Re: [RFC net-next PATCH 05/16] net: phylink: Automatically attach PCS devices

[Sean Anderson]

On 10/5/21 5:48 AM, Russell King (Oracle) wrote:
> On Mon, Oct 04, 2021 at 03:15:16PM -0400, Sean Anderson wrote:
> > This adds support for automatically attaching PCS devices when creating
> > a phylink. To do this, drivers must first register with
> > phylink_register_pcs. After that, new phylinks will attach the PCS
> > device specified by the "pcs" property.
> >
> > At the moment there is no support for specifying the interface used to
> > talk to the PCS. The MAC driver is expected to know how to talk to the
> > PCS. This is not a change, but it is perhaps an area for improvement.
> >
> > I believe this is mostly correct with regard to registering/
> > unregistering. However I am not too familiar with the guts of Linux's
> > device subsystem. It is possible (likely, even) that the current system
> > is insufficient to prevent removing PCS devices which are still in-use.
> > I would really appreciate any feedback, or suggestions of subsystems to
> > use as reference. In particular: do I need to manually create device
> > links? Should I instead add an entry to of_supplier_bindings? Do I need
> > a call to try_module_get?
>
> I think this is an area that needs to be thought about carefully.
> Things are not trivial here.
>
> The first mistake I see below is the use of device links. pl->dev is
> the "struct device" embedded within "struct net_device". This doesn't
> have a driver associated with it, and so using device links is likely
> ineffectual.

So what can the device in net_device be used for?

> Even with the right device, I think careful thought is needed - we have
> network drivers where one "struct device" contains multiple network
> interfaces. Should the removal of a PCS from one network interface take
> out all of them?

Well, it's more of the other way around. We need to prevent removing the
PCS while it is still in-use.

> Alternatively, could we instead use phylink to "unplug" the PCS and
> mark the link down - would that be a better approach than trying to
> use device links?

So here, I think the logic should be: allow phylink to "unplug" the PCS
only when the link is down.

--Sean