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Date:   Tue, 8 Aug 2023 16:25:30 +0200
From:   Andrew Lunn <andrew@lunn.ch>
To:     Bartosz Golaszewski <brgl@bgdev.pl>
Cc:     "Russell King (Oracle)" <linux@armlinux.org.uk>,
        "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>
Subject: Re: [PATCH 0/2] net: stmmac: allow sharing MDIO lines
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> > On Tue, Aug 08, 2023 at 10:13:09AM +0200, Bartosz Golaszewski wrote:
> > > 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?
> >
> > So I think what you're saying is that MAC0 and MAC1's have MDIO bus
> > masters, and the hardware designer decided to tie both together to
> > a single set of clock and data lines, which then go to two PHYs.
> 
> The schematics I have are not very clear on that, but now that you
> mention this, it's most likely the case.

I hope not. That would be very broken. As Russell pointed out, MDIO is
not multi-master. You need to check with the hardware designer if the
schematics are not clear.

> Good point, but it's worse than that: when MAC0 is unbound, it will
> unregister the MDIO bus and destroy all PHY devices. These are not
> refcounted so they will literally go from under MAC1. Not sure how
> this can be dealt with?

unbinding is not a normal operation. So i would just live with it, and
if root decides to shoot herself in the foot, that is her choice.

   Andrew