[net-next,v10,13/13] Documentation: networking: document phy_link_topology

Message ID 20240304151011.1610175-14-maxime.chevallier@bootlin.com
State New
Series Introduce PHY listing and link_topology tracking |

Commit Message

Maxime Chevallier March 4, 2024, 3:10 p.m. UTC
  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>
V10: No changes
V9: No changes
V8: No changes
V7: No changes
V6: No changes
V5: Fixed a lot of typos
V4: No changes
V3: New patch

 Documentation/networking/index.rst            |   1 +
 .../networking/phy-link-topology.rst          | 121 ++++++++++++++++++
 2 files changed, 122 insertions(+)
 create mode 100644 Documentation/networking/phy-link-topology.rst


diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 69f3d6dcd9fd..a2c45a75a4a6 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -88,6 +88,7 @@  Contents:
+   phy-link-topology
diff --git a/Documentation/networking/phy-link-topology.rst b/Documentation/networking/phy-link-topology.rst
new file mode 100644
index 000000000000..1fd8e904ef4b
--- /dev/null
+++ b/Documentation/networking/phy-link-topology.rst
@@ -0,0 +1,121 @@ 
+.. SPDX-License-Identifier: GPL-2.0
+PHY link topology
+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 ethool netlink commands. The base assumption
+when designing these configuration channels were that the link looked
+something like this ::
+  +-----------------------+        +----------+      +--------------+
+  | 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
+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 :
+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
+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>`.
+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
+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
+  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
+  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.