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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>
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Florian Fainelli
David S. Miller
parent
6e9530f4c0
commit
0f22ad45f4
@ -385,3 +385,155 @@ The driver can monitor for updates to arp_tbl using the netevent notifier
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NETEVENT_NEIGH_UPDATE. The device can be programmed with resolved nexthops
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for the routes as arp_tbl updates. The driver implements ndo_neigh_destroy
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to know when arp_tbl neighbor entries are purged from the port.
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Device driver expected behavior
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-------------------------------
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Below is a set of defined behavior that switchdev enabled network devices must
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adhere to.
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Configuration-less state
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^^^^^^^^^^^^^^^^^^^^^^^^
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Upon driver bring up, the network devices must be fully operational, and the
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backing driver must configure the network device such that it is possible to
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send and receive traffic to this network device and it is properly separated
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from other network devices/ports (e.g.: as is frequent with a switch ASIC). How
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this is achieved is heavily hardware dependent, but a simple solution can be to
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use per-port VLAN identifiers unless a better mechanism is available
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(proprietary metadata for each network port for instance).
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The network device must be capable of running a full IP protocol stack
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including multicast, DHCP, IPv4/6, etc. If necessary, it should program the
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appropriate filters for VLAN, multicast, unicast etc. The underlying device
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driver must effectively be configured in a similar fashion to what it would do
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when IGMP snooping is enabled for IP multicast over these switchdev network
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devices and unsolicited multicast must be filtered as early as possible in
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the hardware.
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When configuring VLANs on top of the network device, all VLANs must be working,
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irrespective of the state of other network devices (e.g.: other ports being part
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of a VLAN-aware bridge doing ingress VID checking). See below for details.
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If the device implements e.g.: VLAN filtering, putting the interface in
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promiscuous mode should allow the reception of all VLAN tags (including those
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not present in the filter(s)).
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Bridged switch ports
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^^^^^^^^^^^^^^^^^^^^
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When a switchdev enabled network device is added as a bridge member, it should
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not disrupt any functionality of non-bridged network devices and they
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should continue to behave as normal network devices. Depending on the bridge
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configuration knobs below, the expected behavior is documented.
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Bridge VLAN filtering
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^^^^^^^^^^^^^^^^^^^^^
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The Linux bridge allows the configuration of a VLAN filtering mode (statically,
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at device creation time, and dynamically, during run time) which must be
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observed by the underlying switchdev network device/hardware:
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- with VLAN filtering turned off: the bridge is strictly VLAN unaware and its
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data path will process all Ethernet frames as if they are VLAN-untagged.
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The bridge VLAN database can still be modified, but the modifications should
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have no effect while VLAN filtering is turned off. Frames ingressing the
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device with a VID that is not programmed into the bridge/switch's VLAN table
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must be forwarded and may be processed using a VLAN device (see below).
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- with VLAN filtering turned on: the bridge is VLAN-aware and frames ingressing
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the device with a VID that is not programmed into the bridges/switch's VLAN
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table must be dropped (strict VID checking).
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When there is a VLAN device (e.g: sw0p1.100) configured on top of a switchdev
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network device which is a bridge port member, the behavior of the software
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network stack must be preserved, or the configuration must be refused if that
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is not possible.
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- with VLAN filtering turned off, the bridge will process all ingress traffic
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for the port, except for the traffic tagged with a VLAN ID destined for a
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VLAN upper. The VLAN upper interface (which consumes the VLAN tag) can even
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be added to a second bridge, which includes other switch ports or software
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interfaces. Some approaches to ensure that the forwarding domain for traffic
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belonging to the VLAN upper interfaces are managed properly:
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* If forwarding destinations can be managed per VLAN, the hardware could be
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configured to map all traffic, except the packets tagged with a VID
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belonging to a VLAN upper interface, to an internal VID corresponding to
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untagged packets. This internal VID spans all ports of the VLAN-unaware
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bridge. The VID corresponding to the VLAN upper interface spans the
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physical port of that VLAN interface, as well as the other ports that
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might be bridged with it.
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* Treat bridge ports with VLAN upper interfaces as standalone, and let
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forwarding be handled in the software data path.
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- with VLAN filtering turned on, these VLAN devices can be created as long as
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the bridge does not have an existing VLAN entry with the same VID on any
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bridge port. These VLAN devices cannot be enslaved into the bridge since they
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duplicate functionality/use case with the bridge's VLAN data path processing.
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Non-bridged network ports of the same switch fabric must not be disturbed in any
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way by the enabling of VLAN filtering on the bridge device(s). If the VLAN
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filtering setting is global to the entire chip, then the standalone ports
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should indicate to the network stack that VLAN filtering is required by setting
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'rx-vlan-filter: on [fixed]' in the ethtool features.
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Because VLAN filtering can be turned on/off at runtime, the switchdev driver
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must be able to reconfigure the underlying hardware on the fly to honor the
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toggling of that option and behave appropriately. If that is not possible, the
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switchdev driver can also refuse to support dynamic toggling of the VLAN
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filtering knob at runtime and require a destruction of the bridge device(s) and
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creation of new bridge device(s) with a different VLAN filtering value to
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ensure VLAN awareness is pushed down to the hardware.
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Even when VLAN filtering in the bridge is turned off, the underlying switch
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hardware and driver may still configure itself in a VLAN-aware mode provided
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that the behavior described above is observed.
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The VLAN protocol of the bridge plays a role in deciding whether a packet is
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treated as tagged or not: a bridge using the 802.1ad protocol must treat both
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VLAN-untagged packets, as well as packets tagged with 802.1Q headers, as
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untagged.
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The 802.1p (VID 0) tagged packets must be treated in the same way by the device
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as untagged packets, since the bridge device does not allow the manipulation of
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VID 0 in its database.
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When the bridge has VLAN filtering enabled and a PVID is not configured on the
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ingress port, untagged 802.1p tagged packets must be dropped. When the bridge
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has VLAN filtering enabled and a PVID exists on the ingress port, untagged and
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priority-tagged packets must be accepted and forwarded according to the
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bridge's port membership of the PVID VLAN. When the bridge has VLAN filtering
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disabled, the presence/lack of a PVID should not influence the packet
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forwarding decision.
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Bridge IGMP snooping
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^^^^^^^^^^^^^^^^^^^^
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The Linux bridge allows the configuration of IGMP snooping (statically, at
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interface creation time, or dynamically, during runtime) which must be observed
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by the underlying switchdev network device/hardware in the following way:
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- when IGMP snooping is turned off, multicast traffic must be flooded to all
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ports within the same bridge that have mcast_flood=true. The CPU/management
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port should ideally not be flooded (unless the ingress interface has
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IFF_ALLMULTI or IFF_PROMISC) and continue to learn multicast traffic through
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the network stack notifications. If the hardware is not capable of doing that
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then the CPU/management port must also be flooded and multicast filtering
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happens in software.
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- when IGMP snooping is turned on, multicast traffic must selectively flow
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to the appropriate network ports (including CPU/management port). Flooding of
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unknown multicast should be only towards the ports connected to a multicast
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router (the local device may also act as a multicast router).
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The switch must adhere to RFC 4541 and flood multicast traffic accordingly
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since that is what the Linux bridge implementation does.
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Because IGMP snooping can be turned on/off at runtime, the switchdev driver
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must be able to reconfigure the underlying hardware on the fly to honor the
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toggling of that option and behave appropriately.
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A switchdev driver can also refuse to support dynamic toggling of the multicast
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snooping knob at runtime and require the destruction of the bridge device(s)
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and creation of a new bridge device(s) with a different multicast snooping
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value.
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