Re: [PATCH net-next v17 14/14] Documentation: networking: document phy_link_topology

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 09/07/2024 à 08:30, Maxime Chevallier a écrit :
> The newly introduced phy_link_topology tracks all ethernet PHYs that are
> attached to a netdevice. Document the base principle, internal and
> external APIs. As the phy_link_topology is expected to be extended, this
> documentation will hold any further improvements and additions made
> relative to topology handling.
> 
> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> Reviewed-by: Andrew Lunn <andrew@lunn.ch>

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>

> ---
>   Documentation/networking/ethtool-netlink.rst  |   3 +
>   Documentation/networking/index.rst            |   1 +
>   .../networking/phy-link-topology.rst          | 121 ++++++++++++++++++
>   3 files changed, 125 insertions(+)
>   create mode 100644 Documentation/networking/phy-link-topology.rst
> 
> diff --git a/Documentation/networking/ethtool-netlink.rst b/Documentation/networking/ethtool-netlink.rst
> index d9f0c0dba1e5..81ddb750c1f9 100644
> --- a/Documentation/networking/ethtool-netlink.rst
> +++ b/Documentation/networking/ethtool-netlink.rst
> @@ -2189,10 +2189,13 @@ Retrieve information about a given Ethernet PHY sitting on the link. The DO
>   operation returns all available information about dev->phydev. User can also
>   specify a PHY_INDEX, in which case the DO request returns information about that
>   specific PHY.
> +
>   As there can be more than one PHY, the DUMP operation can be used to list the PHYs
>   present on a given interface, by passing an interface index or name in
>   the dump request.
>   
> +For more information, refer to :ref:`phy_link_topology`
> +
>   Request contents:
>   
>     ====================================  ======  ==========================
> diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
> index d1af04b952f8..c71b87346178 100644
> --- a/Documentation/networking/index.rst
> +++ b/Documentation/networking/index.rst
> @@ -91,6 +91,7 @@ Contents:
>      operstates
>      packet_mmap
>      phonet
> +   phy-link-topology
>      pktgen
>      plip
>      ppp_generic
> diff --git a/Documentation/networking/phy-link-topology.rst b/Documentation/networking/phy-link-topology.rst
> new file mode 100644
> index 000000000000..4dec5d7d6513
> --- /dev/null
> +++ b/Documentation/networking/phy-link-topology.rst
> @@ -0,0 +1,121 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +.. _phy_link_topology:
> +
> +=================
> +PHY link topology
> +=================
> +
> +Overview
> +========
> +
> +The PHY link topology representation in the networking stack aims at representing
> +the hardware layout for any given Ethernet link.
> +
> +An Ethernet interface from userspace's point of view is nothing but a
> +:c:type:`struct net_device <net_device>`, which exposes configuration options
> +through the legacy ioctls and the ethtool netlink commands. The base assumption
> +when designing these configuration APIs were that the link looks something like ::
> +
> +  +-----------------------+        +----------+      +--------------+
> +  | Ethernet Controller / |        | Ethernet |      | Connector /  |
> +  |       MAC             | ------ |   PHY    | ---- |    Port      | ---... to LP
> +  +-----------------------+        +----------+      +--------------+
> +  struct net_device               struct phy_device
> +
> +Commands that needs to configure the PHY will go through the net_device.phydev
> +field to reach the PHY and perform the relevant configuration.
> +
> +This assumption falls apart in more complex topologies that can arise when,
> +for example, using SFP transceivers (although that's not the only specific case).
> +
> +Here, we have 2 basic scenarios. Either the MAC is able to output a serialized
> +interface, that can directly be fed to an SFP cage, such as SGMII, 1000BaseX,
> +10GBaseR, etc.
> +
> +The link topology then looks like this (when an SFP module is inserted) ::
> +
> +  +-----+  SGMII  +------------+
> +  | MAC | ------- | SFP Module |
> +  +-----+         +------------+
> +
> +Knowing that some modules embed a PHY, the actual link is more like ::
> +
> +  +-----+  SGMII   +--------------+
> +  | MAC | -------- | PHY (on SFP) |
> +  +-----+          +--------------+
> +
> +In this case, the SFP PHY is handled by phylib, and registered by phylink through
> +its SFP upstream ops.
> +
> +Now some Ethernet controllers aren't able to output a serialized interface, so
> +we can't directly connect them to an SFP cage. However, some PHYs can be used
> +as media-converters, to translate the non-serialized MAC MII interface to a
> +serialized MII interface fed to the SFP ::
> +
> +  +-----+  RGMII  +-----------------------+  SGMII  +--------------+
> +  | MAC | ------- | PHY (media converter) | ------- | PHY (on SFP) |
> +  +-----+         +-----------------------+         +--------------+
> +
> +This is where the model of having a single net_device.phydev pointer shows its
> +limitations, as we now have 2 PHYs on the link.
> +
> +The phy_link topology framework aims at providing a way to keep track of every
> +PHY on the link, for use by both kernel drivers and subsystems, but also to
> +report the topology to userspace, allowing to target individual PHYs in configuration
> +commands.
> +
> +API
> +===
> +
> +The :c:type:`struct phy_link_topology <phy_link_topology>` is a per-netdevice
> +resource, that gets initialized at netdevice creation. Once it's initialized,
> +it is then possible to register PHYs to the topology through :
> +
> +:c:func:`phy_link_topo_add_phy`
> +
> +Besides registering the PHY to the topology, this call will also assign a unique
> +index to the PHY, which can then be reported to userspace to refer to this PHY
> +(akin to the ifindex). This index is a u32, ranging from 1 to U32_MAX. The value
> +0 is reserved to indicate the PHY doesn't belong to any topology yet.
> +
> +The PHY can then be removed from the topology through
> +
> +:c:func:`phy_link_topo_del_phy`
> +
> +These function are already hooked into the phylib subsystem, so all PHYs that
> +are linked to a net_device through :c:func:`phy_attach_direct` will automatically
> +join the netdev's topology.
> +
> +PHYs that are on a SFP module will also be automatically registered IF the SFP
> +upstream is phylink (so, no media-converter).
> +
> +PHY drivers that can be used as SFP upstream need to call :c:func:`phy_sfp_attach_phy`
> +and :c:func:`phy_sfp_detach_phy`, which can be used as a
> +.attach_phy / .detach_phy implementation for the
> +:c:type:`struct sfp_upstream_ops <sfp_upstream_ops>`.
> +
> +UAPI
> +====
> +
> +There exist a set of netlink commands to query the link topology from userspace,
> +see ``Documentation/networking/ethtool-netlink.rst``.
> +
> +The whole point of having a topology representation is to assign the phyindex
> +field in :c:type:`struct phy_device <phy_device>`. This index is reported to
> +userspace using the ``ETHTOOL_MSG_PHY_GET`` ethtnl command. Performing a DUMP operation
> +will result in all PHYs from all net_device being listed. The DUMP command
> +accepts either a ``ETHTOOL_A_HEADER_DEV_INDEX`` or ``ETHTOOL_A_HEADER_DEV_NAME``
> +to be passed in the request to filter the DUMP to a single net_device.
> +
> +The retrieved index can then be passed as a request parameter using the
> +``ETHTOOL_A_HEADER_PHY_INDEX`` field in the following ethnl commands :
> +
> +* ``ETHTOOL_MSG_STRSET_GET`` to get the stats string set from a given PHY
> +* ``ETHTOOL_MSG_CABLE_TEST_ACT`` and ``ETHTOOL_MSG_CABLE_TEST_ACT``, to perform
> +  cable testing on a given PHY on the link (most likely the outermost PHY)
> +* ``ETHTOOL_MSG_PSE_SET`` and ``ETHTOOL_MSG_PSE_GET`` for PHY-controlled PoE and PSE settings
> +* ``ETHTOOL_MSG_PLCA_GET_CFG``, ``ETHTOOL_MSG_PLCA_SET_CFG`` and ``ETHTOOL_MSG_PLCA_GET_STATUS``
> +  to set the PLCA (Physical Layer Collision Avoidance) parameters
> +
> +Note that the PHY index can be passed to other requests, which will silently
> +ignore it if present and irrelevant.