Documentation: networking: Document the phy_port infrastructure

This documentation aims at describing the main goal of the phy_port
infrastructure.

Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260108080041.553250-15-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

2 files changed, 112 insertions, 0 deletions

diff --git a/Documentation/networking/index.rst b/Documentation/networking/index.rst
index 75db2251649b..49fcfa577711 100644
--- a/Documentation/networking/index.rst
+++ b/Documentation/networking/index.rst
@@ -96,6 +96,7 @@ Contents:
    packet_mmap
    phonet
    phy-link-topology
+   phy-port
    pktgen
    plip
    ppp_generic
diff --git a/Documentation/networking/phy-port.rst b/Documentation/networking/phy-port.rst
new file mode 100644
index 000000000000..6e28d9094bce
--- /dev/null
+++ b/Documentation/networking/phy-port.rst
@@ -0,0 +1,111 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. _phy_port:
+
+=================
+Ethernet ports
+=================
+
+This document is a basic description of the phy_port infrastructure,
+introduced to represent physical interfaces of Ethernet devices.
+
+Without phy_port, we already have quite a lot of information about what the
+media-facing interface of a NIC can do and looks like, through the
+:c:type:`struct ethtool_link_ksettings <ethtool_link_ksettings>` attributes,
+which includes :
+
+ - What the NIC can do through the :c:member:`supported` field
+ - What the Link Partner advertises through :c:member:`lp_advertising`
+ - Which features we're advertising through :c:member:`advertising`
+
+We also have info about the number of pairs and the PORT type. These settings
+are built by aggregating together information reported by various devices that
+are sitting on the link :
+
+  - The NIC itself, through the :c:member:`get_link_ksettings` callback
+  - Precise information from the MAC and PCS by using phylink in the MAC driver
+  - Information reported by the PHY device
+  - Information reported by an SFP module (which can itself include a PHY)
+
+This model however starts showing its limitations when we consider devices that
+have more than one media interface. In such a case, only information about the
+actively used interface is reported, and it's not possible to know what the
+other interfaces can do. In fact, we have very little information about whether
+or not there are any other media interfaces.
+
+The goal of the phy_port representation is to provide a way of representing a
+physical interface of a NIC, regardless of what is driving the port (NIC through
+a firmware, SFP module, Ethernet PHY).
+
+Multi-port interfaces examples
+==============================
+
+Several cases of multi-interface NICs have been observed so far :
+
+Internal MII Mux::
+
+  +------------------+
+  | SoC              |
+  |          +-----+ |           +-----+
+  | +-----+  |     |-------------| PHY |
+  | | MAC |--| Mux | |   +-----+ +-----+
+  | +-----+  |     |-----| SFP |
+  |          +-----+ |   +-----+
+  +------------------+
+
+Internal Mux with internal PHY::
+
+  +------------------------+
+  | SoC                    |
+  |          +-----+ +-----+
+  | +-----+  |     |-| PHY |
+  | | MAC |--| Mux | +-----+   +-----+
+  | +-----+  |     |-----------| SFP |
+  |          +-----+       |   +-----+
+  +------------------------+
+
+External Mux::
+
+  +---------+
+  | SoC     |  +-----+  +-----+
+  |         |  |     |--| PHY |
+  | +-----+ |  |     |  +-----+
+  | | MAC |----| Mux |  +-----+
+  | +-----+ |  |     |--| PHY |
+  |         |  +-----+  +-----+
+  |         |     |
+  |    GPIO-------+
+  +---------+
+
+Double-port PHY::
+
+  +---------+
+  | SoC     | +-----+
+  |         | |     |--- RJ45
+  | +-----+ | |     |
+  | | MAC |---| PHY |   +-----+
+  | +-----+ | |     |---| SFP |
+  +---------+ +-----+   +-----+
+
+phy_port aims at providing a path to support all the above topologies, by
+representing the media interfaces in a way that's agnostic to what's driving
+the interface. the struct phy_port object has its own set of callback ops, and
+will eventually be able to report its own ksettings::
+
+             _____      +------+
+            (     )-----| Port |
+ +-----+   (       )    +------+
+ | MAC |--(   ???   )
+ +-----+   (       )    +------+
+            (_____)-----| Port |
+                        +------+
+
+Next steps
+==========
+
+As of writing this documentation, only ports controlled by PHY devices are
+supported. The next steps will be to add the Netlink API to expose these
+to userspace and add support for raw ports (controlled by some firmware, and directly
+managed by the NIC driver).
+
+Another parallel task is the introduction of a MII muxing framework to allow the
+control of non-PHY driver multi-port setups.