net: fill in MODULE_DESCRIPTION()s for PCS drivers

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the Lynx, XPCS and LynxI PCS drivers.

Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net: pcs: lynxi: implement pcs_disable op

When switching from 10GBase-R/5GBase-R/USXGMII to one of the interface
modes provided by mtk-pcs-lynxi we need to make sure to always perform
a full configuration of the PHYA.

Implement pcs_disable op which resets the stored interface mode to
PHY_INTERFACE_MODE_NA to trigger a full reconfiguration once the LynxI
PCS driver had previously been deselected in favor of another PCS
driver such as the to-be-added driver for the USXGMII PCS found in
MT7988.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Link: https://lore.kernel.org/r/f23d1a60d2c9d2fb72e32dcb0eaa5f7e867a3d68.1692327891.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net: pcs: lynxi: update PCS driver to use neg_mode

Update the Lynxi PCS driver to use neg_mode rather than the mode
argument. This ensures that the link_up() method will always program
the speed and duplex when negotiation is disabled.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1qA8Dz-00EaFY-5A@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net: pcs: add driver for MediaTek SGMII PCS

The SGMII core found in several MediaTek SoCs is identical to what can
also be found in MediaTek's MT7531 Ethernet switch IC.
As this has not always been clear, both drivers developed different
implementations to deal with the PCS.
Recently Alexander Couzens pointed out this fact which lead to the
development of this shared driver.

Add a dedicated driver, mostly by copying the code now found in the
Ethernet driver. The now redundant code will be removed by a follow-up
commit.

Suggested-by: Alexander Couzens <lynxis@fe80.eu>
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Tested-by: Frank Wunderlich <frank-w@public-files.de>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>