Merge branch 'Broadcom-tags-support-for-531x5-539x-families'

Florian Fainelli says:

====================
Broadcom tags support for 531x5/539x families

This patch series finally allows us to enable Broadcom tags on the
BCM531x5/BCM539x switch series which are very often cascaded onto
another on-chip Broadcom switch. Because of that we need to be able to
detect that Broadcom tags are already enabled on our DSA master which
happens to be a DSA slave in that case since they are not part of the
same DSA switch tree, the protocol does not support that.

Due to the way DSA works, get_tag_protocol() is called prior to
ds->ops->setup and we do not have all data structures set-up (in
particular dsa_port::cpu_dp is not filed yet) so doing this at the time
get_tag_protocol() is called and without exporting a helper function is
desirable to limit our footprint into the framework.

Having the core (net/dsa/dsa2.c) return and enforce DSA_TAG_PROTO_NONE
was considered and done initially but this leaves the driver outside of
the decision to force/fallback to a particular protocol, instead of
letting it in control. Also there is no reason to suspect that all
tagging protocols are problematic, e.g.: "inner" Marvell EDSA with
"outer" Broadcom tag may work just fine, and vice versa.

This was tested on:

- Lamobo R1 which now has working Broadcom tags for its external BCM53125 switch
- BCM7445 which has a BCM53125 hanging off one of its internal switch
  port, the BCM53125 still works with DSA_TAG_PROTO_NONE
- BCM7278 which has a peculiar dual CPU port set-up (so dual IMP mode
  needs to be enabled)
- Northstar Plus with DSA_TAG_PROTO_BRCM_PREPEND and no external
  switches hanging off the internal switch
====================

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