Since commit 5d93cfcf ("net: dpaa: Convert to phylink"), we support
the "10gbase-r" phy-mode through a driver-based conversion of "xgmii",
but we still don't actually support it when the device tree specifies
"10gbase-r" proper.

This is because boards such as LS1046A-RDB do not define pcs-handle-names
(for whatever reason) in the ethernet@f0000 device tree node, and the
code enters through this code path:

	err = of_property_match_string(mac_node, "pcs-handle-names", "xfi");
	// code takes neither branch and falls through
	if (err >= 0) {
		(...)
	} else if (err != -EINVAL && err != -ENODATA) {
		goto _return_fm_mac_free;
	}

	(...)

	/* For compatibility, if pcs-handle-names is missing, we assume this
	 * phy is the first one in pcsphy-handle
	 */
	err = of_property_match_string(mac_node, "pcs-handle-names", "sgmii");
	if (err == -EINVAL || err == -ENODATA)
		pcs = memac_pcs_create(mac_node, 0); // code takes this branch
	else if (err < 0)
		goto _return_fm_mac_free;
	else
		pcs = memac_pcs_create(mac_node, err);

	// A default PCS is created and saved in "pcs"

	// This determination fails and mistakenly saves the default PCS
	// memac->sgmii_pcs instead of memac->xfi_pcs, because at this
	// stage, mac_dev->phy_if == PHY_INTERFACE_MODE_10GBASER.
	if (err && mac_dev->phy_if == PHY_INTERFACE_MODE_XGMII)
		memac->xfi_pcs = pcs;
	else
		memac->sgmii_pcs = pcs;

In other words, in the absence of pcs-handle-names, the default
xfi_pcs assignment logic only works when in the device tree we have
PHY_INTERFACE_MODE_XGMII.

By reversing the order between the fallback xfi_pcs assignment and the
"xgmii" overwrite with "10gbase-r", we are able to support both values
in the device tree, with identical behavior.

Currently, it is impossible to make the s/xgmii/10gbase-r/ device tree
conversion, because it would break forward compatibility (new device
tree with old kernel). The only way to modify existing device trees to
phy-interface-mode = "10gbase-r" is to fix stable kernels to accept this
value and handle it properly.

One reason why the conversion is desirable is because with pre-phylink
kernels, the Aquantia PHY driver used to warn about the improper use
of PHY_INTERFACE_MODE_XGMII [1]. It is best to have a single (latest)
device tree that works with all supported stable kernel versions.

Note that the blamed commit does not constitute a regression per se.
Older stable kernels like 6.1 still do not work with "10gbase-r", but
for a different reason. That is a battle for another time.

[1] https://lore.kernel.org/netdev/20240214-ls1046-dts-use-10gbase-r-v1-1-8c2d68547393@concurrent-rt.com/

Fixes: 5d93cfcf ("net: dpaa: Convert to phylink")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
Acked-by: Madalin Bucur <madalin.bucur@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>