linux-kernel - Re: [PATCH v3 3/4] net: stmmac: register parent MDIO node for sun8i-h3-emac

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Date:   Tue, 22 Aug 2017 23:39:22 +0800
From:   Chen-Yu Tsai <wens@...e.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Corentin Labbe <clabbe.montjoie@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        devicetree <devicetree@...r.kernel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH v3 3/4] net: stmmac: register parent MDIO node for sun8i-h3-emac

On Mon, Aug 21, 2017 at 10:23 PM, Andrew Lunn <andrew@...n.ch> wrote:
>> All muxes are mostly always represented the same way afaik, or do you
>> want to simply introduce a new compatible / property?
>
> +         mdio-mux {
> +               compatible = "allwinner,sun8i-h3-mdio-switch";
> +               mdio-parent-bus = <&mdio_parent>;
> +               #address-cells = <1>;
> +               #size-cells = <0>;
> +
> +               internal_mdio: mdio@1 {
>                         reg = <1>;
> -                       clocks = <&ccu CLK_BUS_EPHY>;
> -                       resets = <&ccu RST_BUS_EPHY>;
> +                       #address-cells = <1>;
> +                       #size-cells = <0>;
> +                       int_mii_phy: ethernet-phy@1 {
> +                               compatible = "ethernet-phy-ieee802.3-c22";
> +                               reg = <1>;
> +                               clocks = <&ccu CLK_BUS_EPHY>;
> +                               resets = <&ccu RST_BUS_EPHY>;
> +                               phy-is-integrated;
> +                       };
> +               };
> +               mdio: mdio@0 {
> +                       reg = <0>;
> +                       #address-cells = <1>;
> +                       #size-cells = <0>;
>                 };
>
> Hi Maxim
>
> Anybody who knows the MDIO-mux code/binding, knows that it is a run
> time mux. You swap the mux per MDIO transaction. You can access all
> the PHY and switches on the mux'ed MDIO bus.
>
> However here, it is effectively a boot-time MUX. You cannot change it
> on the fly. What happens when somebody has a phandle to a PHY on the
> internal and a phandle to a phy on the external? Does the driver at
> least return -EINVAL, or -EBUSY? Is there a representation which
> eliminates this possibility?

There is only one controller. Either you use the internal PHY, which
is then directly coupled (no magnetics needed) to the RJ45 port, or
you use an external PHY over MII/RMII/RGMII. You could supposedly
have both on a board, and let the user choose one. But why bother
with the extra complexity and cost? Either you use the internal PHY
at 100M, or an external RGMII PHY for gigabit speeds.

So I think what you are saying is either impossible or engineering-wise
a very stupid design, like using an external MAC with a discrete PHY
connected to the internal MAC's MDIO bus, while using the internal MAC
with the internal PHY.

Now can we please decide on something? We're a week and a half from
the 4.13 release. If mdio-mux is wrong, then we could have two mdio
nodes (internal-mdio & external-mdio).

Regards
ChenYu