net: phy: realtek: support MDI swapping for RTL8226-CG

Add support for configuring swapping of MDI pairs (ABCD->DCBA) when the
property "enet-phy-pair-order" is specified.

Unfortunately, no documentation about this feature is available, but
this implementation still tries to avoid magic numbers and raw register
numbers where it seems clear what is going on.

As it is unknown whether the patching step can be safely reversed, only
enabling MDI swapping is fully supported. A value of "0" for the "enet-
phy-pair-order" property is not accepted if the PHY has already been
patched for MDI swapping (however, this should not occur in practice).

Some other Realtek PHYs also support similar mechanisms:

- RTL8221B-VB-CG allows to configure MDI swapping via the same register,
  but does not need the additional patching step. However, it is unclear
  whether a driver implementation for that PHY is necessary, as it is
  known to support configuration via strapping pins (which is working
  fine at least in Zyxel XGS1210-12 rev B1).

- The patching step seems to match the one for the integrated PHYs of
  some Realtek PCIe/USB NICs (see for example the r8152 driver).

For now, only implement this for the RTL8226-CG PHY, where it is needed
for the switches Zyxel XGS1010-12 rev A1 and XGS1210-12 rev A1.

Signed-off-by: Jan Hoffmann <jan@3e8.eu>
Link: https://patch.msgid.link/20260516190456.387768-1-jan@3e8.eu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions