Documentation: networking: switchdev: clarify device driver behavior

This patch provides details on the expected behavior of switchdev
enabled network devices when operating in a "stand alone" mode, as well
as when being bridge members. This clarifies a number of things that
recently came up during a bug fixing session on the b53 DSA switch
driver.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/switchdev.rst

@@ -385,3 +385,155 @@ The driver can monitor for updates to arp_tbl using the netevent notifier
 NETEVENT_NEIGH_UPDATE.  The device can be programmed with resolved nexthops
 for the routes as arp_tbl updates.  The driver implements ndo_neigh_destroy
 to know when arp_tbl neighbor entries are purged from the port.
+
+Device driver expected behavior
+-------------------------------
+
+Below is a set of defined behavior that switchdev enabled network devices must
+adhere to.
+
+Configuration-less state
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Upon driver bring up, the network devices must be fully operational, and the
+backing driver must configure the network device such that it is possible to
+send and receive traffic to this network device and it is properly separated
+from other network devices/ports (e.g.: as is frequent with a switch ASIC). How
+this is achieved is heavily hardware dependent, but a simple solution can be to
+use per-port VLAN identifiers unless a better mechanism is available
+(proprietary metadata for each network port for instance).
+
+The network device must be capable of running a full IP protocol stack
+including multicast, DHCP, IPv4/6, etc. If necessary, it should program the
+appropriate filters for VLAN, multicast, unicast etc. The underlying device
+driver must effectively be configured in a similar fashion to what it would do
+when IGMP snooping is enabled for IP multicast over these switchdev network
+devices and unsolicited multicast must be filtered as early as possible in
+the hardware.
+
+When configuring VLANs on top of the network device, all VLANs must be working,
+irrespective of the state of other network devices (e.g.: other ports being part
+of a VLAN-aware bridge doing ingress VID checking). See below for details.
+
+If the device implements e.g.: VLAN filtering, putting the interface in
+promiscuous mode should allow the reception of all VLAN tags (including those
+not present in the filter(s)).
+
+Bridged switch ports
+^^^^^^^^^^^^^^^^^^^^
+
+When a switchdev enabled network device is added as a bridge member, it should
+not disrupt any functionality of non-bridged network devices and they
+should continue to behave as normal network devices. Depending on the bridge
+configuration knobs below, the expected behavior is documented.
+
+Bridge VLAN filtering
+^^^^^^^^^^^^^^^^^^^^^
+
+The Linux bridge allows the configuration of a VLAN filtering mode (statically,
+at device creation time, and dynamically, during run time) which must be
+observed by the underlying switchdev network device/hardware:
+
+- with VLAN filtering turned off: the bridge is strictly VLAN unaware and its
+  data path will process all Ethernet frames as if they are VLAN-untagged.
+  The bridge VLAN database can still be modified, but the modifications should
+  have no effect while VLAN filtering is turned off. Frames ingressing the
+  device with a VID that is not programmed into the bridge/switch's VLAN table
+  must be forwarded and may be processed using a VLAN device (see below).
+
+- with VLAN filtering turned on: the bridge is VLAN-aware and frames ingressing
+  the device with a VID that is not programmed into the bridges/switch's VLAN
+  table must be dropped (strict VID checking).
+
+When there is a VLAN device (e.g: sw0p1.100) configured on top of a switchdev
+network device which is a bridge port member, the behavior of the software
+network stack must be preserved, or the configuration must be refused if that
+is not possible.
+
+- with VLAN filtering turned off, the bridge will process all ingress traffic
+  for the port, except for the traffic tagged with a VLAN ID destined for a
+  VLAN upper. The VLAN upper interface (which consumes the VLAN tag) can even
+  be added to a second bridge, which includes other switch ports or software
+  interfaces. Some approaches to ensure that the forwarding domain for traffic
+  belonging to the VLAN upper interfaces are managed properly:
+    * If forwarding destinations can be managed per VLAN, the hardware could be
+      configured to map all traffic, except the packets tagged with a VID
+      belonging to a VLAN upper interface, to an internal VID corresponding to
+      untagged packets. This internal VID spans all ports of the VLAN-unaware
+      bridge. The VID corresponding to the VLAN upper interface spans the
+      physical port of that VLAN interface, as well as the other ports that
+      might be bridged with it.
+    * Treat bridge ports with VLAN upper interfaces as standalone, and let
+      forwarding be handled in the software data path.
+
+- with VLAN filtering turned on, these VLAN devices can be created as long as
+  the bridge does not have an existing VLAN entry with the same VID on any
+  bridge port. These VLAN devices cannot be enslaved into the bridge since they
+  duplicate functionality/use case with the bridge's VLAN data path processing.
+
+Non-bridged network ports of the same switch fabric must not be disturbed in any
+way by the enabling of VLAN filtering on the bridge device(s). If the VLAN
+filtering setting is global to the entire chip, then the standalone ports
+should indicate to the network stack that VLAN filtering is required by setting
+'rx-vlan-filter: on [fixed]' in the ethtool features.
+
+Because VLAN filtering can be turned on/off at runtime, the switchdev driver
+must be able to reconfigure the underlying hardware on the fly to honor the
+toggling of that option and behave appropriately. If that is not possible, the
+switchdev driver can also refuse to support dynamic toggling of the VLAN
+filtering knob at runtime and require a destruction of the bridge device(s) and
+creation of new bridge device(s) with a different VLAN filtering value to
+ensure VLAN awareness is pushed down to the hardware.
+
+Even when VLAN filtering in the bridge is turned off, the underlying switch
+hardware and driver may still configure itself in a VLAN-aware mode provided
+that the behavior described above is observed.
+
+The VLAN protocol of the bridge plays a role in deciding whether a packet is
+treated as tagged or not: a bridge using the 802.1ad protocol must treat both
+VLAN-untagged packets, as well as packets tagged with 802.1Q headers, as
+untagged.
+
+The 802.1p (VID 0) tagged packets must be treated in the same way by the device
+as untagged packets, since the bridge device does not allow the manipulation of
+VID 0 in its database.
+
+When the bridge has VLAN filtering enabled and a PVID is not configured on the
+ingress port, untagged 802.1p tagged packets must be dropped. When the bridge
+has VLAN filtering enabled and a PVID exists on the ingress port, untagged and
+priority-tagged packets must be accepted and forwarded according to the
+bridge's port membership of the PVID VLAN. When the bridge has VLAN filtering
+disabled, the presence/lack of a PVID should not influence the packet
+forwarding decision.
+
+Bridge IGMP snooping
+^^^^^^^^^^^^^^^^^^^^
+
+The Linux bridge allows the configuration of IGMP snooping (statically, at
+interface creation time, or dynamically, during runtime) which must be observed
+by the underlying switchdev network device/hardware in the following way:
+
+- when IGMP snooping is turned off, multicast traffic must be flooded to all
+  ports within the same bridge that have mcast_flood=true. The CPU/management
+  port should ideally not be flooded (unless the ingress interface has
+  IFF_ALLMULTI or IFF_PROMISC) and continue to learn multicast traffic through
+  the network stack notifications. If the hardware is not capable of doing that
+  then the CPU/management port must also be flooded and multicast filtering
+  happens in software.
+
+- when IGMP snooping is turned on, multicast traffic must selectively flow
+  to the appropriate network ports (including CPU/management port). Flooding of
+  unknown multicast should be only towards the ports connected to a multicast
+  router (the local device may also act as a multicast router).
+
+The switch must adhere to RFC 4541 and flood multicast traffic accordingly
+since that is what the Linux bridge implementation does.
+
+Because IGMP snooping can be turned on/off at runtime, the switchdev driver
+must be able to reconfigure the underlying hardware on the fly to honor the
+toggling of that option and behave appropriately.
+
+A switchdev driver can also refuse to support dynamic toggling of the multicast
+snooping knob at runtime and require the destruction of the bridge device(s)
+and creation of a new bridge device(s) with a different multicast snooping
+value.