Question: net: phy: realtek: RTL8211FS SGMII bridge mode support

[Jacky Huang <jackyhuang@nvidia.com>]

Hi,

RTL8211FS is the SGMII-capable variant of RTL8211F. Besides the standard
copper/RGMII mode, it supports hardware-strapped RGMII-to-SGMII bridge mode,
where the MAC side uses RGMII and the line side uses SGMII.

The in-tree realtek driver currently does not handle this mode. RTL8211FS
shares the same PHY ID as RTL8211F (0x001cc916), so the driver binds it
as a standard RTL8211F and follows the existing copper/RGMII path. In
RGMII-to-SGMII bridge mode, this means read_status() does not observe the
SGMII-side link and the link stays down.

We have discussed this with Realtek. They confirmed that the current mode
can be detected from a vendor mode-selection register shared by RTL8211F
variants, and that this case is application-dependent rather than generic
copper PHY behavior.

Our plan is to add RTL8211FS bridge mode support inside the realtek
driver, gated on the detected mode. The driver would only read this
register for detection and would not use it to override the hardware
strapping. For that mode, config_init() would program the SGMII side as
needed, and read_status() would read the SGMII-side link status instead
of relying on the copper path. The existing RTL8211F copper/RGMII behavior
would be left unchanged.

Does this approach sound acceptable for upstream? If you prefer a
different model, or if there is already related work that we should build
on, please let me know.

Thanks,
Jacky C. Huang