[PATCH net-next v2] net: stmmac: don't create a MDIO bus if unnecessary

[Andrew Halaney]

On Wed, Dec 06, 2023 at 05:46:09PM -0600, Andrew Halaney wrote:
> The stmmac_dt_phy() function, which parses the devicetree node of the
> MAC and ultimately causes MDIO bus allocation, misinterprets what
> fixed-link means in relation to the MAC's MDIO bus. This results in
> a MDIO bus being created in situations it need not be.
> 
> Currently a MDIO bus is created if the description is either:
> 
>     1. Not fixed-link
>     2. fixed-link but contains a MDIO bus as well
> 
> The "1" case above isn't always accurate. If there's a phy-handle,
> it could be referencing a phy on another MDIO controller's bus[1]. In
> this case currently the MAC will make a MDIO bus and scan it all
> anyways unnecessarily.
> 
> There's also a lot of upstream devicetrees[2] that expect a MDIO bus to
> be created and scanned for a phy. This case can also be inferred from
> the platform description by not having a phy-handle && not being
> fixed-link. This hits case "1" in the current driver's logic.
> 
> Let's improve the logic to create a MDIO bus if either:
> 
>     - Devicetree contains a MDIO bus
>     - !fixed-link && !phy-handle (legacy handling)
> 
> Below upstream devicetree snippets can be found that explain some of
> the cases above more concretely.
> 
> Here's[0] a devicetree example where the MAC is both fixed-link and
> driving a switch on MDIO (case "2" above). This needs a MDIO bus to
> be created:
> 
>     &fec1 {
>             phy-mode = "rmii";
> 
>             fixed-link {
>                     speed = <100>;
>                     full-duplex;
>             };
> 
>             mdio1: mdio {
>                     switch0: switch0 at 0 {
>                             compatible = "marvell,mv88e6190";
>                             pinctrl-0 = <&pinctrl_gpio_switch0>;
>                     };
>             };
>     };
> 
> Here's[1] an example where there is no MDIO bus or fixed-link for
> the ethernet1 MAC, so no MDIO bus should be created since ethernet0
> is the MDIO master for ethernet1's phy:
> 
>     &ethernet0 {
>             phy-mode = "sgmii";
>             phy-handle = <&sgmii_phy0>;
> 
>             mdio {
>                     compatible = "snps,dwmac-mdio";
>                     sgmii_phy0: phy at 8 {
>                             compatible = "ethernet-phy-id0141.0dd4";
>                             reg = <0x8>;
>                             device_type = "ethernet-phy";
>                     };
> 
>                     sgmii_phy1: phy at a {
>                             compatible = "ethernet-phy-id0141.0dd4";
>                             reg = <0xa>;
>                             device_type = "ethernet-phy";
>                     };
>             };
>     };
> 
>     &ethernet1 {
>             phy-mode = "sgmii";
>             phy-handle = <&sgmii_phy1>;
>     };
> 
> Finally there's descriptions like this[2] which don't describe the
> MDIO bus but expect it to be created and the whole address space
> scanned for a phy since there's no phy-handle or fixed-link described:
> 
>     &gmac {
>             phy-supply = <&vcc_lan>;
>             phy-mode = "rmii";
>             snps,reset-gpio = <&gpio3 RK_PB4 GPIO_ACTIVE_HIGH>;
>             snps,reset-active-low;
>             snps,reset-delays-us = <0 10000 1000000>;
>     };
> 
> [0] https://elixir.bootlin.com/linux/v6.5-rc5/source/arch/arm/boot/dts/nxp/vf/vf610-zii-ssmb-dtu.dts
> [1] https://elixir.bootlin.com/linux/v6.6-rc5/source/arch/arm64/boot/dts/qcom/sa8775p-ride.dts
> [2] https://elixir.bootlin.com/linux/v6.6-rc5/source/arch/arm64/boot/dts/rockchip/rk3368-r88.dts#L164
> 
> Co-developed-by: Bartosz Golaszewski <bartosz.golaszewski at linaro.org>
> Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski at linaro.org>
> Signed-off-by: Andrew Halaney <ahalaney at redhat.com>
> ---

Gah, I failed to describe my changes since Bart's v1 when picking this
up with b4 to make v2. Whoops!

Changes since v1:
    - Handle the fixed-link + mdio case (Andrew Lunn)
    - Reworded commit message
    - Handle the "legacy" case still mentioned in the commit
    - Bit further refactoring of the function