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

From: Sean Anderson <sean.anderson@seco.com>
To: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: netdev@vger.kernel.org, "David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	linux-kernel@vger.kernel.org, Andrew Lunn <andrew@lunn.ch>,
	Heiner Kallweit <hkallweit1@gmail.com>,
	Saravana Kannan <saravanak@google.com>
Subject: Re: [RFC net-next PATCH 05/16] net: phylink: Automatically attach PCS devices
Date: Tue, 5 Oct 2021 12:42:53 -0400	[thread overview]
Message-ID: <61147f21-6de4-d91e-c16f-fdb539e52b42@seco.com> (raw)
In-Reply-To: <YVwfWiMOQH0U5bay@shell.armlinux.org.uk>

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