docs: net: dsa: update information about multiple CPU ports

DSA now supports multiple CPU ports, explain the use cases that are
covered, the new UAPI, the permitted degrees of freedom, the driver API,
and remove some old "hanging fruits".

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 96 insertions, 0 deletions

diff --git a/Documentation/networking/dsa/configuration.rst b/Documentation/networking/dsa/configuration.rst
index 2b08f1a772d3..827701f8cbfe 100644
--- a/Documentation/networking/dsa/configuration.rst
+++ b/Documentation/networking/dsa/configuration.rst
@@ -49,6 +49,9 @@ In this documentation the following Ethernet interfaces are used:
 *eth0*
   the master interface
 
+*eth1*
+  another master interface
+
 *lan1*
   a slave interface
 
@@ -360,3 +363,96 @@ the ``self`` flag) has been removed. This results in the following changes:
 
 Script writers are therefore encouraged to use the ``master static`` set of
 flags when working with bridge FDB entries on DSA switch interfaces.
+
+Affinity of user ports to CPU ports
+-----------------------------------
+
+Typically, DSA switches are attached to the host via a single Ethernet
+interface, but in cases where the switch chip is discrete, the hardware design
+may permit the use of 2 or more ports connected to the host, for an increase in
+termination throughput.
+
+DSA can make use of multiple CPU ports in two ways. First, it is possible to
+statically assign the termination traffic associated with a certain user port
+to be processed by a certain CPU port. This way, user space can implement
+custom policies of static load balancing between user ports, by spreading the
+affinities according to the available CPU ports.
+
+Secondly, it is possible to perform load balancing between CPU ports on a per
+packet basis, rather than statically assigning user ports to CPU ports.
+This can be achieved by placing the DSA masters under a LAG interface (bonding
+or team). DSA monitors this operation and creates a mirror of this software LAG
+on the CPU ports facing the physical DSA masters that constitute the LAG slave
+devices.
+
+To make use of multiple CPU ports, the firmware (device tree) description of
+the switch must mark all the links between CPU ports and their DSA masters
+using the ``ethernet`` reference/phandle. At startup, only a single CPU port
+and DSA master will be used - the numerically first port from the firmware
+description which has an ``ethernet`` property. It is up to the user to
+configure the system for the switch to use other masters.
+
+DSA uses the ``rtnl_link_ops`` mechanism (with a "dsa" ``kind``) to allow
+changing the DSA master of a user port. The ``IFLA_DSA_MASTER`` u32 netlink
+attribute contains the ifindex of the master device that handles each slave
+device. The DSA master must be a valid candidate based on firmware node
+information, or a LAG interface which contains only slaves which are valid
+candidates.
+
+Using iproute2, the following manipulations are possible:
+
+  .. code-block:: sh
+
+    # See the DSA master in current use
+    ip -d link show dev swp0
+        (...)
+        dsa master eth0
+
+    # Static CPU port distribution
+    ip link set swp0 type dsa master eth1
+    ip link set swp1 type dsa master eth0
+    ip link set swp2 type dsa master eth1
+    ip link set swp3 type dsa master eth0
+
+    # CPU ports in LAG, using explicit assignment of the DSA master
+    ip link add bond0 type bond mode balance-xor && ip link set bond0 up
+    ip link set eth1 down && ip link set eth1 master bond0
+    ip link set swp0 type dsa master bond0
+    ip link set swp1 type dsa master bond0
+    ip link set swp2 type dsa master bond0
+    ip link set swp3 type dsa master bond0
+    ip link set eth0 down && ip link set eth0 master bond0
+    ip -d link show dev swp0
+        (...)
+        dsa master bond0
+
+    # CPU ports in LAG, relying on implicit migration of the DSA master
+    ip link add bond0 type bond mode balance-xor && ip link set bond0 up
+    ip link set eth0 down && ip link set eth0 master bond0
+    ip link set eth1 down && ip link set eth1 master bond0
+    ip -d link show dev swp0
+        (...)
+        dsa master bond0
+
+Notice that in the case of CPU ports under a LAG, the use of the
+``IFLA_DSA_MASTER`` netlink attribute is not strictly needed, but rather, DSA
+reacts to the ``IFLA_MASTER`` attribute change of its present master (``eth0``)
+and migrates all user ports to the new upper of ``eth0``, ``bond0``. Similarly,
+when ``bond0`` is destroyed using ``RTM_DELLINK``, DSA migrates the user ports
+that were assigned to this interface to the first physical DSA master which is
+eligible, based on the firmware description (it effectively reverts to the
+startup configuration).
+
+In a setup with more than 2 physical CPU ports, it is therefore possible to mix
+static user to CPU port assignment with LAG between DSA masters. It is not
+possible to statically assign a user port towards a DSA master that has any
+upper interfaces (this includes LAG devices - the master must always be the LAG
+in this case).
+
+Live changing of the DSA master (and thus CPU port) affinity of a user port is
+permitted, in order to allow dynamic redistribution in response to traffic.
+
+Physical DSA masters are allowed to join and leave at any time a LAG interface
+used as a DSA master; however, DSA will reject a LAG interface as a valid
+candidate for being a DSA master unless it has at least one physical DSA master
+as a slave device.