linux-kernel - Re: [PATCH] driver core: Prevent deferred probe loops

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Message-ID: <a52c513c-ff93-4767-a370-3f7c562df7bd@linux.dev>
Date: Tue, 10 Jun 2025 19:44:27 -0400
From: Sean Anderson <sean.anderson@...ux.dev>
To: Saravana Kannan <saravanak@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 "Rafael J . Wysocki" <rafael@...nel.org>, Danilo Krummrich
 <dakr@...nel.org>, linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 Christoph Hellwig <hch@....de>, Rob Herring <robh+dt@...nel.org>,
 Grant Likely <grant.likely@...aro.org>
Subject: Re: [PATCH] driver core: Prevent deferred probe loops

On 6/10/25 19:32, Saravana Kannan wrote:
> On Tue, Jun 10, 2025 at 11:35 AM Sean Anderson <sean.anderson@...ux.dev> wrote:
>>
>> A deferred probe loop can occur when a device returns EPROBE_DEFER after
>> registering a bus with children:
> 
> This is a broken driver. A parent device shouldn't register child
> devices unless it is fully read itself. It's not logical to say the
> child devices are available, if the parent itself isn't fully ready.
> So, adding child devices/the bus should be the last thing done in the
> parent's probe function.
>
> I know there are odd exceptions where the parent depends on the child,
> so they might add the child a bit earlier in the probe

This is exactly the case here. So the bus probing cannot happen any
later than it already does.

> but in those cases, the parent's probe should still do all the checks
> ahead of time.

Such as what? How is the parent going to know the resource is missing
without checking for it?

> Can you be more specific about the actual failure you are seeing?

MAC is looking for a PCS that's on its internal MDIO bus, but that PCS's
driver isn't loaded. The PCS has to be loaded at probe time because
phylink_create needs it, and phylink is necessary to register the
netdev. The latter situation is not ideal, but it would be quite a bit
of work to untangle.

--Sean