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[212.171.11.167]) by smtp.googlemail.com with ESMTPSA id ffacd0b85a97d-49c5f7828a4sm2029768f8f.23.2026.11.12.16.32.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Nov 2026 16:32:53 -0800 (PST) From: Christian Marangi To: Maxime Chevallier , Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Simon Horman , Jonathan Corbet , Shuah Khan , Randy Dunlap , Christian Marangi , Lorenzo Bianconi , Heiner Kallweit , Russell King , Philipp Zabel , Nathan Chancellor , Nick Desaulniers , Bill Wendling , Justin Stitt , netdev@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mediatek@lists.infradead.org, llvm@lists.linux.dev Subject: [PATCH net-next v19 10/15] net: Document PCS subsystem Date: Fri, 13 Nov 2026 01:32:00 +0100 Message-ID: <20261113003217.197757-11-ansuelsmth@gmail.com> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20261113003217.197757-1-ansuelsmth@gmail.com> References: <20261113003217.197757-1-ansuelsmth@gmail.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Add extensive documentation of the new PCS subsystem and the fwnode implementation with producer/consumer API. Also update the sfp-phylink migration guide. Signed-off-by: Christian Marangi --- Documentation/networking/index.rst | 1 + Documentation/networking/pcs.rst | 234 +++++++++++++++++++++++ Documentation/networking/sfp-phylink.rst | 35 ++-- 3 files changed, 246 insertions(+), 24 deletions(-) create mode 100644 Documentation/networking/pcs.rst diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst index 44a422ad3b05..3fce8f6ac089 100644 --- a/Documentation/networking/index.rst +++ b/Documentation/networking/index.rst @@ -28,6 +28,7 @@ Contents: net_failover page_pool phy + pcs sfp-phylink alias bridge diff --git a/Documentation/networking/pcs.rst b/Documentation/networking/pcs.rst new file mode 100644 index 000000000000..2cae70231948 --- /dev/null +++ b/Documentation/networking/pcs.rst @@ -0,0 +1,234 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============= +PCS Subsystem +============= + +The PCS (Physical Coding Sublayer) subsystem handles the registration and lookup +of PCS devices. These devices contain the upper sublayers of the Ethernet +physical layer, generally handling framing, scrambling, and encoding tasks. PCS +devices may also include PMA (Physical Medium Attachment) components. PCS +devices transfer data between the Link-layer MAC device, and the rest of the +physical layer, typically via a serdes. The output of the serdes may be +connected more-or-less directly to the medium when using fiber-optic or +backplane connections (1000BASE-SX, 1000BASE-KX, etc). It may also communicate +with a separate PHY (such as over SGMII) which handles the connection to the +medium (such as 1000BASE-T). + +Remark on usage of .mac_select_pcs and fw_node PCS +-------------------------------------------------- + +There are generally two ways to look up a PCS device. + +1. MAC OP struct .mac_select_pcs (considered deprecated) +2. Internal PCS handling with ``num_possible_pcs`` and ``fill_available_pcs`` + +Implementation 1 leaves the entire handling of the PCS to the MAC +driver with the selection of the PCS driven by .mac_select_pcs. +Custom implementations are required if the PCS is external to the MAC +and needs to be handled by a separate driver. + +Implementation 2 makes the phylink core code to select the PCS +provided by ``num_possible_pcs`` and ``fill_available_pcs`` either +via internal MAC handling or firmware node (fwnode) + +Implementation 1 is considered deprecated and it's suggested to +switch to implementation 2 and where possible switch to firmware +node design. + +.. _pcs_fwnode: + +Looking up PCS Devices (fwnode implementation) +----------------------------------------------- + +The lookup of a PCS device follows the common producer/consumer implementation +used by similar subsystems with a ``#pcs-cells`` on the producer and a +``pcs-handle`` property on the consumer:: + + pcs: pcs { + // ... + #pcs-cells = <0>; + }; + + ethernet-controller { + // ... + pcs-handle = <&pcs>; + }; + +On :c:func:`phylink_create`, phylink will use the ``num_possible_pcs`` +value and ``fill_available_pcs`` helper function in +:c:struct:`phylink_config` to compose the list of available PCS that can be +used for the phylink instance. + +Phylink will then internally handle the selection of the correct PCS for +the requested interface mode based on the interface modes configured in +``pcs_interfaces`` in :c:struct:`phylink_config` struct and +``supported_interfaces`` in :c:struct:`phylink_pcs` struct. + +A PCS is considered eligible when the requested interface mode is present +in both ``pcs_interfaces`` in :c:struct:`phylink_config` struct and +``supported_interfaces`` in :c:struct:`phylink_pcs` struct. + +``supported_interfaces`` describes all interface modes supported by the MAC, +whereas ``pcs_interfaces`` identifies the subset that require PCS selection. + +For the special implementation where the PCS is internal or part of the MAC +and a dedicated driver is not needed, it's possible to leave the implementation +of the PCS to the MAC driver and just implement the ``num_possible_pcs`` +value and ``fill_available_pcs`` helper function in +:c:struct:`phylink_config` referencing the local :c:struct:`phylink_pcs` +struct allocated from the MAC driver. + +.. _pcs_consumer: + +Using PCS Devices +----------------- + +It's mandatory to either implement the ``mac_select_pcs`` callback +of :c:struct:`phylink_mac_ops` or ``num_possible_pcs`` and ``fill_available_pcs`` +of :c:struct:`phylink_config` to use a PCS for a MAC. + +The fwnode implementation exposes simple helpers to parse the PCS from +the fwnode :c:func:`fwnode_phylink_pcs_count` and +:c:func:`fwnode_phylink_pcs_parse`. The :c:func:`fwnode_phylink_pcs_count` helper +takes the fwnode where the ``pcs-handle`` should be parsed and return the +number of PCS entries described in the fwnode. +The :c:func:`fwnode_phylink_pcs_parse` helper takes three arguments, +the fwnode where the ``pcs-handle`` should be parsed, an allocated array +of :c:struct:`phylink_pcs` pointer where to put the parsed PCS from the fwnode +and the maximum number of PCS to parse. +Contrary to :c:func:`fwnode_phylink_pcs_count`, :c:func:`fwnode_phylink_pcs_parse` +helper fills the allocated array with ONLY the available PCS and return the +number of available PCS found. PCS that returns -ENODEV will be skipped and +won't be inserted in the allocated array. + +A phylink instance may use multiple PCS devices. The maximum number is reported +through ``num_possible_pcs``. + +It's mandatory to specify for what interface a PCS is needed. This can be done +by filling the ``pcs_interfaces`` in :c:struct:`phylink_config` struct. +If the requested interface mode is not present in this bitmask, phylink does +not search for a PCS for that specific mode. (example MAC doesn't need a PCS +for SGMII but require one for USXGMII) + +With the use of the :c:func:`fwnode_phylink_pcs_parse` a common implementation +is the following:: + + static int mac_fill_available_pcs(struct phylink_config *config, + struct phylink_pcs **available_pcs, + unsigned int num_possible_pcs) + { + struct device *dev = config->dev; + + return fwnode_phylink_pcs_parse(dev_fwnode(dev), available_pcs, + num_possible_pcs); + } + + static int mac_setup_phylink(struct net_device *netdev) + { + struct phylink_config *config; + + // ... + + config->dev = &netdev->dev; + + // ... + + // Parse possible PCS and fill num_possible_pcs. + config->num_possible_pcs = fwnode_phylink_pcs_count(dev_fwnode(&netdev->dev)); + config->fill_available_pcs = mac_fill_available_pcs; + + __set_bit(PHY_INTERFACE_MODE_INTERNAL, config->supported_interfaces); + __set_bit(PHY_INTERFACE_MODE_SGMII, config->supported_interfaces); + __set_bit(PHY_INTERFACE_MODE_1000BASEX, config->supported_interfaces); + __set_bit(PHY_INTERFACE_MODE_USXGMII, config->supported_interfaces); + + // PCS required only for USXGMII + __set_bit(PHY_INTERFACE_MODE_USXGMII, config->pcs_interfaces); + + phylink = phylink_create(config, //... + +It's worth to mention that it's phylink code that takes care of allocating +the array of :c:struct:`phylink_pcs` pointer for ``fill_available_pcs`` +callback based on the value set in ``num_possible_pcs`` for +:c:struct:`phylink_config` struct. + +The ``fill_available_pcs`` callback must not write more than +``num_possible_pcs`` entries. The third argument may be used to validate +that there is enough space to fill all the available PCS in the passed array +of :c:struct:`phylink_pcs` pointer. + +The ``fill_available_pcs`` callback is called only on :c:func:`phylink_create` +and is used only to compose the initial available PCS list. Ownership of PCS +is held by phylink. + +.. _pcs_producer: + +Writing PCS Drivers +------------------- + +To write a PCS driver, first implement :c:struct:`phylink_pcs_ops`. Then, +register your PCS in your probe function using :c:func:`fwnode_pcs_add_provider`. +The :c:func:`fwnode_pcs_add_provider` takes three arguments, the fwnode where +the PCS provider should be registered to, a xlate function to return the requested +PCS based on ``#pcs-cells`` and a pointer to reference private data for the xlate +function. + +The PCS will then be registered to a global list of PCS provider that the +PCS fwnode implementation will use to parse it. + +For the simple case where the PCS driver expose a single PCS, +:c:func:`fwnode_pcs_simple_xlate` can be used as the xlate function. + +You must call :c:func:`fwnode_pcs_del_provider` from your remove function +on driver detach. + +A devm variant, :c:func:`devm_fwnode_pcs_add_provider`, is available to +automatically release the PCS provider on driver detach. + +It's worth to mention that xlate function MUST reference already allocated +:c:struct:`phylink_pcs` pointer and MUST NOT dynamically allocate new PCS. +The returned pointer is used to identify the PCS across provider lookup and +removal notifications. + +Late PCS registration handling +------------------------------ + +It's possible that a PCS becomes available after the MAC finished probing. +Contrary to the usual producer/consumer implementation, when a PCS is not +registered and can't be found, the fwnode parser helper returns ``-ENODEV`` +instead of ``-EPROBE_DEFER``. + +This is to prevent race condition with particular devices that register +MAC and PCS with USB or PCIe and require the MAC to be registered before +the PCS. + +The phylink logic correctly handle this special case and keep the phylink +instance in a fail condition. + +The PCS fwnode implementation provides a notifier to which each phylink +instance with a non-empty ``pcs_interfaces`` in :c:type:`phylink_config` +registers. When a new PCS provider is registered, the notifier is called +triggering the :c:func:`pcs_provider_notify` function. + +Function :c:func:`pcs_provider_notify` will check if the just added PCS +should be used by the phylink instance. If it should be used then, +it's added to the internal list of available PCS and a phylink major +config is forced. + +If a phylink instance was in a failure state, with the just added PCS +now part of the available PCS internal phylink list, provided all other +conditions are satisfied, the configuration is retried and the failure +condition is cleared. + +API Reference +------------- + +.. kernel-doc:: include/linux/phylink.h + :identifiers: phylink_pcs + +.. kernel-doc:: include/linux/pcs/pcs.h + :internal: + +.. kernel-doc:: include/linux/pcs/pcs-provider.h + :internal: diff --git a/Documentation/networking/sfp-phylink.rst b/Documentation/networking/sfp-phylink.rst index 5bf285d73e8a..306505118eba 100644 --- a/Documentation/networking/sfp-phylink.rst +++ b/Documentation/networking/sfp-phylink.rst @@ -311,33 +311,20 @@ this documentation. priv->pcs = lynx_pcs_create_mdiodev(bus, 0); - Some PCS can be recovered based on firmware information: + PCS that should be recovered based on firmware information should implement + the external PCS as a :ref:`PCS provider ` and reference it + with the use of PCS fwnode helper :ref:`PCS fwnode helper `. - .. code-block:: c - - priv->pcs = lynx_pcs_create_fwnode(of_fwnode_handle(node)); - -12. Populate the :c:func:`mac_select_pcs` callback and add it to your - :c:type:`struct phylink_mac_ops ` set of ops. This function - must return a pointer to the relevant :c:type:`struct phylink_pcs ` - that will be used for the requested link configuration: - - .. code-block:: c - - static struct phylink_pcs *foo_select_pcs(struct phylink_config *config, - phy_interface_t interface) - { - struct foo_priv *priv = container_of(config, struct foo_priv, - phylink_config); - - if ( /* 'interface' needs a PCS to function */ ) - return priv->pcs; +12. Populate the :c:var:`num_possible_pcs` value and + :c:func:`fill_available_pcs` callback and add it to your + :c:type:`struct phylink_config `. - return NULL; - } + Refer to :ref:`Documentation/networking/pcs.rst ` for + further details and examples. - See :c:func:`mvpp2_select_pcs` for an example of a driver that has multiple - internal PCS. + Notice that the previous implementation of populating + :c:func:`mac_select_pcs` is considered deprecated in favor of the new + described above. 13. Fill-in all the :c:type:`phy_interface_t ` (i.e. all MAC to PHY link modes) that your MAC can output. The following example shows a -- 2.55.0