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

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Date:	Wed, 16 Sep 2015 03:04:20 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
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>
Subject: Re: [PATCH] driver core: Ensure proper suspend/resume ordering

On Tuesday, September 15, 2015 10:23:07 AM Alan Stern wrote:
> On Tue, 15 Sep 2015, Rafael J. Wysocki wrote:
> 
> > Hi Alan,
> 
> Hi.
> 
> > On Sat, Sep 12, 2015 at 7:40 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > > On Sat, 12 Sep 2015, Rafael J. Wysocki wrote:
> > >
> > >> On Friday, September 11, 2015 03:01:14 PM Alan Stern wrote:
> > >
> > >> > There's also an issue about other types of dependencies.  For instance,
> > >> > it's conceivable that device B might be discovered and depend on device
> > >> > A, even before A has been bound to a driver.  (B might be discovered by
> > >> > A's subsystem rather than A's driver.)  In that case, moving A to the
> > >> > end of the list would cause B to come before A even though B depends on
> > >> > A.  Of course, deferred probing already has this problem.
> > >>
> > >> That may actually happen for PCIe ports and devices below them AFAICS.
> > >>
> > >> Devices below PCIe ports are discovered by the PCI subsystem and the PCIe
> > >> ports need not be probed before those devices are probed.
> > >
> > > Is it possible to change this?  Make it so that devices below PCIe
> > > ports are discovered by the port driver rather than by the PCI
> > > subsystem?  Or would that be far too difficult?
> > 
> > I don't think it would be really useful.
> > 
> > PCIe ports are PCI bridges from the resource allocation and basic
> > functionality perspective, so they are handled accordingly.
> > 
> > The PCIe port driver provides additional services (such as PME/hotplug
> > interrupt handling and advanced error reporting) that aren't necessary
> > for probing devices below the ports.
> > 
> > I guess the ordering of PCIe ports probing might be changed to happen
> > at the "right" time, but care needs to be taken in that case too.
> 
> Does suspending a PCIe port do anything significant?  In particular, 
> does it cut off normal communication with anything attached through 
> the port?

No, it doesn't.

> If it does then we better not move the port device to the end of the 
> dpm_list after any descendant devices have been probed.
> 
> > >> > An easy way to check for this sort of thing would be to verify that a
> > >> > device about to be probed doesn't have any children.  This wouldn't
> > >> > catch all the potential dependencies, but it would be a reasonable
> > >> > start.
> > >>
> > >> That would address the PCIe ports issue.
> > >
> > > Well, it would _detect_ the PCIe ports issue.
> > 
> > That's what I meant. :-)
> > 
> > >  It might also detect other things.
> > 
> > Right.
> 
> All right, I'll try writing a test patch to report these things.

OK, thanks!

Rafael

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