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From:   Saravana Kannan <saravanak@google.com>
Date:   Mon, 23 Aug 2021 13:48:23 -0700
Message-ID: <CAGETcx8j+bOPL_-qFzHHJkX41Ljzq8HBkbBqtd4E0-2u6a3_Hg@mail.gmail.com>
Subject: Re: [PATCH v2] of: property: fw_devlink: Add support for "phy-handle" property
To:     Andrew Lunn <andrew@lunn.ch>
Cc:     Marek Szyprowski <m.szyprowski@samsung.com>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>, netdev@vger.kernel.org,
        kernel-team@android.com, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Neil Armstrong <narmstrong@baylibre.com>,
        linux-amlogic@lists.infradead.org
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On Mon, Aug 23, 2021 at 12:58 PM Andrew Lunn <andrew@lunn.ch> wrote:
>
> > PHY seems to be one of those cases where it's okay to have the
> > compatible property but also okay to not have it.
>
> Correct. They are like PCI or USB devices. You can ask it, what are
> you? There are two registers in standard locations which give you a
> vendor and product ID. We use that to find the correct driver.

For all the cases of PHYs that currently don't need any compatible
string, requiring a compatible string of type "ethernet-phy-standard"
would have been nice. That would have made PHYs consistent with the
general DT norm of "you need a compatible string to be matched with
the device". Anyway, it's too late to do that now. So I'll have to
deal with this some other way (I have a bunch of ideas, so it's not
the end of the world).

> You only need a compatible when things are not so simple.
>
> 1) The IDs are wrong. Some silicon vendors do stupid things
>
> 2) Chicken/egg problems, you cannot read the ID registers until you
>    load the driver and some resource is enabled.
>
> 3) It is a C45 devices, e.g. part of clause 45 of 802.3, which
>    requires a different protocol to be talked over the bus. So the
>    compatible string tells you to talk C45 to get the IDs.
>
> 4) It is not a PHY, but some sort of other MDIO device, and hence
>    there are no ID registers.

Yeah, I was digging through of_mdiobus_child_is_phy() when I was doing
the mdio-mux fixes and noticed this. But I missed/forgot the mdiobus
doesn't probe part when I sent out the phy-handle patch.

-Saravana