Re: [PATCH] driver core: Prevent deferred probe loops

[Sean Anderson <sean.anderson@linux.dev>]

On 6/17/25 04:50, Greg Kroah-Hartman wrote:
> On Thu, Jun 12, 2025 at 04:40:48PM -0400, Sean Anderson wrote:
>> On 6/12/25 13:56, Saravana Kannan wrote:
>> > On Thu, Jun 12, 2025 at 8:53 AM Sean Anderson <sean.anderson@linux.dev> wrote:
>> >>
>> >> On 6/11/25 08:23, Greg Kroah-Hartman wrote:
>> >> > On Tue, Jun 10, 2025 at 07:44:27PM -0400, Sean Anderson wrote:
>> >> >> On 6/10/25 19:32, Saravana Kannan wrote:
>> >> >> > On Tue, Jun 10, 2025 at 11:35 AM Sean Anderson <sean.anderson@linux.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.
>> >> >
>> >> > Please fix the driver not to do this.
>> >>
>> >> How? The driver needs the PCS to work. And the PCS can live on the MDIO
>> >> bus.
>> > 
>> > Obviously I don't know the full details, but you could implement it as
>> > MFD. So the bus part would not get removed even if the PCS fails to
>> > probe. Then the PCS can probe when whatever it needs ends up probing.
>> 
>> I was thinking about making the MDIO bus a separate device. But I think
>> it will be tricky to get suspend/resume working correctly. And this
>> makes conversions more difficult because you cannot just add some
>> pcs_get/pcs_put calls, you have to split out the MDIO bus too (which is
>> invariably created as a child of the MAC).
>> 
>> And what happens if a developer doesn't realize they have to split off
>> the MDIO bus before converting? Everything works fine, except if there
>> is some problem loading the PCS driver, which they may not test. Is this
>> prohibition against failing after creating a bus documented anywhere? I
>> don't recall seeing it...
> 
> What do you mean "failing after creating a bus"?  If a bus is failed to
> be created, you fail like normal, no difference here.

Creating the bus is successful, but there's an EPROBE_DEFER failure after
that. Which induces the probe loop as described in my initial email.

> And if MFD doesn't work, there's always the aux-bus code, perhaps that
> should be used here instead?

I will have a look. However, I expect both of these approaches to
require fairly invasive conversions for existing drivers. Ideally, I
would like to keep conversions simple.

--Sean