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

[Guenter Roeck]

On 06/17/2014 02:07 PM, Bjorn Helgaas wrote:
> On Tue, Jun 17, 2014 at 1:27 PM, Myron Stowe <myron.stowe@xxxxxxxxxx> 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

I think I have seen that message once in a while in our systems.
Rajat, didn't we talk about this a while ago ?

Guenter

> 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.
>
> 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@xxxxxxxxxx>
> > ---
> >
> >   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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