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(host-87-0-193-91.retail.telecomitalia.it. [87.0.193.91]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-496b4ed5f93sm234143555e9.1.2026.07.26.07.33.32 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 26 Jul 2026 07:33:33 -0700 (PDT) Message-ID: <6a661abd.47e43f48.2d554a.cce9@mx.google.com> X-Google-Original-Message-ID: Date: Sun, 26 Jul 2026 16:33:29 +0200 From: Christian Marangi To: Maxime Chevallier Cc: Andrew Lunn , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Simon Horman , Jonathan Corbet , Shuah Khan , Lorenzo Bianconi , Heiner Kallweit , Russell King , Saravana Kannan , Philipp Zabel , 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 Subject: Re: [PATCH net-next v9 06/12] net: Document PCS subsystem References: <20260717065448.1498335-1-ansuelsmth@gmail.com> <20260717065448.1498335-7-ansuelsmth@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Jul 21, 2026 at 02:11:46PM +0200, Maxime Chevallier wrote: > > > On 7/17/26 08:54, Christian Marangi wrote: > > 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 | 229 +++++++++++++++++++++++++++++ > > 2 files changed, 230 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..98592cdee3ef > > --- /dev/null > > +++ b/Documentation/networking/pcs.rst > > @@ -0,0 +1,229 @@ > > +.. 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. > > The .mac_select_pcs can be deprecated, with the .fill_available_pcs() > mecanism available, we can keep PCS implems inside the MAC driver when > it makes sense, no ? Yes that is the idea but it might take a while to deprecate it. > > If PCSs aren't described in firmware (i.e. DT), we have to use .fill_available_pcs() > without using the fwnode API if I get your code right. Yes everything should be described that way. > > The 'legacy' part is the only remark I have, the rest is all good :) > Any hint on the wording? I remember you already said something about this. I will check previous review. > > + > > +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. > > + > > +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 and :c:func:`phylink_release_pcs` should be used to release > > +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 arguments, 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 ``-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: > -- Ansuel