Message-ID: <CAJZ5v0hq-Qu4ZLw4dcmy-Mm=7un=Yb0pht7TX+3b4NQqQkqXPA@mail.gmail.com>
Date:	Fri, 18 Sep 2015 02:13:39 +0200
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	"Rafael J. Wysocki" <rafael@...nel.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Grygorii Strashko <grygorii.strashko@...com>,
	Tomeu Vizoso <tomeu.vizoso@...labora.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

On Thu, Sep 17, 2015 at 11:06 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> On Thu, 17 Sep 2015, Rafael J. Wysocki wrote:
>
>> Note: Problems also may happen if device A depends on device B and its
>> driver to be present and functional and then the B's driver module is
>> unloaded.  The core doesn't prevent that from happening AFAICS.
>
> It also doesn't prevent B's driver from being unbound from the B
> device.
>
> To some extent the kernel _does_ prevent driver modules from being
> unloaded.  If A's driver uses code resources provided by B's driver
> then the module's refcount would be larger than 0.

Right.

>> I'd like to go back to my initial hunch that the driver knowing about
>> a dependency on another one should tell the core about that, so the
>> core can make the right things happen at various times (like system
>> suspend/resume etc).
>>
>> What if we introduce a mechanism allowing drivers to say "I depend on
>> device X and its driver to be present and functional from now on" and
>> store that information somewhere for the core to use?
>>
>> Some time ago (a few years ago actually IIRC) I proposed something
>> called "PM links".  The idea was to have objects representing such
>> dependencies, although I was not taking the "the driver of the device
>> I depend on should be present and functional going forward" condition.
>>
>> Say, if a driver wants to check the presence of the device+driver it
>> needs to be functional, it will do something like
>>
>> ret = create_pm_link(dev, producer);
>>
>> and that will return -EPROBE_DEFER if the producer device is not
>> functional.  If success is returned, the link has been created and now
>> the core will take it into account.
>>
>> On driver removal the core may just delete the links where the device
>> is the "consumer".  Also there may be a delete_pm_link(dev, producer)
>> operation if needed.
>>
>> The creation of a link may then include the reordering of dpm_list as
>> appropriate so all "producers" are now followed by all of their
>> "consumers".  Going forward, though, the core may use the links to
>> make all "producers" wait for the PM callbacks of their "consumers" to
>> complete during system suspend etc.  It also may use them to prevent
>> drivers being depended on from being unloaded and/or to force the
>> removal of drivers that depend on something being removed.  In
>> principle it may also use those links to coordinate runtime PM
>> transitions, but I guess that's not going to be useful in all cases,
>> so there needs to be an opt-in mechanism for that.
>>
>> Please tell me what you think.
>
> Sounds familiar.  I recall this basic approach from a Plumbers
> conference some years ago -- maybe that was when you first proposed it!
>
> You might want to categorize the dependencies into different types.  I
> can think of three types offhand:
>
>         The target device must be present before the current device
>         can be probed (hard to imagine how that could be stored as a PM
>         link if the target device isn't present, though);

Right, but there is a tricky part here.  The presence of the device
object need not imply that the device is physically present. :-)

>         The target device must be bound to a driver before the current
>         device can be probed;
>
>         The target device must be at full power whenever the current
>         device is.

Or even before attempting to put the current device at full power.

> Maybe you can think of others.
>
> [Oddly enough, the USB subsystem has some dependencies that don't fall
> into any of these categories.  They have to do with the peculiar way in
> which a low- or full-speed device is handed off from a high-speed
> controller to its companion low/full-speed controller, and they apply
> only to system resume, not to normal operation.  (That is, device A
> requires device B to be at full power when A is being resumed from a
> system sleep, but not when A is operating normally or when A is being
> runtime-resumed.)  For such things, we should keep the existing
> device_pm_wait_for_dev() API.]

Absolutely.  The idea is to use the existing APIs for that where it makes sense.

> This sounds like a big change, but it might be worthwhile.

Well, the more I think about that the more it seems to me that some
redesign is needed.

Thanks,
Rafael
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