The MT7531_FORCE_EEE1G and MT7531_FORCE_EEE100 bits let the
PMCR_FORCE_EEE1G and PMCR_FORCE_EEE100 bits determine the 1G/100 EEE
abilities of the MAC. If MT7531_FORCE_EEE1G and MT7531_FORCE_EEE100 are
unset, the abilities are left to be determined by PHY auto polling.

The commit 40b5d2f1 ("net: dsa: mt7530: Add support for EEE features")
made it so that the PMCR_FORCE_EEE1G and PMCR_FORCE_EEE100 bits are set on
mt753x_phylink_mac_link_up(). But it did not set the MT7531_FORCE_EEE1G and
MT7531_FORCE_EEE100 bits. Because of this, the EEE abilities will be
determined by PHY auto polling, regardless of the result of phy_init_eee().

Define these bits and add them to the MT7531_FORCE_MODE mask which is set
in mt7531_setup_common(). With this, there won't be any EEE abilities set
when phy_init_eee() returns a negative value.

Thanks to Russell for explaining when phy_init_eee() could return a
negative value below.

Looking at phy_init_eee(), it could return a negative value when:

1. phydev->drv is NULL
2. if genphy_c45_eee_is_active() returns negative
3. if genphy_c45_eee_is_active() returns zero, it returns -EPROTONOSUPPORT
4. if phy_set_bits_mmd() fails (e.g. communication error with the PHY)

If we then look at genphy_c45_eee_is_active(), then:

genphy_c45_read_eee_adv() and genphy_c45_read_eee_lpa() propagate their
non-zero return values, otherwise this function returns zero or positive
integer.

If we then look at genphy_c45_read_eee_adv(), then a failure of
phy_read_mmd() would cause a negative value to be returned.

Looking at genphy_c45_read_eee_lpa(), the same is true.

So, it can be summarised as:

- phydev->drv is NULL
- there is a communication error accessing the PHY
- EEE is not active

otherwise, it returns zero on success.

If one wishes to determine whether an error occurred vs EEE not being
supported through negotiation for the negotiated speed, if it returns
-EPROTONOSUPPORT in the latter case. Other error codes mean either the
driver has been unloaded or communication error.

In conclusion, determining the EEE abilities by PHY auto polling shouldn't
result in having any EEE abilities enabled, when one of the last two
situations in the summary happens. And it seems that if phydev->drv is
NULL, there would be bigger problems with the device than a broken link. So
this is not a bugfix.
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: David S. Miller <davem@davemloft.net>