net: phy: improve phylib correctness for non-autoneg settings

net: phy: improve phylib correctness for non-autoneg settings

phylib has some undesirable behaviour when forcing a link mode through
ethtool.  phylib uses this code:

	idx = phy_find_valid(phy_find_setting(phydev->speed, phydev->duplex),
			features);

to find an index in the settings table.  phy_find_setting() starts at
index 0, and scans upwards looking for an exact speed and duplex match.
When it doesn't find it, it returns MAX_NUM_SETTINGS - 1, which is
10baseT-Half duplex.

phy_find_valid() then scans from the point (and effectively only checks
one entry) before bailing out, returning MAX_NUM_SETTINGS - 1.

phy_sanitize_settings() then sets ->speed to SPEED_10 and ->duplex to
DUPLEX_HALF whether or not 10baseT-Half is supported or not.  This goes
against all the comments against these functions, and 10baseT-Half may
not even be supported by the hardware.

Rework these functions, introducing a new method of scanning the table.
There are two modes of lookup that phylib wants: exact, and inexact.

- in exact mode, we return either an exact match or failure
- in inexact mode, we return an exact match if it exists, a match at
  the highest speed that is not greater than the requested speed
  (ignoring duplex), or failing that, the lowest supported speed, or
  failure.

The biggest difference is that we always check whether the entry is
supported before further consideration, so all unsupported entries are
not considered as candidates.

This results in arguably saner behaviour, better matches the comments,
and is probably what users would expect.

This becomes important as ethernet speeds increase, PHYs exist which do
not support the 10Mbit speeds, and half-duplex is likely to become
obsolete - it's already not even an option on 10Gbit and faster links.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>