netdev - Re: [PATCH 0/2] net: stmmac: allow sharing MDIO lines

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Date: Tue, 8 Aug 2023 20:26:22 +0200
From: Bartosz Golaszewski <brgl@...ev.pl>
To: Andrew Lunn <andrew@...n.ch>
Cc: Andrew Halaney <ahalaney@...hat.com>, "Russell King (Oracle)" <linux@...linux.org.uk>, 
	"David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Rob Herring <robh+dt@...nel.org>, 
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>, Conor Dooley <conor+dt@...nel.org>, 
	Alexandre Torgue <alexandre.torgue@...s.st.com>, Jose Abreu <joabreu@...opsys.com>, 
	Maxime Coquelin <mcoquelin.stm32@...il.com>, Alex Elder <elder@...aro.org>, 
	Srini Kandagatla <srinivas.kandagatla@...aro.org>, netdev@...r.kernel.org, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-stm32@...md-mailman.stormreply.com, 
	linux-arm-kernel@...ts.infradead.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH 0/2] net: stmmac: allow sharing MDIO lines

On Tue, Aug 8, 2023 at 5:15 PM Andrew Lunn <andrew@...n.ch> wrote:
>
> > 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.
>

When I say "device unbind", I don't just mean manual unbinding using
sysfs. I mean any code path (rmmod, unplugging the USB, etc.) that
leads to the device being detached from its driver. This is a
perfectly normal situation and should work correctly.

I won't be fixing it for this series but may end up looking into
establishing some kind of device links between MACs and their "remote"
PHYs that would allow to safely unbind them.

Bart