realtek: pcs: rtl93xx: fix SerDes polarity configuration for RTL931X

realtek: pcs: rtl93xx: fix SerDes polarity configuration for RTL931X

Commit 56e9a73d0b added support for configuring the SerDes polarity for
both RTL930X and RTL931X. Based on the code in the Realtek SDK in [1]
and [2], in both cases the same register bits are set. Thus, a common
implementation was provided which worked with e.g. USXGMII or 10GBase-R
configured SerDes.

However, after further tests, a strange issue occured where it didn't
work that well for all SerDes configurations. While running fine for
10GBase-R links on two adjacent SerDes, it didn't for 1000Base-X links
on one of two adjacent SerDes with the link not being detected as a
symptom.

Diving into the SDK again revealed that the referenced implementation of
polarity configuration is (by accident or by purpose) misleading. While
[1] and [3] for RTL930X match, [2] and [4] for RTL931X actually don't.
[4] writes the bits for the 1G polarity setting on different background
SerDes, thus in another frontend page.

Split implementations for RTL930X and RTL931X again and adjust the one
for RTL931X according to [4]. This resolves the issues with 1000Base-X
behavior.

[1] https://github.com/plappermaul/realtek-doc/blob/69d2890a2e2d7a03df6e40e2cd2c32ff5b074dcf/sources/rtk-xgs1210/src/hal/phy/phy_rtl9300.c#L1384
[2] https://github.com/plappermaul/realtek-doc/blob/69d2890a2e2d7a03df6e40e2cd2c32ff5b074dcf/sources/rtk-xgs1210/src/hal/phy/phy_rtl9310.c#L3479
[3] https://github.com/plappermaul/realtek-doc/blob/69d2890a2e2d7a03df6e40e2cd2c32ff5b074dcf/sources/rtk-xgs1210/src/dal/longan/dal_longan_construct.c#L2246
[4] https://github.com/plappermaul/realtek-doc/blob/69d2890a2e2d7a03df6e40e2cd2c32ff5b074dcf/sources/rtk-xgs1210/src/dal/mango/dal_mango_construct.c#L1550

Fixes: 56e9a73d0b ("realtek: pcs: rtl93xx: provide proper SerDes polarity
configuration")
Signed-off-by: Jonas Jelonek <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20767
Signed-off-by: Robert Marko <[email protected]>