[RFC PATCH 03/10] phy: rockchip: naneng-combphy: add SGMII MAC selection for RK3568

[Coia Prant]

> [Severity: High]
> Does this break device tree backward compatibility?
>
> When the rockchip,sgmii-mac-sel property is missing from an older device
> tree, priv->sgmii_mac_sel defaults to 0 in rockchip_combphy_parse_dt().
>
> By unconditionally calling rockchip_combphy_param_write() here, the driver
> now actively clears the MAC selection bit to 0 (GMAC0).
>
> Because the hardware reset value is GMAC1, this actively overrides the default
> hardware and bootloader state that older device trees were relying on.
>
> Could the code check if the property was actually present before writing
> to the register, to ensure the hardware default is preserved for existing
> setups?

Thanks for the detailed review. Regarding the backward compatibility
concern for Patch 3:

First, the driver code in the RFC patch set has already been revised
to align with the hardware default: `priv->sgmii_mac_sel = 1` (GMAC1).
So when the property is omitted, the hardware default is preserved.

Second, and more importantly, there is no backward compatibility
issue in practice: SGMII support for RK3568 has never been functional
in mainline before this series. There are no existing device trees
that rely on this path, because the PHY driver simply did not support
SGMII mode at all. Older device trees either didn't enable SGMII,
or were never operational.

So this change only affects new users who explicitly enable SGMII
and set the property as needed. It does not break anything that
was previously working.

Thanks,
Coia