net: phy: aquantia: support phy-mode = "10g-qxgmii" on NXP SPF-30841 (AQR412C)

net: phy: aquantia: support phy-mode = "10g-qxgmii" on NXP SPF-30841 (AQR412C)

The quad port PHYs (AQR4*) have 4 system interfaces, and some of them,
like AQR412C, can be used with a special firmware provisioning which
multiplexes all ports over a single host-side SerDes lane. The protocol
used over this lane is Cisco 10G-QXGMII feature, or "MUSX", as Aquantia
seems to call it.

One such example is the AQR412C PHY from the NXP SPF-30841 10G-QXGMII
add-in card, which uses this firmware file:
https://github.com/nxp-qoriq/qoriq-firmware-aquantia/blob/master/AQR-G3_v4.3.C-AQR_NXP_SPF-30841_MUSX_ID40019_VER1198.cld

There seems to be no disagreement, including from Marvell FAE, that
10G-QXGMII is reported to the host over MDIO as USXGMII and
indistinguishable from it. This includes the registers from the
provisioning based on which the firmware configures a single system
interface (lane C in the case of SPF-30841) to multiplex all ports -
they are also only accessible from the firmware, or over I2C (?!).

However, the Linux MAC and especially SerDes drivers may need to know if
it is using 1 port per lane (USXGMII) or 4 ports per lane (10G-QXGMII).

In the downstream Layerscape SDK we have previously implemented a
simpler scheme where for certain PHY interface modes, we trust the
device tree and never let the PHY driver overwrite phydev->interface:
https://github.com/nxp-qoriq/linux/commit/862694a4961db590c4d8a5590b84791361ca773d

but for upstream, a nicer detection method is implemented, where
although we can not distinguish USXGMII from 10G-QXGMII per se, we
create a whitelist of firmware fingerprints for which USXGMII is
translated into 10G-QXGMII. At the time of writing, it is expected that
this should only happen for the NXP SPF-30841 card, although extending
for more is trivial - just uncomment the phydev_dbg() in
aqr_build_fingerprint().

An advantage of this method is that it doesn't strictly require updates
to arch/arm64/boot/dts/freescale/fsl-ls1028a-qds-13bb.dtso, since the
PHY driver will transition from "usxgmii" to "10g-qxgmii".

All aqr_translate_interface() callers have also previously called
aqr107_probe(), so dereferencing phydev->priv is safe.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20250903130730.2836022-7-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>