linux-kernel - Re: [PATCH v2] PCI: pciehp: Don't enable PME on runtime suspend

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Date:   Tue, 07 Feb 2017 17:15:03 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Bjorn Helgaas <helgaas@...nel.org>,
        Yinghai Lu <yinghai@...nel.org>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PCI: pciehp: Don't enable PME on runtime suspend

On Tuesday, February 07, 2017 07:26:59 AM Lukas Wunner wrote:
> On Mon, Feb 06, 2017 at 10:52:12PM +0100, Rafael J. Wysocki wrote:
> > On Monday, February 06, 2017 10:20:41 PM Lukas Wunner wrote:
> > > On Mon, Feb 06, 2017 at 11:54:05AM -0600, Bjorn Helgaas wrote:
> > > > On Mon, Feb 06, 2017 at 06:54:37AM +0100, Lukas Wunner wrote:
> > > > > Since commit 68db9bc81436 ("PCI: pciehp: Add runtime PM support for PCIe
> > > > > hotplug ports") we runtime suspend a hotplug port to D3hot when all its
> > > > > children are runtime suspended or none are present.
> > > > > 
> > > > > When runtime suspending the port the PCI core automatically enables PME:
> > > > >     pci_pm_runtime_suspend()
> > > > >         pci_finish_runtime_suspend()
> > > > >             __pci_enable_wake()
> > > > > 
> > > > > According to the PCI Express Base Specification, section 6.7.3.4:
> > > > >    "Note that PME and Hot-Plug Event interrupts (when both are
> > > > >     implemented) always share the same MSI or MSI-X vector [...]
> > > > >     If wake generation is required by the associated form factor
> > > > >     specification, a hot-plug capable Downstream Port must support
> > > > >     generation of a wakeup event (using the PME mechanism) on hotplug
> > > > >     events that occur when the system is in a sleep state or the Port
> > > > >     is in device state D1, D2, or D3Hot."
> > > > > 
> > > > > Thus, if the port is runtime suspended even though it is still occupied,
> > > > > it may immediately be woken by a PME interrupt.
> > > > 
> > > > The spec goes on to say that a wakeup event should be generated when
> > > > all three of these conditions occur:
> > > > 
> > > >   - status register for an enabled [hotplug] event transitions from
> > > >     not set to set
> > > > 
> > > >   - Port is in D1, D2, or D3hot,
> > > > 
> > > >   - PME_En is set
> > > > 
> > > > I think you're saying that if we put a hotplug-capable port that
> > > > controls an occupied slot into D3hot, the port may immediately
> > > > generate a wakeup PME.
> > > > 
> > > > What is the hotplug event that causes generation of this wakeup event?
> > > 
> > > If you had read all e-mails in this thread or looked at the bugzilla
> > > entry I've created, you wouldn't have to ask this question.
> > > 
> > > I think it's disappointing that you're asking me to jump through
> > > various hoops like creating a bugzilla entry, as well as threatening
> > > to revert my patch, but are unwilling to even look at the bugzilla
> > > entry or read the entire thread.  It is equally disappointing that
> > > the reporter of the regression was unwilling or unable to provide
> > > dmesg output for both machines so that we've got no real idea what
> > > we're dealing with.
> > > 
> > > It's a Link Up event.
> > > 
> > > Which doesn't make sense of course because per the spec PME is only
> > > supposed to be signaled when the Link Status *changes* from 0 to 1,
> > 
> > BTW, where exactly did you find this bit in the spec?
> 
> PCIe Base Spec, section 6.7.3.4:

That whole section isn't particularly clear to me to be honest.  At least it
may be subject to interpretation leading to different types of behavior IMO.

> "For form factors that support wake generation, a wakeup event must be
>  generated if all three of the following conditions occur:
> 
>  * The status register for an enabled event transitions from not set to set
>                                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Are you sure that the "status register" here refers to Link Status?

>  * The Port is in device state D1, D2, or D3Hot, and
>  * The PME_En bit in the Port's Power Management Control/Status register is
>    set"

Also it doesn't say when PMEs can be generated.  It only says when it is
required to generate them.
 
> 
> My point was that the Link Status shouldn't arbitrarily change from 0 to 1
> after powering off the slot.

That's a fair point, but I'm not sure this is what actually happens.

Thanks,
Rafael