[PATCH net-next v6 06/12] net: Document PCS subsystem

[Christian Marangi <ansuelsmth@gmail.com>]

Add extensive documentation of the new PCS subsystem and the fwnode
implementation with producer/consumer API.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
---
 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