treewide: remove redundant IS_ERR() before error code check

[linux/fpc-iii.git] / Documentation / networking / sfp-phylink.rst

.. SPDX-License-Identifier: GPL-2.0

=======
phylink
=======

Overview
========

phylink is a mechanism to support hot-pluggable networking modules
directly connected to a MAC 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;
        struct phylink_config phylink_config;

   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;
        priv->phylink_config.dev = &dev.dev;
        priv->phylink_config.type = PHYLINK_NETDEV;

        phylink = phylink_create(&priv->phylink_config, 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. If a MAC is unable to provide these interrupts, then
    it should set ``priv->phylink_config.pcs_poll = true;`` in step 9.

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``