doc: add phylink documentation to the networking book

   failover

   net_failover

   phy

   sfp-phylink

   alias

   bridge

   snmp_counter

.. SPDX-License-Identifier: GPL-2.0

=======

phylink

=======

Overview

========

phylink is a mechanism to support hot-pluggable networking modules

without needing to re-initialise the adapter on hot-plug events.

phylink supports conventional phylib-based setups, fixed link setups

and SFP (Small Formfactor Pluggable) modules at present.

Modes of operation

==================

phylink has several modes of operation, which depend on the firmware

settings.

1. PHY mode

   In PHY mode, we use phylib to read the current link settings from

   the PHY, and pass them to the MAC driver.  We expect the MAC driver

   to configure exactly the modes that are specified without any

   negotiation being enabled on the link.

2. Fixed mode

   Fixed mode is the same as PHY mode as far as the MAC driver is

   concerned.

3. In-band mode

   In-band mode is used with 802.3z, SGMII and similar interface modes,

   and we are expecting to use and honor the in-band negotiation or

   control word sent across the serdes channel.

By example, what this means is that:

.. code-block:: none

  &eth {

    phy = <&phy>;

    phy-mode = "sgmii";

  };

does not use in-band SGMII signalling.  The PHY is expected to follow

exactly the settings given to it in its :c:func:`mac_config` function.

The link should be forced up or down appropriately in the

:c:func:`mac_link_up` and :c:func:`mac_link_down` functions.

.. code-block:: none

  &eth {

    managed = "in-band-status";

    phy = <&phy>;

    phy-mode = "sgmii";

  };

uses in-band mode, where results from the PHY's negotiation are passed

to the MAC through the SGMII control word, and the MAC is expected to

acknowledge the control word.  The :c:func:`mac_link_up` and

:c:func:`mac_link_down` functions must not force the MAC side link

up and down.

Rough guide to converting a network driver to sfp/phylink

=========================================================

This guide briefly describes how to convert a network driver from

phylib to the sfp/phylink support.  Please send patches to improve

this documentation.

1. Optionally split the network driver's phylib update function into

   three parts dealing with link-down, link-up and reconfiguring the

   MAC settings. This can be done as a separate preparation commit.

   An example of this preparation can be found in git commit fc548b991fb0.

2. Replace::

	select FIXED_PHY

	select PHYLIB

   with::

	select PHYLINK

   in the driver's Kconfig stanza.

3. Add::

	#include <linux/phylink.h>

   to the driver's list of header files.

4. Add::

	struct phylink *phylink;

   to the driver's private data structure.  We shall refer to the

   driver's private data pointer as ``priv`` below, and the driver's

   private data structure as ``struct foo_priv``.

5. Replace the following functions:

   .. flat-table::

    :header-rows: 1

    :widths: 1 1

    :stub-columns: 0

    * - Original function

      - Replacement function

    * - phy_start(phydev)

      - phylink_start(priv->phylink)

    * - phy_stop(phydev)

      - phylink_stop(priv->phylink)

    * - phy_mii_ioctl(phydev, ifr, cmd)

      - phylink_mii_ioctl(priv->phylink, ifr, cmd)

    * - phy_ethtool_get_wol(phydev, wol)

      - phylink_ethtool_get_wol(priv->phylink, wol)

    * - phy_ethtool_set_wol(phydev, wol)

      - phylink_ethtool_set_wol(priv->phylink, wol)

    * - phy_disconnect(phydev)

      - phylink_disconnect_phy(priv->phylink)

   Please note that some of these functions must be called under the

   rtnl lock, and will warn if not. This will normally be the case,

   except if these are called from the driver suspend/resume paths.

6. Add/replace ksettings get/set methods with:

   .. code-block:: c

    static int foo_ethtool_set_link_ksettings(struct net_device *dev,

					     const struct ethtool_link_ksettings *cmd)

    {

	struct foo_priv *priv = netdev_priv(dev);

	return phylink_ethtool_ksettings_set(priv->phylink, cmd);

    }

    static int foo_ethtool_get_link_ksettings(struct net_device *dev,

					     struct ethtool_link_ksettings *cmd)

    {

	struct foo_priv *priv = netdev_priv(dev);

	return phylink_ethtool_ksettings_get(priv->phylink, cmd);

    }

7. Replace the call to:

	phy_dev = of_phy_connect(dev, node, link_func, flags, phy_interface);

   and associated code with a call to:

	err = phylink_of_phy_connect(priv->phylink, node, flags);

   For the most part, ``flags`` can be zero; these flags are passed to

   the of_phy_attach() inside this function call if a PHY is specified

   in the DT node ``node``.

   ``node`` should be the DT node which contains the network phy property,

   fixed link properties, and will also contain the sfp property.

   The setup of fixed links should also be removed; these are handled

   internally by phylink.

   of_phy_connect() was also passed a function pointer for link updates.

   This function is replaced by a different form of MAC updates

   described below in (8).

   Manipulation of the PHY's supported/advertised happens within phylink

   based on the validate callback, see below in (8).

   Note that the driver no longer needs to store the ``phy_interface``,

   and also note that ``phy_interface`` becomes a dynamic property,

   just like the speed, duplex etc. settings.

   Finally, note that the MAC driver has no direct access to the PHY

   anymore; that is because in the phylink model, the PHY can be

   dynamic.

8. Add a :c:type:`struct phylink_mac_ops <phylink_mac_ops>` instance to

   the driver, which is a table of function pointers, and implement

   these functions. The old link update function for

   :c:func:`of_phy_connect` becomes three methods: :c:func:`mac_link_up`,

   :c:func:`mac_link_down`, and :c:func:`mac_config`. If step 1 was

   performed, then the functionality will have been split there.

   It is important that if in-band negotiation is used,

   :c:func:`mac_link_up` and :c:func:`mac_link_down` do not prevent the

   in-band negotiation from completing, since these functions are called

   when the in-band link state changes - otherwise the link will never

   come up.

   The :c:func:`validate` method should mask the supplied supported mask,

   and ``state->advertising`` with the supported ethtool link modes.

   These are the new ethtool link modes, so bitmask operations must be

   used. For an example, see drivers/net/ethernet/marvell/mvneta.c.

   The :c:func:`mac_link_state` method is used to read the link state

   from the MAC, and report back the settings that the MAC is currently

   using. This is particularly important for in-band negotiation

   methods such as 1000base-X and SGMII.

   The :c:func:`mac_config` method is used to update the MAC with the

   requested state, and must avoid unnecessarily taking the link down

   when making changes to the MAC configuration.  This means the

   function should modify the state and only take the link down when

   absolutely necessary to change the MAC configuration.  An example

   of how to do this can be found in :c:func:`mvneta_mac_config` in

   drivers/net/ethernet/marvell/mvneta.c.

   For further information on these methods, please see the inline

   documentation in :c:type:`struct phylink_mac_ops <phylink_mac_ops>`.

9. Remove calls to of_parse_phandle() for the PHY,

   of_phy_register_fixed_link() for fixed links etc. from the probe

   function, and replace with:

   .. code-block:: c

	struct phylink *phylink;

	phylink = phylink_create(dev, node, phy_mode, &phylink_ops);

	if (IS_ERR(phylink)) {

		err = PTR_ERR(phylink);

		fail probe;

	}

	priv->phylink = phylink;

   and arrange to destroy the phylink in the probe failure path as

   appropriate and the removal path too by calling:

   .. code-block:: c

	phylink_destroy(priv->phylink);

10. Arrange for MAC link state interrupts to be forwarded into

    phylink, via:

    .. code-block:: c

	phylink_mac_change(priv->phylink, link_is_up);

    where ``link_is_up`` is true if the link is currently up or false

    otherwise.

11. Verify that the driver does not call::

	netif_carrier_on()

	netif_carrier_off()

   as these will interfere with phylink's tracking of the link state,

   and cause phylink to omit calls via the :c:func:`mac_link_up` and

   :c:func:`mac_link_down` methods.

Network drivers should call phylink_stop() and phylink_start() via their

suspend/resume paths, which ensures that the appropriate

:c:type:`struct phylink_mac_ops <phylink_mac_ops>` methods are called

as necessary.

For information describing the SFP cage in DT, please see the binding

documentation in the kernel source tree

``Documentation/devicetree/bindings/net/sff,sfp.txt``