Merge branch 'dsa-add-cross-chip-VLAN-support'

Vivien Didelot says:

====================
net: dsa: add cross-chip VLAN support

The current code in DSA does not support cross-chip VLAN. This means
that in a multi-chip environment such as this one (similar to ZII Rev B)

         [CPU].................... (mdio)
    (eth0) |   :       :          :
          _|_____    _______    _______
         [__sw0__]--[__sw1__]--[__sw2__]
          |  |  |    |  |  |    |  |  |
          v  v  v    v  v  v    v  v  v
          p1 p2 p3   p4 p5 p6   p7 p8 p9

adding a VLAN to p9 won't be enough to reach the CPU, until at least one
port of sw0 and sw1 join the VLAN as well and become aware of the VID.

This patchset makes the DSA core program the VLAN on the CPU and DSA
links itself, which brings seamlessly cross-chip VLAN support to DSA.

With this series applied*, the hardware VLAN tables of a 3-switch setup
look like this after adding a VLAN to only one port of the end switch:

    # cat /sys/class/net/br0/bridge/default_pvid
    42
    # cat /sys/kernel/debug/mv88e6xxx/sw{0,1,2}/vtu
    # ip link set up master br0 dev lan6
    # cat /sys/kernel/debug/mv88e6xxx/sw{0,1,2}/vtu
     VID  FID  SID  0  1  2  3  4  5  6
      42    1    0  x  x  x  x  x  =  =
     VID  FID  SID  0  1  2  3  4  5  6
      42    1    0  x  x  x  x  x  =  =
     VID  FID  SID  0  1  2  3  4  5  6  7  8  9
      42    1    0  u  x  x  x  x  x  x  x  x  =

('x' is excluded, 'u' is untagged, '=' is unmodified DSA and CPU ports.)

Completely removing a VLAN entry (which is currently the responsibility
of drivers anyway) is not supported yet since it requires some caching.

(*) the output is shown from this out-of-tree debugfs patch:
https://github.com/vivien/linux/commit/7b61a684b9d6b6a499135a587c7f62a1fddceb8b.patch

Changes in v2:
  - canonical incrementation (port++ instead of ++port)
  - check CPU and DSA ports before purging a VLAN
  - add Reviewed-by tags
====================

Signed-off-by: David S. Miller <davem@davemloft.net>