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

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Date:   Tue, 8 Aug 2023 16:10:30 +0100
From:   "Russell King (Oracle)" <linux@...linux.org.uk>
To:     Andrew Halaney <ahalaney@...hat.com>
Cc:     Bartosz Golaszewski <brgl@...ev.pl>, Andrew Lunn <andrew@...n.ch>,
        "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 08, 2023 at 09:44:16AM -0500, Andrew Halaney wrote:
> On Tue, Aug 08, 2023 at 04:30:05PM +0200, Bartosz Golaszewski wrote:
> > On Tue, Aug 8, 2023 at 4:25 PM Andrew Lunn <andrew@...n.ch> wrote:
> > >
> > > > > 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.
> > 
> > Sorry, it was not very clear. It's the case that two MDIO masters
> > share the MDC and data lines.
> 
> 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.

That all sounds sane, thanks.

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