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

[Russell King (Oracle)]

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.

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?

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?

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