Date:   Tue, 15 Aug 2017 17:42:56 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Keith Busch <keith.busch@...el.com>
Cc:     linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Mayurkumar Patel <mayurkumar.patel@...el.com>
Subject: Re: [PATCH] pciehp: Fix infinite interupt handler loop

On Tue, Aug 15, 2017 at 03:48:25PM -0500, Bjorn Helgaas wrote:
> On Mon, Aug 14, 2017 at 06:11:23PM -0400, Keith Busch wrote:
> > On Mon, Aug 14, 2017 at 03:59:48PM -0500, Bjorn Helgaas wrote:
> > > On Tue, Aug 01, 2017 at 03:11:52AM -0400, Keith Busch wrote:
> > > > We've encountered a particular platform that under some circumstances
> > > > always has the power fault detected status raised. The pciehp irq handler
> > > > would loop forever because it thinks it is handling new events when in
> > > > fact the power fault is not new. This patch fixes that by masking off
> > > > the power fault status from new events if the driver hasn't seen the
> > > > power fault clear from the previous handling attempt.
> > > 
> > > Can you say which platform this is?  If this is a hardware defect,
> > > it'd be interesting to know where it happens.
> > > 
> > > But I'm not sure we handle PCI_EXP_SLTSTA correctly.  We basically
> > > have this:
> > > 
> > >   pciehp_isr()
> > >   {
> > >     pcie_capability_read_word(pdev, PCI_EXP_SLTSTA, &status);
> > >     events = status & (<events we care about>);
> > >     pcie_capability_write_word(pdev, PCI_EXP_SLTSTA, events);
> > >     <queue event handling>
> > >   }
> > > 
> > > The write to PCI_EXP_SLTSTA clears PCI_EXP_SLTSTA_PFD because it's
> > > RW1C.  But we haven't done anything that would actually change the
> > > situation that caused a power fault, so I don't think it would be
> > > surprising if the hardware immediately reasserted it.
> > > 
> > > So maybe this continual assertion of power fault is really a software
> > > bug, not a hardware problem?
> > 
> > I *think* it's a software bug for the exact reason you provided, but I'm
> > sure it must be isolated to certain conditions with certain hardware. We'd
> > have heard about this regression during 4.9 if it was more wide-spread.
> 
> OK, I think this makes pretty good sense.
> 
> > > > Fixes: fad214b0aa72 ("PCI: pciehp: Process all hotplug events before
> > > > looking for new ones")
> 
> > This is on a PEX8733 bridge, and it reports power fault detected status as
> > long as the power fault exists. While we can write 1 to clear the event,
> > that just rearms the port to retrigger power fault detected status for as
> > long as the power controller detects its faulted. The status is cleared
> > for good only when the power fault no longer exists rather than when
> > it is acknowledged. The spec seems to support that view (Table (7-21:
> > Slot Status Register):
> > 
> >   Power Fault Detected – If a Power Controller that supports power fault
> >   detection is implemented, this bit is Set when the Power Controller
> >   detects a power fault at this slot.
> 
> Interesting: 5651c48cfafe ("PCI pciehp: fix power fault interrupt
> storm problem") turned off PCI_EXP_SLTCTL_PFDE in 2009 for basically
> the same problem.
> 
> AFAICS, we *still* never turn PCI_EXP_SLTCTL_PFDE on, so in your case,
> you're probably taking an interrupt for some other reason, and
> coincidentally noticing that PCI_EXP_SLTSTA_PFD is set.
> 
> Maybe it's time to turn PCI_EXP_SLTCTL_PFDE back on along with your
> fix to prevent the loop?  It seems like kind of a hole that we will
> never notice power faults unless some other interrupt happens.  Or am
> I missing something?
> 
> I'd like to move your PCI_EXP_SLTSTA_PFD out on its own so there's
> room for a comment.  What do you think of the following?  I really
> don't think this is specific to a particular platform (other than
> maybe timing-wise), so I dropped that from the changelog too.
> 
> 
> commit 7612b3b28c0b900dcbcdf5e9b9747cc20a1e2455
> Author: Keith Busch <keith.busch@...el.com>
> Date:   Tue Aug 1 03:11:52 2017 -0400
> 
>     PCI: pciehp: Report power fault only once until we clear it
>     
>     When a power fault occurs, the power controller sets Power Fault Detected
>     in the Slot Status register, and pciehp_isr() queues an INT_POWER_FAULT
>     event to handle it.
>     
>     It also clears Power Fault Detected, but since nothing has yet changed to
>     correct the power fault, the power controller will likely set it again
>     immediately, which may cause an infinite loop when pcie_isr() rechecks
>     Slot Status.
>     
>     Fix that by masking off Power Fault Detected from new events if the driver
>     hasn't seen the power fault clear from the previous handling attempt.
>     
>     Fixes: fad214b0aa72 ("PCI: pciehp: Process all hotplug events before looking for new ones")
>     Signed-off-by: Keith Busch <keith.busch@...el.com>
>     [bhelgaas: changelog, pull test out and add comment]
>     Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
>     Cc: Mayurkumar Patel <mayurkumar.patel@...el.com>
>     Cc: stable@...r.kernel.org      # 4.9+
> 
> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
> index 026830a138ae..e5d5ce9e3010 100644
> --- a/drivers/pci/hotplug/pciehp_hpc.c
> +++ b/drivers/pci/hotplug/pciehp_hpc.c
> @@ -586,6 +586,14 @@ static irqreturn_t pciehp_isr(int irq, void *dev_id)
>  	events = status & (PCI_EXP_SLTSTA_ABP | PCI_EXP_SLTSTA_PFD |
>  			   PCI_EXP_SLTSTA_PDC | PCI_EXP_SLTSTA_CC |
>  			   PCI_EXP_SLTSTA_DLLSC);
> +
> +	/*
> +	 * If we've already reported a power fault, don't report it again
> +	 * until we've done something to handle it.
> +	 */
> +	if (ctrl->power_fault_detected)
> +		events &= ~PCI_EXP_SLTSTA_PFD;
> +
>  	if (!events)
>  		return IRQ_NONE;
>  

This is on pci/hotplug for v4.14.

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