linux-kernel - Re: [PATCH] driver core: Ensure proper suspend/resume ordering

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Date:	Wed, 16 Sep 2015 15:27:55 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Grygorii Strashko <grygorii.strashko@...com>
cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thierry Reding <thierry.reding@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	<linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

On Wed, 16 Sep 2015, Grygorii Strashko wrote:

> I think, It should prohibited to probe devices during suspend/hibernation.
> And solution introduced in this patch might help to fix it -
> in general, we could do :
> - add sync point on suspend enter: wait_for_device_probe() and
> - prohibit probing: move all devices which will request probing into
> deferred_probe list
> - one suspend exit: allow probing and do driver_deferred_probe_trigger

That could work; it's a good idea.

> I'd like to mention here that this patch will work only
> if dmp_list will be filled according device creation order ("parent<-child" dependencies)
> *AND* according device's probing order ("supplier<-consumer").
> So, if there is the case when Parent device can be probed AFTER its children
> - it will not work, because "parent<-child" dependencies will not be tracked
> any more :( Sry, I could not even imagine that such crazy case exist :'(

If we avoid moving devices to the end of the dpm_list when they already 
have children, then we should be okay, right?

> Are there any other subsystems with the same behavior like PCI?

I don't know.

> If not - probably, it could be fixed in PCI subsystem using device_pm_move_after() or 
> device_move() in PCIe ports probe.
> if yes - ... maybe we can scan/re-check and reorder dpm_list on suspend enter and
> restore ("parent<-child" dependencies).

> Truth is that smth. need to be done 100%. Personally, I was hit by this issue also,
>  and it cost me 3 hours of debugging and I came up with the same patch as
> Bill Huang, then spent some time trying to understand what is wrong with PCI
> - finally, I've just changed the order of my devices in DT :)
> 
> Also, I think, it will be good to have this patch in -next to collect more feedbacks.

I like the idea of forcing all probes during a sleep transition to be 
deferred.  We could carry them out just before unfreezing the user 
threads.  That combined with the change mentioned above ought to be 
worth testing.

Alan Stern

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