linux-kernel - Re: [PATCH] PCI: pciehp: Include the Data Link Layer State Changed bit when clearing the Slot Status register's event bits

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Date:	Mon, 7 Jul 2014 14:26:37 -0600
From:	Myron Stowe <myron.stowe@...il.com>
To:	Bjorn Helgaas <bhelgaas@...gle.com>
Cc:	Myron Stowe <myron.stowe@...hat.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	Kenji Kaneshige <kaneshige.kenji@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rajat Jain <rajatjain.linux@...il.com>
Subject: Re: [PATCH] PCI: pciehp: Include the Data Link Layer State Changed
 bit when clearing the Slot Status register's event bits

On Tue, Jul 1, 2014 at 1:29 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
> On Mon, Jun 30, 2014 at 10:49 AM, Myron Stowe <myron.stowe@...il.com> wrote:
>> On Tue, Jun 17, 2014 at 3:07 PM, Bjorn Helgaas <bhelgaas@...gle.com> wrote:
>>> On Tue, Jun 17, 2014 at 1:27 PM, Myron Stowe <myron.stowe@...hat.com> wrote:
>>>> During PCIe hot-plug initialization - pciehp_probe - data structures
>>>> related to slot capabilities are set up.  As part of this set up, ISRs are
>>>> put in place to handle slot events and all event bits are cleared out.
>>>>
>>>> This patch adds the Data Link Layer State Changed (PCI_EXP_SLTSTA_DLLSC)
>>>> Slot Status bit to the event bits that are cleared out during
>>>> initialization.
>>>
>>> I think this is related to e48f1b67f668 ("PCI: pciehp: Use link change
>>> notifications for hot-plug and removal").  Prior to that, pcie_isr()
>>> didn't look at the PCI_EXP_SLTSTA_DLLSC bit.
>>>
>>> Apparently there's a non-public report of spurious messages like this
>>> at boot-time, with no actual hotplug events:
>>>
>>>   pciehp 0000:82:04.0:pcie24: slot(4): Link Up event
>>>   pciehp 0000:82:04.0:pcie24: Device 0000:83:00.0 already exists at
>>> 0000:83:00, cannot hot-add
>>>   pciehp 0000:82:04.0:pcie24: Cannot add device at 0000:83:00
>>>
>>> Device 0000:83:00.0 was enumerated previously, during the normal PCI
>>> device enumeration.  I suspect DLLSC was set by the hardware when the
>>> link came up after power-on, and it remained set until Linux booted.
>>> Then when we take the first pciehp interrupt, we notice DLLSC is set
>>> and think it's a new link state change.
>>>
>>> This might be system-specific, because some BIOSes might clear DLLSC
>>> before handing off to the OS.  Or my theory might be all wet.
>>>
>>> But I think we should clear DLLSC in any case.  We clear all the other
>>> RW1C bits in Slot Status, and I think we should clear DLLSC as well.
>>>
>>> I'd like to include a bugzilla or mailing list reference for the
>>> spurious message, and I'd also like confirmation that this changes
>>> actually fixes it.
>>
>> I just got confirmation from the reporter that the patch fixes the
>> issue they were encountering.
>>
>> I've also asked them numerous time if I could make the bug public as I
>> expect others may be hitting it and could benefit from a BZ and
>> analysis/explanation.  I just repeated my request - if they reply
>> positively I'll follow through with some documentation.
>
> I'd be glad to merge this, as soon as we get more details about the
> problem it fixes.  I think it's good to include some specifics, e.g.,
> "Unexpected Link Up event," etc., because it helps other people with
> the same problem to find the solution.  It's OK if details about
> pre-production machines and so on are removed, but it's better if
> changes to generic code are supported by public evidence that
> everybody can evaluate.
>
> If you want to pull out the public stuff into a kernel.org bugzilla,
> that would be fine, too.  That would actually be better because then
> we don't have to depend on Red Hat maintaining it.

Agreed (but I couldn't get the vendor to agree to open up the Red Hat
BZ).  I've opened https://bugzilla.kernel.org/show_bug.cgi?id=79611
and included a scrubbed 'dmesg' log there along with an analysis of
how we are hitting the issue.

>
> Bjorn
>
>>>> Reference:
>>>>   PCI-SIG.  PCI Express Base Specification Revision 4.0 Version 0.3
>>>>   (PCI-SIG, 2014): 7.8.11. Slot Status Register (Offset 1Ah).
>>>>
>>>> Signed-off-by: Myron Stowe <myron.stowe@...hat.com>
>>>> ---
>>>>
>>>>  drivers/pci/hotplug/pciehp_hpc.c |    2 +-
>>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/drivers/pci/hotplug/pciehp_hpc.c b/drivers/pci/hotplug/pciehp_hpc.c
>>>> index 42914e0..0568416 100644
>>>> --- a/drivers/pci/hotplug/pciehp_hpc.c
>>>> +++ b/drivers/pci/hotplug/pciehp_hpc.c
>>>> @@ -794,7 +794,7 @@ struct controller *pcie_init(struct pcie_device *dev)
>>>>         pcie_capability_write_word(pdev, PCI_EXP_SLTSTA,
>>>>                 PCI_EXP_SLTSTA_ABP | PCI_EXP_SLTSTA_PFD |
>>>>                 PCI_EXP_SLTSTA_MRLSC | PCI_EXP_SLTSTA_PDC |
>>>> -               PCI_EXP_SLTSTA_CC);
>>>> +               PCI_EXP_SLTSTA_CC | PCI_EXP_SLTSTA_DLLSC);
>>>>
>>>>         /* Disable software notification */
>>>>         pcie_disable_notification(ctrl);
>>>>
>>> --
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