Re: [PATCH v17 09/12] PCI/AER: Allow clearing Error Status Register in FF mode

[Kuppuswamy Sathyanarayanan]

Hi,

On 3/12/20 2:52 PM, Bjorn Helgaas wrote:
> On Thu, Mar 12, 2020 at 02:29:58PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > Hi,
> >
> > On 3/12/20 2:02 PM, Austin.Bolen@xxxxxxxx wrote:
> > > On 3/12/2020 2:53 PM, Bjorn Helgaas wrote:
> > > > On Wed, Mar 11, 2020 at 04:07:59PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > > > > On 3/11/20 3:23 PM, Bjorn Helgaas wrote:
> > > > > > Is any synchronization needed here between the EDR path and the
> > > > > > hotplug/enumeration path?
> > > > > If we want to follow the implementation note step by step (in
> > > > > sequence) then we need some synchronization between EDR path and
> > > > > enumeration path. But if it's OK to achieve the same end result by
> > > > > following steps out of sequence then we don't need to create any
> > > > > dependency between EDR and enumeration paths. Currently we follow
> > > > > the latter approach.
> > > > What would the synchronization look like?
> > > >
> > > > Ideally I think it would be better to follow the order in the
> > > > flowchart if it's not too onerous.  That will make the code easier to
> > > > understand.  The current situation with this dependency on pciehp and
> > > > what it will do leaves a lot of things implicit.
> > > >
> > > > What happens if CONFIG_PCIE_EDR=y but CONFIG_HOTPLUG_PCI_PCIE=n?
> > > >
> > > > IIUC, when DPC triggers, pciehp is what fields the DLLSC interrupt and
> > > > unbinds the drivers and removes the devices.  If that doesn't happen,
> > > > and Linux clears the DPC trigger to bring the link back up, will those
> > > > drivers try to operate uninitialized devices?
> > > >
> > > > Does EDR need a dependency on CONFIG_HOTPLUG_PCI_PCIE?
> > >    From one of Sathya's other responses:
> > >
> > > "If hotplug is not supported then there is support to enumerate
> > > devices via polling  or ACPI events. But a point to note
> > > here is, enumeration path is independent of error handler path, and
> > > hence there is no explicit trigger or event from error handler path
> > > to enumeration path to kick start the enumeration."
> > >
> > > The EDR standard doesn't have any dependency on hot-plug. It sounds like
> > > in the current implementation there's some manual intervention needed if
> > > hot-plug is not supported?
> > No, there is no need for manual intervention even in non hotplug
> > cases.
> >
> > For ACPI events case, we would rely on ACPI event to kick start the
> > enumeration. And for polling model, there is an independent polling
> > thread which will kick start the enumeration.
> I'm guessing the ACPI case works via hotplug_is_native(): if
> CONFIG_HOTPLUG_PCI_PCIE=n, pciehp_is_native() returns false, and
> acpiphp manages hotplug.
>
> What if CONFIG_HOTPLUG_PCI_ACPI=n also?
If none of the auto scans are enabled then we might need some
manual trigger ( rescan). But this would be needed in native
DPC case as well.
> Where is the polling thread?
drivers/pci/hotplug/pciehp_hpc.c
> > Above both enumeration models are totally independent and has
> > no dependency on error handler thread.
> I see they're currently independent from the EDR thread, but it's not
> clear to me that there's no dependency.  After all, both EDR and the
> hotplug paths are operating on the same devices at roughly the same
> time, so we should have some story about what keeps them from getting
> in each other's way.
>
> > We will decide which model to use based on hardware capability and
> > _OSC negotiation or kernel command line option.

--
Sathyanarayanan Kuppuswamy
Linux kernel developer