• Vladimir Oltean's avatar
    net: dsa: Optional VLAN-based port separation for switches without tagging · f9bbe447
    Vladimir Oltean authored
    This patch provides generic DSA code for using VLAN (802.1Q) tags for
    the same purpose as a dedicated switch tag for injection/extraction.
    It is based on the discussions and interest that has been so far
    expressed in https://www.spinics.net/lists/netdev/msg556125.html.
    
    Unlike all other DSA-supported tagging protocols, CONFIG_NET_DSA_TAG_8021Q
    does not offer a complete solution for drivers (nor can it). Instead, it
    provides generic code that driver can opt into calling:
    - dsa_8021q_xmit: Inserts a VLAN header with the specified contents.
      Can be called from another tagging protocol's xmit function.
      Currently the LAN9303 driver is inserting headers that are simply
      802.1Q with custom fields, so this is an opportunity for code reuse.
    - dsa_8021q_rcv: Retrieves the TPID and TCI from a VLAN-tagged skb.
      Removing the VLAN header is left as a decision for the caller to make.
    - dsa_port_setup_8021q_tagging: For each user port, installs an Rx VID
      and a Tx VID, for proper untagged traffic identification on ingress
      and steering on egress. Also sets up the VLAN trunk on the upstream
      (CPU or DSA) port. Drivers are intentionally left to call this
      function explicitly, depending on the context and hardware support.
      The expected switch behavior and VLAN semantics should not be violated
      under any conditions. That is, after calling
      dsa_port_setup_8021q_tagging, the hardware should still pass all
      ingress traffic, be it tagged or untagged.
    
    For uniformity with the other tagging protocols, a module for the
    dsa_8021q_netdev_ops structure is registered, but the typical usage is
    to set up another tagging protocol which selects CONFIG_NET_DSA_TAG_8021Q,
    and calls the API from tag_8021q.h. Null function definitions are also
    provided so that a "depends on" is not forced in the Kconfig.
    
    This tagging protocol only works when switch ports are standalone, or
    when they are added to a VLAN-unaware bridge. It will probably remain
    this way for the reasons below.
    
    When added to a bridge that has vlan_filtering 1, the bridge core will
    install its own VLANs and reset the pvids through switchdev. For the
    bridge core, switchdev is a write-only pipe. All VLAN-related state is
    kept in the bridge core and nothing is read from DSA/switchdev or from
    the driver. So the bridge core will break this port separation because
    it will install the vlan_default_pvid into all switchdev ports.
    
    Even if we could teach the bridge driver about switchdev preference of a
    certain vlan_default_pvid (task difficult in itself since the current
    setting is per-bridge but we would need it per-port), there would still
    exist many other challenges.
    
    Firstly, in the DSA rcv callback, a driver would have to perform an
    iterative reverse lookup to find the correct switch port. That is
    because the port is a bridge slave, so its Rx VID (port PVID) is subject
    to user configuration. How would we ensure that the user doesn't reset
    the pvid to a different value (which would make an O(1) translation
    impossible), or to a non-unique value within this DSA switch tree (which
    would make any translation impossible)?
    
    Finally, not all switch ports are equal in DSA, and that makes it
    difficult for the bridge to be completely aware of this anyway.
    The CPU port needs to transmit tagged packets (VLAN trunk) in order for
    the DSA rcv code to be able to decode source information.
    But the bridge code has absolutely no idea which switch port is the CPU
    port, if nothing else then just because there is no netdevice registered
    by DSA for the CPU port.
    Also DSA does not currently allow the user to specify that they want the
    CPU port to do VLAN trunking anyway. VLANs are added to the CPU port
    using the same flags as they were added on the user port.
    
    So the VLANs installed by dsa_port_setup_8021q_tagging per driver
    request should remain private from the bridge's and user's perspective,
    and should not alter the VLAN semantics observed by the user.
    
    In the current implementation a VLAN range ending at 4095 (VLAN_N_VID)
    is reserved for this purpose. Each port receives a unique Rx VLAN and a
    unique Tx VLAN. Separate VLANs are needed for Rx and Tx because they
    serve different purposes: on Rx the switch must process traffic as
    untagged and process it with a port-based VLAN, but with care not to
    hinder bridging. On the other hand, the Tx VLAN is where the
    reachability restrictions are imposed, since by tagging frames in the
    xmit callback we are telling the switch onto which port to steer the
    frame.
    
    Some general guidance on how this support might be employed for
    real-life hardware (some comments made by Florian Fainelli):
    
    - If the hardware supports VLAN tag stacking, it should somehow back
      up its private VLAN settings when the bridge tries to override them.
      Then the driver could re-apply them as outer tags. Dedicating an outer
      tag per bridge device would allow identical inner tag VID numbers to
      co-exist, yet preserve broadcast domain isolation.
    
    - If the switch cannot handle VLAN tag stacking, it should disable this
      port separation when added as slave to a vlan_filtering bridge, in
      that case having reduced functionality.
    
    - Drivers for old switches that don't support the entire VLAN_N_VID
      range will need to rework the current range selection mechanism.
    Signed-off-by: default avatarVladimir Oltean <olteanv@gmail.com>
    Reviewed-by: default avatarFlorian Fainelli <f.fainelli@gmail.com>
    Reviewed-by: default avatarVivien Didelot <vivien.didelot@gmail.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    f9bbe447
Kconfig 3.32 KB