net: phy: realtek: convert RTL8226-CG to c45 only

Short: Convert the RTL8226-CG to c45 so it can be used in its
Realtek based ecosystems.

Long: The RTL8226-CG can be mainly found on devices of the
Realtek Otto switch platform. Devices like the Zyxel XGS1210-12
are based on it. These implement a hardware based phy polling
in the background to update SoC status registers.

The hardware provides 4 smi busses where phys are attached to.
For each bus one can decide if it is polled in c45 or c22 mode.
See https://svanheule.net/realtek/longan/register/smi_glb_ctrl
With this setting the register access will be limited by the
hardware. This is very complex (including caching and special
c45-over-c22 handling). But basically it boils down to "enable
protocol x and SoC will disable register access via protocol y".

Mainline already gained support for the rtl9300 mdio driver
in commit 24e31e474769 ("net: mdio: Add RTL9300 MDIO driver").

It covers the basic features, but a lot effort is still needed
to understand hardware properly. So it runs a simple setup by
selecting the proper bus mode during startup.

/* Put the interfaces into C45 mode if required */
glb_ctrl_mask = GENMASK(19, 16);
for (i = 0; i < MAX_SMI_BUSSES; i++)
if (priv->smi_bus_is_c45[i])
glb_ctrl_val |= GLB_CTRL_INTF_SEL(i);
...
err = regmap_update_bits(regmap, SMI_GLB_CTRL,
glb_ctrl_mask, glb_ctrl_val);

To avoid complex coding later on, it limits access by only
providing either c22 or c45:

bus->name = "Realtek Switch MDIO Bus";
if (priv->smi_bus_is_c45[mdio_bus]) {
bus->read_c45 = rtl9300_mdio_read_c45;
bus->write_c45 = rtl9300_mdio_write_c45;
} else {
bus->read = rtl9300_mdio_read_c22;
bus->write = rtl9300_mdio_write_c22;
}

Because of these limitations the existing RTL8226 phy driver
is not working at all on Realtek switches. Convert the driver
to c45-only.

Luckily the RTL8226 seems to support proper MDIO_PMA_EXTABLE
flags. So standard function genphy_c45_pma_read_abilities() can
call genphy_c45_pma_read_ext_abilities() and 10/100/1000 is
populated right. Thus conversion is straight forward.

Outputs before - REMARK: For this a "hacked" bus was used that
toggles the mode for each c22/c45 access. But that is slow and
produces unstable data in the SoC status registers).

Settings for lan9:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseT/Full
Supported pause frame use: Symmetric Receive-only
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseT/Full
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Port: Twisted Pair
PHYAD: 24
Transceiver: external
Auto-negotiation: on
MDI-X: Unknown
Supports Wake-on: d
Wake-on: d
Link detected: no

Outputs with this commit:

Settings for lan9:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseT/Full
Supported pause frame use: Symmetric Receive-only
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
2500baseT/Full
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: Yes
Advertised FEC modes: Not reported
Speed: Unknown!
Duplex: Unknown! (255)
Port: Twisted Pair
PHYAD: 24
Transceiver: external
Auto-negotiation: on
MDI-X: Unknown
Supports Wake-on: d
Wake-on: d
Link detected: no

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://patch.msgid.link/20250813054407.1108285-1-markus.stockhausen@gmx.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>