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Date:   Thu, 30 Aug 2018 05:04:20 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     Moritz Fischer <mdf@...nel.org>
Cc:     davem@...emloft.net, keescook@...omium.org, f.fainelli@...il.com,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        alex.williams@...com
Subject: Re: [PATCH net-next 1/3] net: nixge: Add support for fixed-link
 subnodes

On Wed, Aug 29, 2018 at 05:40:44PM -0700, Moritz Fischer wrote:
> Add support for fixed-link cases where no MDIO is
> actually required to run the device.
> In that case no MDIO bus is instantiated since the
> actual registers are not available in hardware.

Hi Moritz

There are a few different use cases here:

The hardware is missing MDIO - You need fixed-link.

The hardware has MDIO, but you don't have a PHY connected on it, and
use fixed link.

The hardware has MDIO, and it is used e.g. for an Ethernet switch, or
a PHY for another Ethernet interface. Plus you need fixed link.

The binding typically looks like:

&fec1 {
        phy-mode = "rmii";
        pinctrl-names = "default";
        pinctrl-0 = <&pinctrl_fec1>;
        status = "okay";

        fixed-link {
                speed = <100>;
                full-duplex;
        };

        mdio1: mdio {
                #address-cells = <1>;
                #size-cells = <0>;
                status = "okay";

                switch0: switch0@0 {
                        compatible = "marvell,mv88e6085";
                        pinctrl-names = "default";
                        pinctrl-0 = <&pinctrl_switch>;
                        reg = <0>;
                        eeprom-length = <512>;
                        interrupt-parent = <&gpio3>;

It is important you have the mdio subnode, with PHYs and switches as
children. The driver currently gets this wrong, it uses
pdev->dev.of_node.

So the first patch should be to extend this behaviour. Look for a
child node called mdio. If it exists, call nixge_mdio_setup() passing
that child. Otherwise continue using pdev->dev.of_node, so you don't
break backwards compatibility.

Then a patch adding support for fixed-link. If the mdio child node
exists, you still need to register the MDIO bus. If there is no child
node, but there is a fixed-link, skip registering the mdio bus with
pdev->dev.of_node.

	Andrew

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