Merge branch 'Add-ethtool-extended-link-state'

Ido Schimmel says:

====================
Add ethtool extended link state

Amit says:

Currently, device drivers can only indicate to user space if the network
link is up or down, without additional information.

This patch set provides an infrastructure that allows these drivers to
expose more information to user space about the link state. The
information can save users' time when trying to understand why a link is
not operationally up, for example.

The above is achieved by extending the existing ethtool LINKSTATE_GET
command with attributes that carry the extended state.

For example, no link due to missing cable:

$ ethtool ethX
...
Link detected: no (No cable)

Beside the general extended state, drivers can pass additional
information about the link state using the sub-state field. For example:

$ ethtool ethX
...
Link detected: no (Autoneg, No partner detected)

In the future the infrastructure can be extended - for example - to
allow PHY drivers to report whether a downshift to a lower speed
occurred. Something like:

$ ethtool ethX
...
Link detected: yes (downshifted)

Patch set overview:

Patches #1-#3 move mlxsw ethtool code to a separate file
Patches #4-#5 add the ethtool infrastructure for extended link state
Patches #6-#7 add support of extended link state in the mlxsw driver
Patches #8-#10 add test cases

Changes since v1:

* In documentation, show ETHTOOL_LINK_EXT_STATE_* and
  ETHTOOL_LINK_EXT_SUBSTATE_* constants instead of user-space strings
* Add `_CI_` to cable_issue substates to be consistent with
  other substates
* Keep the commit messages within 75 columns
* Use u8 variable for __link_ext_substate
* Document the meaning of -ENODATA in get_link_ext_state() callback
  description
* Do not zero data->link_ext_state_provided after getting an error
* Use `ret` variable for error value

Changes since RFC:

* Move documentation patch before ethtool patch
* Add nla_total_size() instead of sizeof() directly
* Return an error code from linkstate_get_ext_state()
* Remove SHORTED_CABLE, add CABLE_TEST_FAILURE instead
* Check if the interface is administratively up before setting ext_state
* Document all sub-states
====================

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