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

[Bartosz Golaszewski]

On Tue, Aug 8, 2023 at 3:26 PM Russell King (Oracle)
<linux@xxxxxxxxxxxxxxx> 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.

>
> In that case, I would strongly advise only registering one MDIO bus,
> and avoid registering the second one - thereby preventing any issues
> caused by both MDIO bus masters trying to talk at the same time.
>

I sent a patch for that earlier today.

> The PHYs should be populated in firmware on just one of the buses.
>
> You will also need to ensure that whatever registers the bus does
> make sure that the clocks necessary for communicating on the bus
> are under control of the MDIO bus code and not the ethernet MAC
> code. We've run into problems in the past where this has not been
> the case, and it means - taking your example above - that when MAC1
> wants to talk to its PHY, if MAC0 isn't alive it can't.

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?

>
> So just be aware of the clocking situation and make sure that your
> MDIO bus code is managing the clocks necessary for the MDIO bus
> master to work.

Doesn't seem like stmmac is ready for it as it is now so this is going
to be fun...

Bartosz

>
> In regard to sharing of the MDIO bus signals between two bus
> masters, I do not believe that is permissible - there's no
> collision detection in hardware like there is on I涎. So
> having two MDIO bus masters talking at the same time would
> end up corrupting the MDC (clock) and MDIO (data) signals if
> both were active at the same time.
>