On Wed, Nov 17, 2021 at 11:50:43PM +0100, Marek Behún wrote:

Sometimes, an ethernet PHY may communicate with ethernet controller with
multiple different PHY connection types, and the software should be able
to choose between them.

Russell King says:
  conventionally phy-mode has meant "this is the mode we want to operate
  the PHY interface in" which was fine when PHYs didn't change their
  mode depending on the media speed
This is no longer the case, since we have PHYs that can change PHY mode.

Existing example is the Marvell 88X3310 PHY, which supports connecting
the MAC with the PHY with `xaui` and `rxaui`. The MAC may also support
both modes, but it is possible that a particular board doesn't have
these modes wired (since they use multiple SerDes lanes).

Another example is one SerDes lane capable of `1000base-x`, `2500base-x`
and `sgmii` when connecting Marvell switches with Marvell ethernet
controller. Currently we mention only one of these modes in device-tree,
and software assumes the other modes are also supported, since they use
the same SerDes lanes. But a board may be able to support `1000base-x`
and not support `2500base-x`, for example due to the higher frequency
not working correctly on a particular board.

In order for the kernel to know which modes are supported on the board,
we need to be able to specify them all in the device-tree.

Change the type of property `phy-connection-type` of an ethernet
controller to be an array of the enumerated strings, instead of just one
string. Require at least one item defined.

Signed-off-by: Marek Behún <kabel@kernel.org>

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew