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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Simon Horman , Jonathan Corbet , Shuah Khan , Christian Marangi , Lorenzo Bianconi , Heiner Kallweit , Russell King , Saravana Kannan , 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 v6 06/12] net: Document PCS subsystem Date: Tue, 9 Jun 2026 17:12:02 +0200 Message-ID: <20260609151212.29469-7-ansuelsmth@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260609151212.29469-1-ansuelsmth@gmail.com> References: <20260609151212.29469-1-ansuelsmth@gmail.com> Precedence: bulk X-Mailing-List: devicetree@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Add extensive documentation of the new PCS subsystem and the fwnode implementation with producer/consumer API. Signed-off-by: Christian Marangi --- Documentation/networking/index.rst | 1 + Documentation/networking/pcs.rst | 228 +++++++++++++++++++++++++++++ 2 files changed, 229 insertions(+) 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..9436ba43cebd --- /dev/null +++ b/Documentation/networking/pcs.rst @@ -0,0 +1,228 @@ +.. 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 legacy) +2. firmware node (fwnode) PCS entirely handled by phylink + +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. + +This implementation is considered legacy and it's suggested to +switch to the new fwnode PCS. + +Looking up PCS Devices (fwnode implementation) +----------------------------------------------- + +The lookup of a PCS device follows the common producer/consumer implementation +used by similar subsystem 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_available_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_available_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. + +Using PCS Devices +----------------- + +It's mandatory to either implement the ``mac_select_pcs`` callback +of :c:struct:`phylink_mac_ops` or ``num_available_pcs`` and ``fill_available_pcs`` of :c:struct:`phylink_config` to use a PCS +for a MAC. + +The fwnode implementation expose a simple helper to parse the PCS from +the fwnode :c:func:`fwnode_phylink_pcs_parse`. The 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 a pointer to the maximum number of PCS to parse. The helper can also be used +to obtain the number of PCS parsed (without filling the array) by passing +``NULL`` for the second arg. In such case, the third arg will be set to the +number of PCS parsed in the fwnode. + +A phylink instance may use multiple PCS devices. The maximum number is reported +through ``num_available_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_available_pcs) + { + struct device *dev = config->dev; + + return fwnode_phylink_pcs_parse(dev_fwnode(dev), available_pcs, + &num_available_pcs); + } + + static int mac_setup_phylink(struct net_device *netdev) + { + struct phylink_config *config; + + // ... + + config->dev = &netdev->dev; + + // ... + + // Parse available PCS and fill num_available_pcs. + err = fwnode_phylink_pcs_parse(dev_fwnode(&netdev->dev), NULL, + &config->num_available_pcs); + if (err) + return err; + + 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_available_pcs`` for +:c:struct:`phylink_config` struct. + +The ``fill_available_pcs`` callback must not write more than +``num_available_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 and :c:func:`phylink_release_pcs` should be used to relase +them. + +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 arg, the fwnode where the PCS +provider should be registered to, a get function to return the requested PCS +based on ``#pcs-cells`` and a pointer to reference private data for the get +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_get` can be used as the get function. + +You must call :c:func:`fwnode_pcs_del_provider` from your remove function and +release the PCS from any phylink instance under RTNL lock with +:c:func:`phylink_release_pcs`:: + + fwnode_pcs_del_provider(dev_fwnode(&pdev->dev)); + + rtnl_lock(); + + for (i = 0; i < data->num_port; i++) { + struct pcs_port *port = &priv->ports[i]; + + phylink_release_pcs(&port->pcs); + } + + rtnl_unlock(); + +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 ``-EINVAL`` +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: -- 2.53.0