Merge branch 'dsa-tag-override'

Tobias Waldekranz says:

====================
net: dsa: Allow default tag protocol to be overridden from DT

This is a continuation of the work started in this patch:
https://lore.kernel.org/netdev/20210323102326.3677940-1-tobias@waldekranz.com/

In addition to the mv88e6xxx support to dynamically change the
protocol, it is now possible to override the protocol from the device
tree. This means that when a board vendor finds an incompatibility,
they can specify a working protocol in the DT, and users will not have
to worry about it.

Some background information:

In a system using an NXP T1023 SoC connected to a 6390X switch, we
noticed that TO_CPU frames where not reaching the CPU. This only
happened on hardware port 8. Looking at the DSA master interface
(dpaa-ethernet) we could see that an Rx error counter was bumped at
the same rate. The logs indicated a parser error.

It just so happens that a TO_CPU coming in on device 0, port 8, will
result in the first two bytes of the DSA tag being one of:

00 40
00 44
00 46

My guess was that since these values looked like 802.3 length fields,
the controller's parser would signal an error if the frame length did
not match what was in the header.

This was later confirmed using two different workarounds provided by
Vladimir. Unfortunately these either bypass or ignore the hardware
parser and thus robs working combinations of the ability to do RSS and
other nifty things. It was therefore decided to go with the option of
a DT override.

v1 -> v2:
  - Fail if the device does not support changing protocols instead of
    falling back to the default. (Andrew)
  - Only call change_tag_protocol on CPU ports. (Andrew/Vladimir)
  - Only allow changing the protocol on chips that have at least
    "undocumented" level of support for EDSA. (Andrew).
  - List the supported protocols in the binding documentation. I opted
    for only listing the protocols that I have tested. As more people
    test their drivers, they can add them. (Rob)

v2 -> v3:
  - Rename "dsa,tag-protocol" -> "dsa-tag-protocol". (Rob)
  - Some cleanups to 4/5. (Vladimir)
  - Add a comment detailing how tree/driver agreement on the tag
    protocol is enforced. (Vladimir).
====================

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