtool pause API is defined as a **wish policy** for
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IEEE 802.3 link-wide PAUSE only. A user request is always accepted
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as the preferred configuration, but it may not be possible to apply
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it in all link states.
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Key constraints:
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- Link-wide PAUSE is not valid on half-duplex links.
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- Link-wide PAUSE cannot be used together with Priority-based Flow Control
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(PFC, IEEE 802.1Q Clause 36).
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- If autonegotiation is active and the link is currently down, the future
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mode is not yet known.
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Because of these constraints, the kernel stores the requested setting
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and applies it only when the link is in a compatible state.
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Implications for userspace:
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1. Set once (the "wish"): the requested Rx/Tx PAUSE policy is
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remembered even if it cannot be applied immediately.
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2. Applied conditionally: when the link comes up, the kernel enables
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PAUSE only if the active mode allows it.
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Component Roles in Flow Control
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||||
===============================
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The configuration of flow control involves several components, each with a
|
||||
distinct role.
|
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|
||||
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.
|
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For link-wide PAUSE, MACs can vary in their support for symmetric (both
|
||||
directions) or asymmetric (independent TX/RX) flow control.
|
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|
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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.
|
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Many MACs also implement automatic PAUSE frame transmission based on the fill
|
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level of their internal RX FIFO. This is typically configured with two
|
||||
thresholds:
|
||||
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* **FLOW_ON (High Water Mark)**: When the RX FIFO usage reaches this
|
||||
threshold, the MAC automatically transmits a PAUSE frame to stop the sender.
|
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* **FLOW_OFF (Low Water Mark)**: When the RX FIFO usage drops below this
|
||||
threshold, the MAC transmits a PAUSE frame with a quantum of zero to tell
|
||||
the sender it can resume transmission.
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|
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The PHY (Physical Layer Transceiver)
|
||||
------------------------------------
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||||
The PHY's role is distinct for each flow control mechanism:
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||||
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* **Link-wide PAUSE**: During the autonegotiation process, the PHY is
|
||||
responsible for advertising the device's flow control capabilities. See the
|
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"Link-wide PAUSE Autonegotiation Details" section for more information.
|
||||
|
||||
* **Half-Duplex Collision-Based Flow Control**: The PHY is fundamental to the
|
||||
CSMA/CD process. It performs carrier sensing (checking if the line is idle)
|
||||
and collision detection, which is the mechanism leveraged to throttle the
|
||||
sender.
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||||
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||||
* **Priority-based Flow Control (PFC)**: The PHY is not directly involved in
|
||||
negotiating PFC capabilities. Its role is to establish the physical link.
|
||||
PFC negotiation happens at a higher layer via the Data Center Bridging
|
||||
Capability Exchange Protocol (DCBX).
|
||||
|
||||
User Space Interface
|
||||
====================
|
||||
The primary user space tools are ``ethtool`` for link-wide PAUSE and ``dcb`` for
|
||||
PFC. They communicate with the kernel to configure the network device driver
|
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and underlying hardware.
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**Link-wide PAUSE Netlink Interface (``ethtool``)**
|
||||
|
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See the ethtool Netlink spec (``Documentation/netlink/specs/ethtool.yaml``)
|
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for the authoritative definition of the Pause control and Pause statistics
|
||||
attributes. The generated UAPI is in
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||||
``include/uapi/linux/ethtool_netlink_generated.h``.
|
||||
|
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**PFC Netlink Interface (``dcb``)**
|
||||
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||||
The authoritative definitions for DCB/PFC netlink attributes and commands are in
|
||||
``include/uapi/linux/dcbnl.h``. See also the ``dcb(8)`` manual page and the DCB
|
||||
subsystem documentation for userspace configuration details.
|
||||
|
||||
@ -55,7 +55,6 @@ Contents:
|
||||
eql
|
||||
fib_trie
|
||||
filter
|
||||
flow_control
|
||||
generic-hdlc
|
||||
generic_netlink
|
||||
../netlink/specs/index
|
||||
|
||||
@ -343,8 +343,16 @@ Some of the interface modes are described below:
|
||||
Pause frames / flow control
|
||||
===========================
|
||||
|
||||
For detailed link-wide PAUSE and PFC behavior and configuration, see
|
||||
flow_control.rst.
|
||||
The PHY does not participate directly in flow control/pause frames except by
|
||||
making sure that the SUPPORTED_Pause and SUPPORTED_AsymPause bits are set in
|
||||
MII_ADVERTISE to indicate towards the link partner that the Ethernet MAC
|
||||
controller supports such a thing. Since flow control/pause frames generation
|
||||
involves the Ethernet MAC driver, it is recommended that this driver takes care
|
||||
of properly indicating advertisement and support for such features by setting
|
||||
the SUPPORTED_Pause and SUPPORTED_AsymPause bits accordingly. This can be done
|
||||
either before or after phy_connect() and/or as a result of implementing the
|
||||
ethtool::set_pauseparam feature.
|
||||
|
||||
|
||||
Keeping Close Tabs on the PAL
|
||||
=============================
|
||||
|
||||
@ -953,48 +953,9 @@ struct kernel_ethtool_ts_info {
|
||||
* @get_pause_stats: Report pause frame statistics. Drivers must not zero
|
||||
* statistics which they don't report. The stats structure is initialized
|
||||
* to ETHTOOL_STAT_NOT_SET indicating driver does not report statistics.
|
||||
*
|
||||
* @get_pauseparam: Report the configured policy for link-wide PAUSE
|
||||
* (IEEE 802.3 Annex 31B). Drivers must fill struct ethtool_pauseparam
|
||||
* such that:
|
||||
* @autoneg:
|
||||
* This refers to **Pause Autoneg** (IEEE 802.3 Annex 31B) only
|
||||
* and is independent of generic link autonegotiation configured
|
||||
* via ethtool -s.
|
||||
* true -> the device follows the negotiated result of pause
|
||||
* autonegotiation (Pause/Asym);
|
||||
* false -> the device uses a forced MAC state independent of
|
||||
* negotiation.
|
||||
* @rx_pause/@tx_pause:
|
||||
* represent the desired policy (preferred configuration).
|
||||
* In autoneg mode they describe what is to be advertised;
|
||||
* in forced mode they describe the MAC state to apply.
|
||||
*
|
||||
* Drivers (and/or frameworks) should persist this policy across link
|
||||
* changes and reapply appropriate MAC programming when link parameters
|
||||
* change.
|
||||
*
|
||||
* @set_pauseparam: Apply a policy for link-wide PAUSE (IEEE 802.3 Annex 31B).
|
||||
* If @autoneg is true:
|
||||
* Arrange for pause advertisement (Pause/Asym) based on
|
||||
* @rx_pause/@tx_pause and program the MAC to follow the
|
||||
* negotiated result (which may be symmetric, asymmetric, or off
|
||||
* depending on the link partner).
|
||||
* If @autoneg is false:
|
||||
* Do not rely on autonegotiation; force the MAC RX/TX pause
|
||||
* state directly per @rx_pause/@tx_pause.
|
||||
*
|
||||
* Implementations that integrate with PHYLIB/PHYLINK should cooperate
|
||||
* with those frameworks for advertisement and resolution; MAC drivers are
|
||||
* still responsible for applying the required MAC state.
|
||||
*
|
||||
* Return: 0 on success or a negative errno. Return -EOPNOTSUPP if
|
||||
* link-wide PAUSE is unsupported. If only symmetric pause is supported,
|
||||
* reject unsupported asymmetric requests with -EINVAL (or document any
|
||||
* coercion policy).
|
||||
*
|
||||
* See also: Documentation/networking/flow_control.rst
|
||||
*
|
||||
* @get_pauseparam: Report pause parameters
|
||||
* @set_pauseparam: Set pause parameters. Returns a negative error code
|
||||
* or zero.
|
||||
* @self_test: Run specified self-tests
|
||||
* @get_strings: Return a set of strings that describe the requested objects
|
||||
* @set_phys_id: Identify the physical devices, e.g. by flashing an LED
|
||||
|
||||
@ -375,7 +375,7 @@ enum {
|
||||
ETHTOOL_A_COALESCE_MAX = (__ETHTOOL_A_COALESCE_CNT - 1)
|
||||
};
|
||||
|
||||
enum ethtool_a_pause_stat {
|
||||
enum {
|
||||
ETHTOOL_A_PAUSE_STAT_UNSPEC,
|
||||
ETHTOOL_A_PAUSE_STAT_PAD,
|
||||
ETHTOOL_A_PAUSE_STAT_TX_FRAMES,
|
||||
@ -385,7 +385,7 @@ enum ethtool_a_pause_stat {
|
||||
ETHTOOL_A_PAUSE_STAT_MAX = (__ETHTOOL_A_PAUSE_STAT_CNT - 1)
|
||||
};
|
||||
|
||||
enum ethtool_a_pause {
|
||||
enum {
|
||||
ETHTOOL_A_PAUSE_UNSPEC,
|
||||
ETHTOOL_A_PAUSE_HEADER,
|
||||
ETHTOOL_A_PAUSE_AUTONEG,
|
||||
|
||||
@ -27,8 +27,6 @@
|
||||
*
|
||||
* Priority-based Flow Control (PFC) - provides a flow control mechanism which
|
||||
* can work independently for each 802.1p priority.
|
||||
* See Documentation/networking/flow_control.rst for a high level description
|
||||
* of the user space interface for Priority-based Flow Control (PFC).
|
||||
*
|
||||
* Congestion Notification - provides a mechanism for end-to-end congestion
|
||||
* control for protocols which do not have built-in congestion management.
|
||||
|
||||
@ -1,9 +1,5 @@
|
||||
// SPDX-License-Identifier: GPL-2.0-only
|
||||
|
||||
/* See Documentation/networking/flow_control.rst for a high level description of
|
||||
* the userspace interface.
|
||||
*/
|
||||
|
||||
#include "netlink.h"
|
||||
#include "common.h"
|
||||
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user