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

[Bartosz Golaszewski]

On Tue, Aug 8, 2023 at 4:25 PM Andrew Lunn <andrew@xxxxxxx> 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.

>
> > 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. Less so for
platform devices but still, it is there for a reason and should be
expected to work correctly. Or at the very least not crash and burn
the system.

On the other hand, I like your approach because I may get away without
having to fix it. But if I were to fix it - I would reference the MDIO
bus from the secondary mac by phandle and count its references before
dropping it. :)

Bartosz