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Date:   Tue, 8 Aug 2023 17:15:45 +0200
From:   Andrew Lunn <andrew@lunn.ch>
To:     Andrew Halaney <ahalaney@redhat.com>
Cc:     Bartosz Golaszewski <brgl@bgdev.pl>,
        "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>,
        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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> I'll make the water muddier (hopefully clearer?). I have access to the
> board schematic (not SIP/SOM stuff though), but that should help here.
> 
> MAC0 owns its own MDIO bus (we'll call it MDIO0). It is pinmuxed to
> gpio8/gpio9 for mdc/mdio. MAC1 owns its own bus (MDIO1) which is
> pinmuxed to gpio21/22.
> 
> On MDIO0 there are two SGMII ethernet phys. One is connected to MAC0,
> one is connected to MAC1.
> 
> MDIO1 is not connected to anything on the board. So there is only one
> MDIO master, MAC0 on MDIO0, and it manages the ethernet phy for both
> MAC0/MAC1.
> 
> Does that make sense? I don't think from a hardware design standpoint
> this is violating anything, it isn't a multimaster setup on MDIO.

Thanks for taking a detailed look at the schematics. This is how i
would expect it to be.

> > > > 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.
> > >
> > 
> > I disagree. Unbinding is very much a normal operation.

What do you use it for?

I don't think i've ever manually done it. Maybe as part of a script to
unbind the FTDI driver from an FTDI device in order to use user space
tools to program the EEPROM? But that is about it.

I actually expect many unbind operations are broken because it is very
rarely used.

       Andrew