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References: <20230807193102.6374-1-brgl@bgdev.pl> <54421791-75fa-4ed3-8432-e21184556cde@lunn.ch>
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From:   Bartosz Golaszewski <brgl@bgdev.pl>
Date:   Tue, 8 Aug 2023 10:13:09 +0200
Message-ID: <CAMRc=Mc6COaxM6GExHF2M+=v2TBpz87RciAv=9kHr41HkjQhCg@mail.gmail.com>
Subject: Re: [PATCH 0/2] net: stmmac: allow sharing MDIO lines
To:     Andrew Lunn <andrew@lunn.ch>
Cc:     "David S . Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Rob Herring <robh+dt@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Conor Dooley <conor+dt@kernel.org>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>,
        Jose Abreu <joabreu@synopsys.com>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Andrew Halaney <ahalaney@redhat.com>,
        Alex Elder <elder@linaro.org>,
        Srini Kandagatla <srinivas.kandagatla@linaro.org>,
        netdev@vger.kernel.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        linux-stm32@st-md-mailman.stormreply.com,
        linux-arm-kernel@lists.infradead.org,
        Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
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On Mon, Aug 7, 2023 at 9:50=E2=80=AFPM Andrew Lunn <andrew@lunn.ch> wrote:
>
> On Mon, Aug 07, 2023 at 09:31:00PM +0200, Bartosz Golaszewski wrote:
> > From: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
> >
> > Two MACs may share MDIO lines to the PHYs. Let's allow that in the
> > stmmac driver by providing a new device-tree property allowing one MAC
> > node to reference the MDIO bus defined on a second MAC node.
>
> I don't understand why this is needed. phy-handle can point to a phy
> on any MDIO bus. So it is no problem for one MAC to point to the other
> MACs MDIO bus as is.
>
> You do sometimes get into ordering problems, especially if MAC0 is
> pointing to a PHY on MAC1 MDIO bus. But MAC0 should get a
> -EPROBE_DEFER, MAC1 then probes, creating its MDIO bus and the two
> PHYs on it, and then later MAC0 is probes again and is successful.
>
>      Andrew

Ok so upon some further investigation, the actual culprit is in stmmac
platform code - it always tries to register an MDIO bus - independent
of whether there is an actual mdio child node - unless the MAC is
marked explicitly as having a fixed-link.

When I fixed that, MAC1's probe is correctly deferred until MAC0 has
created the MDIO bus.

Even so, isn't it useful to actually reference the shared MDIO bus in some =
way?

If the schematics look something like this:

--------           -------
| MAC0 |--MDIO-----| PHY |
-------- |     |   -------
         |     |
-------- |     |   -------
| MAC1 |--     ----| PHY |
--------           -------

Then it would make sense to model it on the device tree?

Anyway, this can be discussed later, I will drop this for now and send
a fix for stmmac mdio code instead to get this upstream.

Bart